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Loss of reactive elements during electroslag processing of iron-base alloys Etienne, Michel 1970

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THE LOSS OF REACTIVE ELEMENTS DURING ELECTROSLAG PROCESSING OF IRON-BASE ALLOYS  BY  MICHEL ETIENNE I n g e n i e u r C i v i l M e t a l l u r g i s t s , L i e g e 1965. M . A . S c , E c o l e P o l y t e c h n i q u e , U . de M. , 1966.  A THESIS SUBMITTED IN PARTIAL FULFILMENT OF THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY  i n the  Department of  METALLURGY  We a c c e p t t h i s t h e s i s as conforming to the standards  THE UNIVERSITY OF BRITISH COLUMBIA October,  1970  required  In p r e s e n t i n g t h i s  thesis  an advanced degree at the L i b r a r y I  the U n i v e r s i t y  s h a l l make i t  f u r t h e r agree  in p a r t i a l  freely  f u l f i l m e n t o f the  requirements  of B r i t i s h ' C o l u m b i a ,  available  for  I agree  for  that  r e f e r e n c e and s t u d y .  that permission for extensive copying of t h i s  thesis  f o r s c h o l a r l y purposes may be g r a n t e d by the Head o f my Department o r by h i s of  this  written  representatives.  It  thesis for financial  i s understood that copying o r p u b l i c a t i o n gain shall  permission.  Department o f The U n i v e r s i t y o f B r i t i s h Vancouver 8, Canada  Date  Columbia  not  be allowed without my  - i i RESUME Ce t r a v a i l c o n s t i t u e une etude de l ' o x y d a t i o n d ' E l e m e n t s  d'alliage  pendant l a r e f u s i o n d ' a c i e r s a l l i e s sous l a i t i e r E l e c t r o c o n d u c t e u r .  Les  cas p a r t i c u l i e r s t r a i t e * s sont l e s p e r t e s de t i t a n e dans l ' a c i e r i n o x y d a b l e A I S I 321 et l ' a c i e r maraging type 300 e t l e s p e r t e s d ' a l u m i n i u m dans l ' a c i e r au chrome 1409 A l . Les p r i n c i p a u x parametres de l a r e f u s i o n dont on e"tudie sont:  l a p r e s s i o n p a r t i e l l e d'oxygene dans 1 ' a t m o s p h e r e ,  1'influence  l a composition  du l a i t i e r en ce q u ' e l l e a f f e c t e l e p o t e n t i e l oxygene e t l e t r a n s f e r t  de  m a t i e r e , l e type e t l a p o l a r i t e " du c o n r a n t d l e c t r i q u e , l a v i t e s s e de fusion.  On a f a i t l a d i s t i n c t i o n e n t r e l e s v a r i a t i o n s de c o m p o s i t i o n  q u i r d s u l t e n t de l ' o x y d a t i o n e t c e l l e s q u i r e s u l t e n t de 1 ' E l i m i n a t i o n d'inclusions. D i v e r s modeles c i u e t i q u e s sont proposes q u i de"crivent l e  transfert  par d i f f u s i o n de l ' o x y g & n e et des Elements d ' a l l i a g e aux d i v e r s i n t e r faces entre phases.  Ces modules sont compare's aux r d s u l t a t s expErimentaux  obtenus avec l a machine de l a b o r t o i r e e t ceux q u i sont d i s p o n i b l e s dans l a l i t e r a t u r e .  - i i i ABSTRACT O x i d a t i v e l o s s e s of r e a c t i v e  elements  during Electroslag  R e m e l t i n g have been i n v e s t i g a t e d i n the a l l o y s A I S I 321 s t a i n l e s s  steel  and Maraging 300 s t e e l w h i c h c o n t a i n t i t a n i u m and i n 1409 A l s t e e l , containing aluminium. A t t e n t i o n has been devoted to the i n f l u e n c e of v a r i o u s m e l t p a r a meters,  i.e.,  the p a r t i a l p r e s s u r e of oxygen i n the atmosphere,  the  c o m p o s i t i o n of the s l a g i n r e l a t i o n to i t s oxygen p o t e n t i a l and mass transport properties, the m e l t r a t e .  the type of c u r r e n t ,  the p o l a r i t y of o p e r a t i o n and  A d i s t i n c t i o n i s made between a c t u a l o x i d a t i v e l o s s e s  and c o m p o s i t i o n v a r i a t i o n s w h i c h may r e s u l t d u r i n g the r e m e l t i n g  from the removal of  process-  K i n e t i c models are proposed f o r the r a t e of t r a n s f e r and a l l o y i n g elements  inclusions  between the v a r i o u s phases p r e s e n t ,  of oxygen • and are  compared  w i t h the e x p e r i m e n t a l d a t a o b t a i n e d i n t h i s work or c o l l e c t e d from the literature.  - iv  -  TABLE OF CONTENTS VOLUME I  P a  TITLE PAGE . . ,  ?  e  1  ABSTRACT  1 1  TABLE OF CONTENTS  i v  LIST OF FIGURES  ix  LIST OF TABLES  xii  LIST OF SYMBOLS  xiii  ACKNOWLEDGEMENTS  CHAPTER I .  xvi  INTRODUCTION  1  1.1  The ESR p r o c e s s  1  1.2  Statement of  3  1.3  Choice of  the problem  the m a t e r i a l s  ^ o  1.4  Preliminary dicussion 1.4.1  8  1.4.2  9  1.4.3  1 0  1.4.4  1 0  1.4.5  1 0  CHAPTER I I . 11.1  THE U . B . C . ELECTROSLAG UNIT  Generalities  ,  I  1  ^ 1 o  11.2  Framework and e l e c t r o d e t r a v e l mechanism  11.3  Electrode holder  I I . 4 Mold baseplate  1 3  1 3  - v Page II. 5 II.6  Mold and water j a c k e t Atmosphere c o n t r o l  14 15  II. 7  Continuous s l a g o r m e t a l a d d i t i o n s  16  11.8  Other r e m e l t i n g equipment  16  11.9  D . C . power s u p p l y  16  11.10  A . C . power s u p p l y  17  11.11  D.C. bias  18  11.12  D.C. controls  19  11.13  A . C . controls  19  11.14  Common c o n t r o l s  20  11.15  P a r t i c u l a r i t i e s of t h e d e s i g n  20  11.16  S t a r t i n g procedure  22  CHAPTER I I I .  REMELTING OF INGOTS  24  111.1  P r o c e d u r e and p r e c a u t i o n s  24  111.2  C o m p o s i t i o n of t h e a l l o y s s t u d i e d  25  111.3  Slags  26  111.4  R e m e l t i n g c o n d i t i o n s f o r 321 s t a i n l e s s  111.5  R e m e l t i n g c o n d i t i o n s f o r Maraging 300 s t e e l  39  111.6  R e m e l t i n g c o n d i t i o n s f o r 1409 A l s t e e l  43  III. 7  R e m e l t i n g of F e r r o v a c E and m i l d s t e e l  46  111.8  A n a l y s i s and s a m p l i n g of i n g o t s  48  111.9  I n c l u s i o n counts and r a t i n g s  49  steel  28  I I I . 10 A n a l y s i s and s a m p l i n g of t h e s l a g  51  111.11 C o m p o s i t i o n and m a t e r i a l b a l a n c e o f 321 S . S . i n g o t s . .  53  111.12 C o m p o s i t i o n and m a t e r i a l b a l a n c e of M a r a g i n g 300 s t e e l ingots  ^  •A  y  - vi -  Page I I I . 1 3 C o m p o s i t i o n o f 1409 A l s t e e l i n g o t s  75  I I I . 14 Rate of o x i d a t i o n o f i r o n  79  111.15 S o l i d i f i c a t i o n p a t t e r n s  80  111.16 O b s e r v a t i o n of c o n v e c t i v e motions  82  I I I . 17 E l e c t r o d e temperature p r o f i l e s  83  CHAPTER I V .  8 5  IV. 1  IV.2  DISCUSSION  Q u a n t i t a t i v e e v a l u a t i o n of o x i d a t i o n causes  86  IV.1.1  A t m o s p h e r i c oxygen  86  IV.1.2  O x i d a t i o n o f the e l e c t r o d e  8 7  IV.1.3  Electrolysis  90  IV.1.4  Thermochemical d a t a  91  IV.1.5  E q u i l i b r i u m of d e o x i d a t i o n w i t h t i t a n t i u m . .  94  IV.1.6  E q u i l i b r i u m of d e o x i d a t i o n w i t h a l u m i n i u m . . .  97  IV.1.7  S t a t e of o x i d a t i o n of t i t a n i u m i n the s l a g . .  99  P h y s i c a l d e s c r i p t i o n of t h e p r o c e s s IV.2.1  100  Temperature g r a d i e n t s and n a t u r a l c o n v e c t i o n : i n the s l a g  1 0 0  IV.2.2  D i f f u s i o n a t the s l a g atmosphere i n t e r f a c e . .  1 0 2  IV.2.3  D i f f u s i o n i n an e l e c t r i c f i e l d  1 0 8  IV.2.4  Flow of m e t a l on the e l e c t r o d e  1 1 0  IV.2.5  D i f f u s i o n of oxygen i n the e l e c t r o d e  IV.2.6  (reverse p o l a r i t y ) D i f f u s i o n o f a l l o y i n g elements i n t h e electrode f i l m (reverse p o l a r i t y )  IV.2,7  film  D i f f u s i o n i n the m e t a l a t t h e s l a g - i n g o t interface (direct polarity)  l± ixf to  l  2  - ^  - vii Page IV.2.8  D i f f u s i o n i n the s l a g a t the s l a g - m e t a l interfaces ,  IV.2.9  IV.3  Comparison between oxygen and a l l o y i n g  122 element  d i f f u s i o n i n the m e t a l  124  IV.2.10  Dependence upon time o r W^/W  125  IV.2.11  M a c r o s c o p i c assessment o f t h e  process  (Oxidant i n l i m i t e d s u p p l y - A . C . )  127  I V . 2.12  M e l t r a t e and s e g r e g a t i o n  131  IV.2.13  Comparison between s m a l l and l a r g e u n i t s . .  133  Review of the e x p e r i m e n t a l r e s u l t s w i t h r e s p e c t  to  c o m p o s i t i o n changes  138  IV.3.1  I n f l u e n c e of the atmosphere  138  IV.3.2  I n f l u e n c e of s l a g c o m p o s i t i o n on 321  S.S. -I  ingots I n f l u e n c e o f s l a g c o m p o s i t i o n on M a r . 300 ingots I n f l u e n c e o f s l a g c o m p o s i t i o n on 1409 A l ingots I n f l u e n c e o f s l a g c o m p o s i t i o n on the  IV.3.3 IV.3.4 IV.3.5  O Q  x J O  l ^  2  i  3  o x i d a t i o n o f Fe IV.3.6  I  I n f l u e n c e o f the m e l t r a t e  IV.3.7  4  . I n f l u e n c e o f the oxygen p o t e n t i a l o f the s l a g  i  4 4  4  4  145  IV.3.8  I n f l u e n c e of r e m e l t i n g p o l a r i t y  i  IV.3.9  I n c l u s i o n and oxygen c o n t e n t s  I ''  IV.3.10  Importance of the a n a l y s i s methods  1^  CHAPTER V .  4  6  4  151  CONCLUSIONS  VOLUME I I Appendix I .  Flow of m e t a l on the  electrode  154  - v i i i Page Al.l  C a l c u l a t i o n of t ,  A1.2  Momentum t r a n s f e r w i t h the s l a g . ,  e  Appendix I I .  Segregation  Appendix I I I .  A c t i v i t y of T 1 0  A4.2  i n ESR  2  154 155  158  i n CaF^CaO  161  A3.1  D e s c r i p t i o n of the experiment  161  A3.2  R e l a t i o n to ESR  1 6 2  A3.3  Reactions  1 6 3  A3.4  Range of the experiments  A3.5  Activity coefficient  A3. 6  Discussion  Appendix I V . A4.1  free flowing f i l m  of T i C  164  of TiC>  2  165 168  A n a l y t i c a l methods  1^0  A n a l y s i s of s l a g s  1^0  A4.1.1  T i t a n i u m and I r o n  I  A4.1.2  Chromium  i 7 2  A4.1.3  Fluorine  i 7 2  A n a l y s i s of the s t e e l s A4.2.1  7 0  ,  T o t a l m e t a l l i c elements  i 7 3  1 TO A4.2.2  M a t r i x m e t a l l i c elements  X / J  A4.2.3  Oxygen  1 7 5  BIBLIOGRAPHY FIGURES  1 7 7  182  y  - ix  -  LIST OF FIGURES Figure  Page  CHAPTER I 1  P r i n c i p l e of E l e c t r o s l a g R e m e l t i n g  182  CHAPTER I I 2  Cold s t a r t c o n f i g u r a t i o n  182  3  Commercial ESR u n i t (Consarc C o r p o r a t i o n )  183  4  E l e c t r o d e d r i v e mechanism  184  5  Schematic  185  6  Electrode holder  186  7  B a s e p l a t e arrangement  187  8  Mold and water j a c k e t  188  9  Fume hood  188  10  Atmospheric s h i e l d  189  11  S l a g and m e t a l a d d i t i o n d e v i c e  190  12  Non consumable e l e c t r o d e  191  13  Sampling of the s l a g  191  14  View of the l a b o r a t o r y  190  15  Schematic  192  16  Welders output c h a r a c t e r i s t i c s  CHAPTER  d r i v e motor c i r c u i t  configuration  power c i r c u i t s  193  III  17  Sampling of i n g o t and s l a g  19^  18  C o m p o s i t i o n p r o f i l e - Ingot S l , S2,  19  C o m p o s i t i o n p r o f i l e - Ingot S6  20  C o m p o s i t i o n p r o f i l e - Ingot S7,  S8  21  C o m p o s i t i o n p r o f i l e - I n g o t S9,  S10,  S3  i 9 5  1^6 1 9 7  Sll  i 9 8  -  X  -  Figure  Page  22  Composition p r o f i l e .  I n g o t S12  199  23  Composition p r o f i l e .  Ingot S13  200  24  Composition p r o f i l e .  Ingot S14  201  25  Composition p r o f i l e .  I n g o t S15  202  26  Composition p r o f i l e . '  Ingot S16  203  27  Composition p r o f i l e . -  Ingot S17  204  28  Composition p r o f i l e .  I n g o t M l , M3, M4  205  29  Composition p r o f i l e .  Ingot M2  206  30  Composition p r o f i l e .  I n g o t M5, M6  207  31  Composition p r o f i l e .  Ingot M7  208  32  Macrograph, i n g o t S17  209  33  Macrograph, i n g o t S14  209  34  M a c r o g r a p h , i n g o t M3  210  35  Macrograph, i n g o t M7  211  36  Macrograph, i n g o t A l  210  37  P a t t e r n of g r a p h i t e  particles  on the s u r f a c e of  the  slag  212  38  Suggested c o n v e c t i o n  currents  213  39  Thermocouple  40  Electrode  temperature g r a d i e n t s  (stainless steel)  41  Electrode  temperature g r a d i e n t s  (mild s t e e l )  arrangement ( e l e c t r o d e )  213 ..  214 215  CHAPTER IV 42  D e o x i d a t i o n of 321 S.S  216  43  D e o x i d a t i o n of m i l d s t e e l  217  44  D e o x i d a t i o n e q u i l i b r i a ' i n f u n c t i o n of p 0  45  D i f f u s i o n i n the s l a g  2  218 219  - xi  -  Figure  Page  46  Flow on the e l e c t r o d e  219  47  F i l m of m e t a l on the e l e c t r o d e  220  48  I n t e r f a c i a l v e l o c i t y of the m e t a l on the e l e c t r o d e  49  Oxygen d i f f u s i o n i n f:he e l e c t r o d e  50  V a r i a t i o n of t  51  e O x i d a t i o n of major a l l o y i n g elements  .  film  (on the e l e c t r o d e )  220 221  with 0 i n the  221 electrode  film  222  52  Equation (IV.19)  223  53  I n f l u e n c e of m e l t r a t e on A [ T i ] e l - v e  54  I n f l u e n c e of the s l a g c o m p o s i t i o n on the oxygen content  321 S . S .  .....  224 225  Appendix I Al.l  Momentum t r a n s f e r between the e l e c t r o d e  f i l m and the  slag Appendix A2.1  226  II C o m p o s i t i o n p r o f i l e due to m e l t r a t e d i s c o n t i n u i t y .  Appendix I I I  227  '  A3.1  E q u i l i b r a t i o n apparatus  228  A3.2  A c t i v i t y of t i t a n i u m o x i d e s under 1 atm C 0 / C ( g r )  A3.3  R a t i o of a c t i v i t i e s of t i t a n i u m i o n s  230  A3.4  y°  231  = f u n c t i o n of T  ...  229  2 Appendix IV A4.1  C a l i b r a t i o n of s p e c i f i c f l u o r i d e i o n e l e c t r o d e  232  A4.2  C a l i b r a t i o n of the e l e c t r o n probe f o r  2^3  [Ti]  - xii LIST OF TABLES Table  Page  CHAPTER I I I 1 to 19  R e m e l t i n g c o n d i t i o n s f o r i n g o t s S l t o S19 (321 S . S . )  30-38  20 t o 26  R e m e l t i n g c o n d i t i o n s f o r i n g o t s M l t o M7 ( M a r . 300)  39-42  27 to 30  R e m e l t i n g c o n d i t i o n s f o r i n g o t s A l t o A4 (1409 A l )  44-45  31 t o 35  R e m e l t i n g c o n d i t i o n s f o r i n g o t s F l t o F5 ( F e r r o v a c -E and 1018 M . S . )  46-47  36  C r y s t a l l i n e compounds of T i i n s l a g s  ->2  37 to 39  C o m p o s i t i o n of i n g o t s S l to S3  54-55  40 t o 51  C o m p o s i t i o n of i n g o t s S6 t o S17  56-68  52 to 58  C o m p o s i t i o n o f i n g o t s M l to M7  69-74  59 to 62  C o m p o s i t i o n of i n g o t s A l t o A4  76-78  63  O x i d a t i o n of i r o n  :  .  79  CHAPTER IV 64  Sources of o x i d a n t  86  65  O x i d a t i o n a t the s l a g - i n g o t i n t e r f a c e  APPENDIX I I I A.l  Free e n t h a l p i e s of f o r m a t i o n ( A I I I )  A. 2  Range o f experiments  A. 3  Experimental r e s u l t s , Y ^  l ^  (AIII) i 0  3  1^ (AIII)  166  APPENDIX IV A.4  A n a l y s i s of t o t a l m e t a l l i c elements i n s t e e l s  (AIV).  173  - xiii  -  LIST OF SYMBOLS  a.  1  A  a c t i v i t y of  i  t o t a l area of s l a g - m e t a l c o n t a c t ,  C C C o d D  -3 c o n c e n t r a t i o n o f d i f f u s i n g s p e c i e s i n the s l a g , g cm -3 i n t e r f a c i a l c o n c e n t r a t i o n o f same, g cm -3 b u l k c o n c e n t r a t i o n of same, g cm t h i c k n e s s of s l a g boundary l a y e r , cm diffusion coefficient,  D. . erf x  cm  2  2 -1 cm sec  2 d i f f u s i o n c o e f f i c i e n t o f i i n d i l u t e s o l u t i o n i n i , cm sec e r r o r f u n c t i o n of x  e~! i  i n t e r a c t i o n c o e f f i c i e n t of i on i  f_^  Henryan a c t i v i t y c o e f f i c i e n t of  F  Faraday's  i  constant -2  g  a c c e l e r a t i o n of g r a v i t y , cm sec  I  electric current, A -2  J  d i f f u s i v e mass f l u x per u n i t a r e a , g cm  k k  constant effective distribution coefficient  k  e  -1 sec  (solidification)  k' P K  equilibrium distribution coefficient (solidification) 2 -1 p a r a b o l i c c o n s t a n t of o x i d a t i o n cm sec . r e a c t i o n constant  K  r e a c t i o n constant  P  (partial  pressures)  H  i n g o t - e l e c t r o d e gap, cm  L  depth of p e n e t r a t i o n i n unsteady s t a t e d i f f u s i o n , cm  m  subscript for metal  - xiv -  (M)  _3 c o n c e n t r a t i o n of M i n the m e t a l , g cm _3 c o n c e n t r a t i o n of ^ i n the s l a g , g cm  [M]  c o n c e n t r a t i o n of M i n the e l e c t r o d e m e t a l , g c m "  [M]  (M)  o x i d a n t i n the s l a g e q u i v a l e n t to  N. x  mole f r a c t i o n of  p^  p a r t i a l p r e s s u r e qf i , atm.  i  i n t e g r a t e d mass f l u x of i , g sec r  radius  R  r a d i u s of the e l e c t r o d e ,  R^  r a d i u s of the mol4, cm  s  subscript for slag  S  interfacial  t  time,  t  1  coordinate cm  area  sec  exposure t i m e ,  e  [M], g cm  sec  T  absolute temperature,  u. x  m o b i l i t y of i o n i  v ,v r 6  v e l o c i t i e s i n polar coordinates  v ,v ,v x y z  v e l o c i t i e s i n Cartesian coordinates,  v  t e r m i n a l v e l o c i t y , cm sec  °K  ' *  oo  V  process v o l t a g e , V  W , aw Wm= 3- t—  mass of r e m e l t e d m e t a l , g ' m e l t r a t e , g sec  .V W m  m Ie l t 'r a- t e ,  m  w  P  W  m — m  _3  •»  6  •  - 1 cm 3sec  mass of the s l a g b a t h , g  1  (slag),  cm sec - 1  cm sec  1  3  -  x,y,z  XV  -  C a r t e s i a n c o o r d i n a t e s , cm  vY = _m W w  Raoultian a c t i v i t y coefficient  of  i  B,r  Eulerian functions  6  T h i c k n e s s of l i q u i d m e t a l f i l m on e l e c t r o d e ,  AF°  Standard f r e e e n t h a l p y of r e a c t i o n , c a l mol  AH  E n t h a l p y of r e a c t i o n , c a l mole  AS  E n t r o p y of r e a c t i o n c a l mole  G  B a s a l a n g l e of the e l e c t r o d e mass t r a n s f e r  X  e l e c t r i c a l c o n d u c t i v i t y , cm fi viscosity,  x  X  °K  tip  K  u(with subscript)  coefficient,  X  cm  cm sec  X  cone X  x  poise  -4  p(without s u b s c r i p t ) II  10  cm  product  p=p -p m  P >P t  s  g cm  - 3  -3  x  d e n s i t y of l i q u i d m e t a l , o f l i q u i d s l a g , g cm summation u 1 - 2 shear s t, r e s s , g cm - sec  $  electric  p KA s (j) =— W m  dimensionless  %  weight  m  s  field,  percent  proportional  to  v o l t cm  x  - xvi -  ACKNOWLEDGEMENT The a u t h o r i s i n d e b t e d to D r . A . M i t c h e l l f o r h i s a d v i c e and a s s i s t a n c e throughout the d u r a t i o n of t h i s w o r k . Thanks are a l s o due to D r . K . Brimacombe and f e l l o w g r a d u a t e s t u d e n t s f o r innumerable h e l p f u l d i s c u s s i o n s . The a s s i s t a n c e of the t e c h n i c a l s t a f f d u r i n g the e x p e r i m e n t a l program i s g r e a t l y  appreciated.  The f i n a n c i a l a s s i s t a n c e of the Canada C o u n c i l and the N a t i o n a l Research C o u n c i l , under the form of s c h o l a r s h i p s and r e s e a r c h i s g r a t e f u l l y acknowledged.  grants  CHAPTER I INTRODUCTION 1.1  The ESR process  The electroslag process (ESR) i s one of the few processes which compete for the production of high quality alloys, especially forging alloys.  Its chief competitors are vacuum arc remelting (VAR) and,  to a lesser extent the Electron Beam Remelting and the Vacuum Induction Melting processes. The main qualities of ESR remelted material may be summarized as follows: 1.  Good surface quality of the ingot, readily usable for forging  (a main advantage on VAR) 2.  Absence of big inclusions  3.  Absence of radial and axial macrosegregation in the remelted  product 4.  A certain degree of refining (mainly of sulphur) i s possible  5.  Solidification pattern which i s extremely favorable to  forgeability as the dendrites are oriented along the axis of the ingot 6.  Absence of porosity.  In a VAR furnace, the environment, water cooled copper mold and vacuum, is inert to the metal being remelted.  Any refining effect is  -  2 -  therefore l i m i t e d to some pressure sensitive reactions and the f l o a t i n g out of inclusions.  The schematic diagram of figure 1 shows immediately  that the s i t u a t i o n i s very d i f f e r e n t i n a t y p i c a l ESR unit.  There are  several interfaces of contact between the metal and i t s environment, resulting i n many possible s i t e s for chemical reactions.  The most  important s i t e s are the electrode-atmosphere, the slag-atmosphere, the electrode-slag and the ingot pool-slag interfaces. While i t i s usually accepted that Electroslag Remelting complements Vacuum Arc Remelting by i t s a b i l i t y to change ingot chemistry as well as structure, the cost factor does not allow to make a clear choice' for i n d u s t r i a l operation higher production  (1).  The lower c a p i t a l cost and  slightly  rate of electroslag -melting equipment are o f f s e t by  the higher s p e c i f i c power required and the cost of the slag i t s e l f . Both processes are capable pf producing the high grade of steels and alloys required mostly by the a i r c r a f t industry and the difference i n the l e v e l pf development attained i n various countries i s more the result of t r a d i t i o n or of the available technology at a given time than of a deliberate choice.  ESR has a clear advantage i n the production  shaped ingots (hollows, slabs) which cannot be made by VAR,  of  mainly  because of the pressure d i f f e r e n t i a l to which the copper mold i s subjected between the vacuum and the cooling water  (2).  The world production of ESR s t e e l i s very unevenly d i s t r i b u t e d among developed countries. production of ESR  Duckworth and Hoyle (3)  report  that,the  s t e e l i n the USSR i s the largest i n the world:  because i t provides a substitute for VAR whereas i n the-rest of the world, ESR  material i n that country,  i s regarded as competitive  or  - 3 complementary, depending on the  circumstances.  ESR i n g o t s have a l r e a d y exceeded the maximum s i z e of VAR i n g o t s , as a p r o d u c e r r e p o r t s h a v i n g produced i n g o t s of 23 tons  (4).  further increase  is  to about 70 tons  (  1.8 m i n d i a m e t e r )  w i t h o u t i n v o l v i n g major t e c h n o l o g i c a l  A foreseen  changes.  The optimum r a t e o f p r o d u c t i o n i s l i m i t e d by the type of  solidifi-  c a t i o n p a t t e r n w h i c h i s d e s i r e d , the a v a i l a b l e power a n d , i n VAR, the maximum r a t e at w h i c h heat can be e x t r a c t e d  t h r o u g h the copper m o l d .  T y p i c a l l y , the r e m e l t i n g of a 23 t o n i n g o t of 1 m i n d i a m e t e r  (4) may  take 24 hours and the l i n e a r r a t e of advance of the i n g o t w i l l o n l y increase  s l o w l y w i t h d e c r e a s i n g s i z e (see  Section IV. 2.13),  l e n g t h of the i n g o t b e i n g then the main f a c t o r l i m i t i n g the production. to 50%  the possible  VAR i n g o t s are s o l i d i f i e d a t a s l i g h t l y l o w e r r a t e  (25  less).  T y p i c a l power consumption i s 1200 to 2000 KWh p e r t o n of m e t a l f o r ESR, and somewhat l e s s f o r V A R ( l )  (5).  Both p r o c e s s e s  are  r e c o g n i z e d to add a p p r o x i m a t e l y the same c o s t per t o n of m e t a l , dependi n g upon the s i z e of the o p e r a t i o n  1.2 Inmost  (5).  Statement of the problem  instances,  the E l e c t r o s l a g p r o c e s s  i s to be c o n s i d e r e d  s t r i c t l y as a r e m e l t i n g p r o c e s s where the c o m p o s i t i o n of the  final  i n g o t i s e s s e n t i a l l y the same as t h a t of the o r i g i n a l consumable  electrode  minus i t s most c o n s p i c u o u s i n c l u s i o n s . ft  An e x c e p t i o n to t h i s would be the " A r c o s " p r o c e s s (5) where c o n t i n u o u s a d d i t i o n of a l l o y powder supplements the base m e t a l brought i n by the e l e c t r o d e to a c h i e v e the r e q u i r e d c o m p o s i t i o n . The o r i g i n a l ESR p a t e n t s a l s o c a l l e d f o r powder a d d i t i o n s of a l l o y i n g e l e m e n t s .  -  Nevertheless,  4  -  t h i s g o a l i s n o t always a c h i e v e d , e s p e c i a l l y  h i g h l y o x i d i z a b l e elements are p r e s e n t .  where  Losses of t i t a n i u m , aluminum,  molybdenum, s i l i c o n ( 7 , 8 , 9 ) have o f t e n been observed and r e p o r t e d . T h i s can have i m p o r t a n t consequences i n c a u s i n g the f i n a l p r o d u c t lose i t s  required properties  to  and hence f a i l to meet commercial  specification. A few workers have devoted t h e i r i n t e r e s t to p r a c t i c a l of p r e v e n t i n g o x i d a t i o n . argon atmosphere resistant  Buzek and a l .  methods  (7) m e n t i o n the use of an  o r the c o a l i n g of the e l e c t r o d e s w i t h o x i d a t i o n  paints.  Most o f t e n i t i s c l a i m e d t h a t a d j u s t i n g the  composition w i l l r e s u l t i n l i m i t i n g o x i d a t i o n .  Medovar (9)  slag  suggests  t h a t the a d d i t i o n of TiO^ to the s l a g w i l l h i n d e r the o x i d a t i o n of titanium.  E o l z g r u b e r (10)  recommends t h a t r e d u c i b l e o x i d e s s h o u l d be  a v o i d e d i n the s l a g used f o r r e m e l t i n g m e t a l s w i t h r e a c t i v e A more r e c e n t p u b l i c a t i o n (11) TiO^  and A ^ O ^  and aluminum.  a  ^  e  reports  that "balanced"slags,  containing  used f o r r e m e l t i n g a l l o y s c o n t a i n i n g b o t h t i t a n i u m  There remains however the q u e s t i o n of whether o r not an  i n c r e a s e i n the t i t a n i u m o x i d e c o n t e n t of the s l a g p r e v e n t s by s h i f t i n g the c h e m i c a l e q u i l i b r i u m . t i t a n i u m or  elements.  The apparent  aluminium may r e f l e c t an i n c r e a s e d  oxidation  retention  c o n t e n t of  of  oxide  inclusions. I t has a l s o been suggested  (11)  that e l e c t r o l y t i c  removal of  ions  w i t h m u l t i p l e v a l e n c y s t a t e s by i n s e r t i n g p o s i t i v e a u x i l i a r y e l e c t r o d e s i n the s l a g would p r e v e n t to the  the t r a n s p o r t  of oxygen from the  atmosphere  slag.  The g e n e r a l  concern f o r o x i d a t i v e l o s s e s i s not s u r p r i s i n g ,  since  - 5 -  some a l l o y s , where a s m a l l c o n c e n t r a t i o n of an e s s e n t i a l a l l o y i n g element  i s present,  are p a r t i c u l a r l y s e n s i t i v e to c o m p o s i t i o n f l u c t u a -  tions.  I n Maraging S t e e l type 300,  f o r i n s t a n c e , which contains  0.8% t i t a n i u m , a l o s s of 0.1% i n the t i t a n i u m c o n t e n t w i l l 7 y i e l d strengths  to drop by 6 x 10  about  cause the  2 N/m  (10,000 p s i )  (12).  The carbon l e v e l of s t e e l s r e l y i n g on c a r b i d e p r e c i p i t a t i o n as a s t r e n g t h e n i n g mechanism has a l s o to be c o n t r o l l e d w i t h i n narrow limits  (.38  to .43% i n the case of A I S I 4340  which  precipitates  chromium c a r b i d e s ) . As our experiments and those r e p o r t e d by v a r i o u s u s e r s of process  seem to i n d i c a t e t h a t l o s s e s  of r e a c t i v e elements  the  cannot be  t o t a l l y a v o i d e d , i t i s of paramount importance t h a t t h e s e  losses  s h o u l d a t l e a s t be known i n advance and reproduced from m e l t to m e l t . The s a m p l i n g and a n a l y s i s of the r e m e l t e d m e t a l d u r i n g the i s not a p r a c t i c a l p r o p o s i t i o n . at by a d j u s t i n g the i n i t i a l  process  The f i n a l c o m p o s i t i o n i s to be a r r i v e d  (electrode)  any change d u r i n g the r e m e l t i n g p r o c e s s .  c o m p o s i t i o n to compensate In i n d u s t r i a l p r a c t i c e ,  for this  c o r r e c t i o n i s a r r i v e d a t e m p i r i c a l l y by o b s e r v a t i o n of p r e v i o u s m e l t s . I t has been f e l t t h a t the a p p l i c a t i o n of m a t h e m a t i c a l models to mass transfer  c o n d i t i o n s i n the E l e c t r o s l a g p r o c e s s  c o u l d r e p l a c e the  and e r r o r method" i n d e t e r m i n i n g c o m p o s i t i o n changes  i n the  "trial  remelted  m a t e r i a l and a l s o g i v e i n f o r m a t i o n about the r e p r o d u c i b i l i t y of t h e s e changes. In t h i s c o n t e x t ,  i t s h o u l d be noted t h a t low r e p r o d u c i b i l i t y i n  the f i n a l c o m p o s i t i o n w i l l l e a d not o n l y to v a r i a t i o n s from i n g o t to i n g o t but a l s o to l o n g i t u d i n a l f l u c t u a t i o n s  ( " s e g r e g a t i o n " ) i n a, g i v e n  - 6 ingot. I n summary, the purpose of t h i s study was to answer the  following  points: - Does a s i g n i f i c a n t l o s s of r e a c t i v e elements electroslag  occur during  r e m e l t i n g , w h i c h cannot be accounted f o r by the s i m p l e  removal of o x i d e  inclusions?  - can a m a t e r i a l b a l a n c e be w r i t t e n f o r the elements  in  consideration? - what i s the r o l e p l a y e d by the  atmosphere?  - what i s the r o l e p l a y e d by the s l a g - are the l o s s e s e l e c t r o c h e m i c a l - where are the r e a c t i o n  composition?  i n nature?  sites?  - to what e x t e n t can we develop q u a n t i t a t i v e k i n e t i c models?  1.3  C h o i c e of  materials  For the purpose of i n v e s t i g a t i n g the i n f l u e n c e of v a r i o u s p a r a meters on c o m p o s i t i o n changes, the U . B . C . E l e c t r o s l a g u n i t . adapted to the r e q u i r e m e n t s be d e s c r i b e d i n Chapter  experiments have been c a r r i e d out w i t h The d e s i g n of t h i s u n i t i s  specifically  of a range of r e s e a r c h p r o j e c t s .  It  will  II.  Thermochemical and p h y s i c a l p r o p e r t i e s of v a r i o u s s l a g s  have  been c a l c u l a t e d o r a l t e r n a t i v e l y measured u s i n g a p p r o p r i a t e l y d e s i g n e d equipment; appendix 3 ) ,  they are r e f e r r e d to i n the t e x t  and w i l l be the b a s i s  (see  f o r the choice of c o m p o s i t i o n s  to be c o m p a t i b l e w i t h the c h e m i c a l r e q u i r e m e n t s of the metals.  also likely  remelted  T h i s w i l l be done by a d d i n g to the b a s i c CaF^ s l a g v a r i o u s  amounts of the o x i d e of the element which c o n t r o l s  the oxygen p o t e n t i a l  - 7 of t h e m e t a l l i c phase ( ( T i C ^ )  f o r [ T i ] or ( A l ^ ) f o r  An i m p o r t a n t problem has been t h e s e l e c t i o n f o r t h e purpose pf the s t u d y . (exact compositions 321 S t a i n l e s s  [Al]...)  of s u i t a b l e  The f o l l o w i n g ones have been s e l e c t e d  are g i v e n i n Chapter I I I ,  2).  Steel  T h i s a l l o y has a b a s i c  c o m p o s i t i o n 18% C r , 10% N i w i t h t i t a n i u m  added as a c a r b i d e s t a b i l i z e r .  The t i t a n i u m c o n t e n t s h o u l d be a t  l e a s t f i v e times carbon and i s u s u a l l y kept around 0.5%. i n the e l e c t r o d e  The p r e s e n c e  m a t e r i a l o f 0.56% s i l i c o n , 1.86% manganese and  chromium c r e a t e s i n t e r e s t i n g problems as t o t h e a c t u a l reactions  alloys  deoxidation  and the c o m p o s i t i o n o f i n c l u s i o n s .  18% N i c k e l Maraging S t e e l (300 S e r i e s ) T i t a n i u m (0.77%) and molybdenum (4.95%) a r e the p r i n c i p a l elements s u b j e c t e d to o x i d a t i o n w h i l e t h e m a t r i x i s b a s i c a l l y  iron-  18% N i .  The d e o x i d a t i o n p r o d u c t s s h o u l d h e r e be e s s e n t i a l l y  titanium  oxides.  The c o m p o s i t i o n changes a r e t o be compared w i t h those of t h e  p r e v i o u s m a t e r i a l , b o t h q u a l i t a t i v e l y and q u a n t i t a t i v e l y . 1409  Al A high concentration  (3.74%) of aluminium i s p r e s e n t i n t h i s  alloy  a l o n g w i t h 0.58% s i l i c o n and 0.58% manganese - most of the 0.49% T i i s a l r e a d y i n the form of c a r b i d e i n c l u s i o n s .  Losses of aluminium  a l o n g w i t h t h e r e a c t i o n between aluminium and t i t a n i u m o x i d e s d u r i n g r e m e l t i n g i s of  interest.  - 8 Ferrovac E (Fe,  0.01% C, 0.001% Mn, 0.006%  Si...)  The r a t e of o x i d a t i o n of pure i r o n i n s i m i l a r r e m e l t i n g can be compared w i t h the b e h a v i o u r of the o t h e r  conditions  alloys.  Preliminary discussion  1.4  The major p a r t of the a l l o y i n g elements l o s t d u r i n g r e m e l t i n g i s found as o x i d e s d i s s o l v e d i n the s l a g a f t e r the o p e r a t i o n been c o m p l e t e d .  It  has  i s r e a s o n a b l e to assume t h a t the mechanism by  w h i c h they are removed from the m e t a l i s , e i t h e r  their  i n the form of o x i d i z e d i n c l u s i o n s and p r e c i p i t a t e s , o x i d a t i o n at a s l a g - m e t a l  interface.  or  dissolution their  D i r e c t d i s s o l u t i o n of  metallic  elements f o l l o w e d by t h e i r o x i d a t i o n i n the b u l k of the s l a g or a t slag-atmosphere  interface  w i l l be r u l e d out when i t can be  the  argued  t h a t the a c t u a l oxygen p o t e n t i a l of the s l a g i s h i g h and the s o l u b i l i t y of m e t a l s  i n i t i s low.  T h i s w i l l be d i s c u s s e d i n more d e t a i l  later.  1.4.1  A t m o s p h e r i c oxygen i s one i m p o r t a n t f a c t o r can e n t e r 1.  of o x i d a t i o n .  It  the system i n two ways: The oxygen reaches the s l a g s u r f a c e and i s d i s s o l v e d , 2-  presumably i n the form of 0  2(Fe  2(Ti  2 +  3 +  ions.  A few p o s s i b l e  )  +  l/20 "-  •  2CFe )  +  (0~)  )  +  l/20  •  2(Ti )  +  (0~)  +  1/20  •  (Ca )  (Ca).  ?  2  3 +  4 +  2 +  +  (o"~)  reactions  are:  _ 9 -  2.  The e l e c t r o d e  the m o l t e n s l a g .  itself  i s oxidized before i t p h y s i c a l l y enters  The e x t e n t of t h i s r e a c t i o n can be d i r e c t l y  observed o r c a l c u l a t e d w i t h the h e l p of temperature on the e l e c t r o d e s  0  o  2  + .  (Chapter XV,  Fe  »-  FeO  p r o f i l e s measured  1,2).  v  (solid) '  ^ FeO [0] {  (slag)  L  1.4.2 When the s l a g e x h i b i t s an oxygen p o t e n t i a l h i g h e r than r e q u i r e d by the e q u i l i b r i u m w i t h the m e t a l , a r e a c t i o n between m e t a l and s l a g to t a k e p l a c e .  I n most of the a l l o y s we c o n s i d e r h e r e ,  d e o x i d a t i o n p r o d u c t i s the pure o x i d e of the r e a c t i v e Al).  is  the normal  element  ( T i and  The removal of the p r p d u c t of the o x i d a t i o n r e a c t i o n o r  the  s u p p l y of oxygen through a s l a g boundary l a y e r i n c o n t a c t w i t h the m e t a l may b o t h p l a y a r o l e i n the o v e r a l l k i n e t i c s of the o x i d a t i o n . ~. , , x 0 (slag)  heterogeneous —; reaction  . , . •> 0 (metal)  D i f f u s i o n of b o t h oxygen and r e a c t i v e elements  i n the m e t a l  itself  near the same s l a g - m e t a l i n t e r f a c e may a l s o be i m p o r t a n t :  3  W  slag  —  ^ m e t a l  *  3  ^  +  2  ^  —  A 1  2°3  I n t h i s c a s e , d i f f u s i o n r a t e s of b o t h oxygen and aluminium (or t i t a n i u m . . . ) may combine to produce the o v e r a l l r a t e of  /  i  reaction.  - 10  -  1.4.3 I n the p a r t i c u l a r case of 1409 pressure  steel,  a substantial  of aluminium d e v e l o p s i n the l i q u i d m e t a l .  of aluminium i s t h e r e f o r e to be  vapour  Direct  evaporation  considered.  1.4.4 The d i r e c t removal of i n c l u s i o n s and p r e c i p i t a t e s are o x i d e s or c a r b i d e s , t i o n change.  is translated  of the f i n i s h e d p r o d u c t .  It  they  i n a composi-  I n numerous c a s e s , the r e s u l t i n g e f f e c t w i l l be  to the p r o p e r t i e s to d i s s o c i a t e  sulphides, e t c . . .  whether  i s important  favourable  therefore  c o m p o s i t i o n changes due to o x i d a t i o n of m a t r i x elements  and c o m p o s i t i o n changes due to the removal of  inclusions.  1.4.5 S o l v i n g the v a r i o u s problems a s s o c i a t e d reaction requires the s y s t e m .  These are a c t i v i t i e s  i n the l i q u i d phases e t c . . . discussed.  of;  a knowledge of p h y s i c a l and c h e m i c a l parameters i n  the m e t a l , d i f f u s i o n c o e f f i c i e n t s ,  a l s o be  with l i m i t i n g rates  of the c o n s t i t u e n t s viscosities,  T h e i r measurement  i n the s l a g and  convection  patterns  or computation w i l l .  CHAPTER I I THE U . B . C . ELECTROSLAG UNIT II.I  Generalities  E l e c t r o s l a g r e m e l t i n g i s one of the few m e t a l l u r g i c a l p r o c e s s e s which can be s c a l e d down w i t h o u t l o s i n g i n t r i n s i c  characteristics.  S m a l l l a b o r a t o r y u n i t s can produce i n g o t s w h i c h p r e s e n t the main q u a l i t i e s l i s t e d i n (1.1) scale  six  to an e x t e n t comparable w i t h the  large  ingots. R e s i s t a n c e h e a t i n g of the s l a g i s the main mode of power  d i s s i p a t i o n i n the p r o c e s s . densities,  intermittent  As s m a l l u n i t s r e q u i r e h i g h e r  a r c i n g becomes d i f f i c u l t to a v o i d i n s m a l l  s i z e u n i t s , i n t r o d u c i n g a l o w e r l i m i t on w o r k a b l e i n g o t (around 5 cm, see a l s o  diameters  IV.2.13).  Commercial o p e r a t i o n i s done on i n g o t s range i n s i z e from 0.15  current  to 1 m i n diameter  l i m i t seems to be a v e r y temporary one.  (or s l a b s e t c . . ) w h i c h (1970) a l t h o u g h the upper  A t y p i c a l furnace,  by the Consarc C o r p o r a t i o n i s shown i n f i g u r e  produced  3.  Research u n i t s w i l l produce i n g o t s up to .10 or .15 m ( 6 " ) , i n diameter w i t h o u t exceeding the f a c i l i t i e s - power, h a n d l i n g , o v e r h e a d , f l o o r l o a d s - a v a i l a b l e i n a medium s i z e d r e s e a r c h  laboratory. •  I n the p r e s e n t c a s e , the d e s i g n was to f u l f i l  the f o l l o w i n g  requirements:  - 12  -  - A r e l a t i v e l y s i m p l e framework must a c c e p t v a r i o u s shapes of molds i n s i z e s up to .10 m (4")  i n diameter along w i t h  f o r the c o n t r o l of the atmosphere, deoxidant,  auxiliaries  the c o n t i n u o u s a d d i t i o n of s l a g  the p r o b i n g and s a m p l i n g of the  or  bath.  - The power can be e i t h e r D . C . w i t h the e l e c t r o d e  of  either,  p o l a r i t y o r A . C . o r a m i x t u r e o f b o t h ; b o t h v o l t a g e and c u r r e n t  are  to be v a r i e d c o n t i n u o u s l y . - Instrumentation measurement  and a c c e s s p o i n t s have to be p r o v i d e d f o r  of a l a r g e number of parameters w h i c h can a f f e c t  the  the  process.  II.2  Framework and e l e c t r o d e  The framework i s d e s i g n e d molds and a c c e s s o r i e s .  It  t r a v e l mechanism  ( f i g u r e 4)  to accommodate  i s r i g i d enough t o s u p p o r t  various  electrodes  w e i g h i n g 100 kg and 2.5m i n l e n g t h w i t h o u t s i g n i f i c a n t b e n d i n g of s u p p o r t i n g beam ( l e s s than 1.5 mm d e f l e c t i o n at the t i p of the The e l e c t r o d e  the  electrode).  h o l d e r i s s e c u r e d on a t r o l l e y and moves up and  down an aluminium r a i l manufactured f o r a commercial a u t o m a t i c  welder.  (Weld T o o l i n g C o r p , P i t t s b u r g h 15204, ARR-0 r a i l and BUG-2700  Car).  The normal r a c k and p i n i o n d r i v e of the r a i l i s not u s e d . p l a c e the moving t r o l l e y i s d r i v e n by a c o n t i n u o u s 3 / 4 "  In  its  p i t c h chain  loop a l l o w i n g the use of a 1/8 Hp v a r i a b l e speed motor l o c a t e d b e h i n d the u n i t  (Boston Gear R a t i o t r o l BG29005).  geared down i n 2 stages by a f a c t o r c o n t r o l over the e l e c t r o d e The c a r r i a g e  The 1750  RPM motor  is  of 2000, thereby p r o v i d i n g a good  t r a v e l v e l o c i t y between 0.25  and 12 cm min  t r a v e l i s l i m i t e d a t b o t h ends of the r a i l by m i c r o -  \  -  13  -  s w i t c h e s w h i c h a u t o m a t i c a l l y shut o f f the motor ( f i g u r e 5 ) .  Similarly,  a s p r i n g and m i c r o s w i t c h arrangement i n s e r t e d between the c h a i n and the lower s i d e of the t r o l l e y l i m i t s the downwards f o r c e on the to a p p r o x i m a t e l y 100 k g f . and c h a i n attachment  electrode  T h i s p r e v e n t s damage to the t r a n s m i s s i o n  systems w h i l e a l l o w i n g the a p p l i c a t i o n of a  s i z a b l e downward p r e s s u r e on the s t a r t i n g compact.  Provision for  r e s t a r t i n g the motor when cut o f f by a m i c r o s w i t c h i s  II.  3  Electrode holder (figure  provided.  6)  The e l e c t r o d e i s h e l d from an arm o v e r h a n g i n g 0.21 m from the beam, permanently a l i g n e d and s e c u r e d on the moving t r o l l e y . electrode  The  i s clamped by a s p l i t copper p l a t e , w a t e r c o o l e d and c o r e d  to an i n s i d e diameter of 1 . 5 " .  B i g g e r d i a m e t e r s of e l e c t r o d e s  accommodated by w e l d i n g a stub a t the h o l d i n g e n d ; s m a l l e r by i n s e r t i n g s p l i t b u s h i n g s of the a p p r o p r i a t e s i z e . fed through four i n s u l a t e d c a b l e s ,  are  diameters  Power i s  each r a t e d a t 500 Ampere  and  s u p p o r t e d by a c r o s s member s e c u r e d to the main v e r t i c a l beam.  II.  4  Mold b a s e p l a t e ( f i g u r e  7)  The b a s e p l a t e i s cut i n a 25 x 300 x 300 mm copper p l a t e .  Six  t h r e a d e d b l i n d h o l e s on a 159 mm c i r c l e r e c e i v e s t u d s used to clamp down the mold on top of the  plate.  Power i s c a r r i e d to the b a s e p l a t e by a 1" d i a m e t e r copper r o d threaded at b o t h ends and s e c u r e d i n a b l i n d h o l e a t the bottom c e n t e r of the p l a t e . end of the r o d .  Four 500 A p e r e power l e a d s are connected to the lower m  - 14 -  A water j a c k e t covers the bottom f a c e o f the p l a t e .  To p r e v e n t  f o r m a t i o n of any a i r or steam p o c k e t on the w a t e r c o o l e d s i d e of baseplate,  the  t h i s has been t u r n e d i n t o a s h a l l o w i n v e r t e d cone w i t h  water o u t l e t near the base of the cone.  Two t a n g e n t i a l water  the  iniets  p r o v i d e a s w i r l m o t i o n i n the j a c k e t w i t h the maximum v e l o c i t y near the  baseplate. P r o t e c t i o n to the upper f a c e o f the b a s e p l a t e  i s a f f o r d e d by an  a d d i t i o n a l copper p l a t e , a p p r o x i m a t e l y 5 mm t h i c k , w h i c h i s clamped underneath the m o l d .  T h i s p l a t e can be r e s u r f a c e d i f i t has been  damaged by a r c i n g w i t h the base of the i n g o t . The i n g o t bottom i t s e l f i s made of two dummy p l a t e s s i t t i n g i n s i d e the m o l d .  They a r e i n s u l a t e d from the mold by a l a y e r of a s b e s t o s and  p r e v e n t o v e r h e a t i n g of the copper b a s e . The f a c e s of the dummy p l a t e s a r e s u r f a c e ground to p r o v i d e a good e l e c t r i c a l c o n t a c t .  I n the event t h a t the e l e c t r o d e i s withdrawn  under power a f t e r w e l d i n g t o the upper dummy p l a t e , a r c i n g w i l l o n l y o c c u r between the two dummy p l a t e s ,  t h e r e b y s a v i n g the s u r f a c e of  the  copper.  II.5  Mold and w a t e r j a c k e t  (figure  8)  To m i n i m i z e the danger of e x p l o s i o n i n the event of mold p u n c t u r e , a f r e e f l o o d i n g system has been c h o s e n .  The water i n t a k e i s p o s i t i o n e d  on b o t h s i d e s a t the base of the j a c k e t ,  w i t h t w i n o u t l e t s of  large  d i a m e t e r near the t o p . Warm water (30°C) i s used to a v o i d c o n d e n s a t i o n i n s i d e the m o l d . Most molds are -made of a copper tube welded to a t h i c k copper  - 15 flange.  -  The mold i s e l e c t r i c a l l y  p l a t e by an a s b e s t o s g a s k e t . i n t h i s c a s e , the t h i c k n e s s  insulated  ( 10 Kfi) from the b a s e -  M i l d s t e e l molds have a l s o been t r i e d ; of the w a l l i s reduced to a maximum of  3 mm to a v o i d o v e r h e a t i n g of the i n s i d e s u r f a c e .  This l a t t e r  condition  may r e s u l t i n the s l a g s k i n becoming too c o n d u c t i v e and hence l e a d p r o c e s s i n s t a b i l i t y and a r c i n g . i n useful height  to  Mold s i z e s range from 30 to 80 cm  (40 to 90 cm o v e r a l l )  and 5 to 10 cm i n i n s i d e  diameter.  II.6  Atmosphere  Two types of hoods are i n u s e : b l a n k e t and e x t r a c t i o n  control one, w h i c h p r o v i d e s an argon  of fumes i s shown on f i g u r e  9.  A more e l a b o r a t e d e s i g n i n c l u d e s a s e a l e d chamber i n w h i c h the electrode  i s h e l d onto a w a t e r c o o l e d s t u b .  A rubber b e l l o w s  clamped  t o the stub a t the top of the assembly p r o v i d e s the moving s e a l ( f i g u r e 10),  S p r i n g clamps and blowout windows a l l o w f o r a q u i c k  r e l e a s e of i n s i d e p r e s s u r e This l a s t  i n case of  explosion.  d e s i g n p r o v i d e s an atmosphere  the q u a l i t y of w h i c h depends  almost e x c l u s i v e l y upon the p u r i t y of the argon u s e d . i n i t i a l p u r g e , a v e r y s m a l l f l o w (200 ml h r  x  )  After  is sufficient  the to .  p r e v e n t back d i f f u s i o n of a i r t h r o u g h the s m a l l exhaust and the g a s k e t at the base of the m o l d . of the argon b l a n k e t f l o w rate of 100  1/hr.  indicates  By c o n t r a s t ,  chromatographic  asbestos  analysis  t h a t i t may c o n t a i n up to 1% 0^ w i t h a  - 16 II.7  Continuous s l a g o r m e t a l a d d i t i o n ( f i g u r e 11)  A s p e c i a l l y d e s i g n e d r o t a t i n g t a b l e a l l o w s the d e l i v e r y of s l a g , d e o x i d a t i o n or a l l o y i n g m a t e r i a l during the m e l t .  A vertical  cannister  whose base i s c l o s e d by t h e r o t a t i n g p l a t e d e l i v e r s the m a t e r i a l (powder o r s m a l l g r a n u l e s )  through a c a l i b r a t e d gate.  The stream of  m a t e r i a l i s then wiped o v e r t h e edge o f t h e p l a t e i n t o the m o l d . p l a t e i s powered by a v a r i a b l e speed motor ( E l e c t r o c r a f t  The  C o r p . Motomatic  E5503) geared down by a f a c t o r of 3000:1 t h e r e b y a l l o w i n g good c o n t r o l f o r the d e l i v e r y o f 0 t o 10 g / m i n o f m a t e r i a l . rotating table:  (Maximum RPM of .  1.7)  II.8  Other r e m e l t i n g equipment  A water c o o l e d e l e c t r o d e h o l d e r , made of c o p p e r , and a l s o used w i t h the t i g h t a t m o s p h e r i c s h i e l d ( I I . 6 ) consumable e l e c t r o d e  experiments  i s available f o r non-  (figure 12).  A movable arm w h i c h can be brought near t h e e l e c t r o d e p r o v i d e s a g u i d e f o r a s l a g temperature p r o b e , p a s s i v e e l e c t r o d e o r s l a g sampler ( f i g u r e 1 3 ) .  The v e r t i c a l m o t i o n o f the arm i s c o n t r o l l e d  m a n u a l l y o r w i t h an e l e c t r i c a l m o t o r . The whole e l e c t r o s l a g r i g i s e n c l o s e d on t h r e e s i d e s to p r o t e c t the o p e r a t o r and the l a b o r a t o r y i n t h e event o f an e x p l o s i o n  (Picture  f i g u r e 14).  II.9  D . C . Power s u p p l y ( f i g u r e 15)  D . C . power i s s u p p l i e d by two commercial E o b a r t Welders RC750) connected i n p a r a l l e l .  (Hobart  Maximum c u r r e n t i s l i m i t e d t o 1500  - 17  -  Amperes by the a v a i l a b l e mains power.  The o p e r a t i n g v o l t a g e can be  v a r i e d from a p p r o x i m a t e l y 10 to 50 V (open c i r c u i t v o l t a g e 55 V ) .  The  w e l d e r s are c o n t r o l l e d by a s i n g l e r h e o s t a t w h i c h v a r i e s the degree of s a t u r a t i o n of t h e i r r e s p e c t i v e  reactors  a d j u s t e d by v a r y i n g the power f a c t o r ) .  (the p r i m a r y power i s The s l o p e of the  current  v o l t a g e curve on the secondary a l l o w s t h e s e u n i t s to o p e r a t e d u r i n g a c o l d s l a g s t a r t when the o v e r a l l r e s i s t a n c e of the v a r i e s u n p r e d i c t a b l y ( f i g u r e 16)  satifactorily process  (11.16).  R e c t i f i c a t i o n i s done by a s e t p a r t of the u n i t .  therefore  of s i l i c o n r e c t i f i e r s w h i c h  The r e c t i f i e d c u r r e n t c o n t a i n s  an A . C . component  (360 Hz) of a p p r o x i m a t e l y 10% (RMS) of the t o t a l D . C . c u r r e n t . i s c o n s i d e r e d to have a n e g l i b i b l e i n f l u e n c e on the  c o n s i d e r a b l y above any c o n c e i v a b l e d e c o m p o s i t i o n p o t e n t i a l of F u r t h e r m o r e , i t has been observed  p o l a r i z a t i o n p r o c e s s e s on the e l e c t r o d e s  This  electrochemical  p r o c e s s e s t a k i n g p l a c e i n the u n i t as the minimum peak v a l u e  s l a g components.  are  (13)  remains the  that  are slow compared w i t h  the  p e r i o d of the A . C . component. The n e g a t i v e s i d e of the w e l d e r s i s grounded.  Each u n i t has  its  own c i r c u i t b r e a k e r w h i l e the s t a r t i n g r e l a y s are o p e r a t e d from a s i n g l e s w i t c h on the c o n t r o l p a n e l .  11.10  A . C . power s u p p l y ( f i g u r e  15)  The A . C . stepdown t r a n s f o r m e r o p e r a t e s from one phase of mains.  The t r a n s f o r m e r e x h i b i t s n e a r l y c o n s t a n t  under l o a d .  A d i s c o n t i n u o u s o f f - l o a d adjustment  a v a i l a b l e t h r o u g h a set of  voltage  the  characteristics  of the v o l t a g e  t a p p i n g s on the p r i m a r y c o i l .  This  is  - 18 p r o v i d e s 4 v o l t a g e s o f 35, 30, 24.7  and 20 V r e s p e c t i v e l y .  Continuous adjustment of the v o l t a g e between n a r r o w e r l i m i t s  is  done by v a r y i n g the p r i m a r y v o l t a g e of the s t e p down t r a n s f o r m e r .  A  b o o s t e r t r a n s f o r m e r whose p r i m a r y c o i l i s f e d by a v a r i a c (0 t o 220 V) opposes a v a r i a b l e E . M . F .  to the mains v o l t a g e (0 to 45 V a p p r o x i m a t e l y ) .  One of the f o l l o w i n g v o l t a g e ranges i s t h e r e f o r e a v a i l a b l e a t secondary of the s t e p down t r a n s f o r m e r f o r the o p e r a t i o n of  the  the  ESR r i g :  28.3  to  35 V  24.3  to  30 V  20  t o 24.7 V  16.2  to  20 V  Maximum power a v a i l a b l e i s l i m i t e d by the p r i m a r y c u r r e n t w h i c h can be drawn from the mains (230 amps).  T h i s c o r r e s p o n d s to a p p r o x i -  m a t e l y 50 KW ( t y p i c a l l y 1600 amps, 24 v o l t s ) on the s e c o n d a r y .  11.11  D.C. bias  A r e c e n t a d d i t i o n to the machine i s the i n t r o d u c t i o n of an a d d i t i o n a l set of s i l i c o n r e c t i f i e r s to p r o v i d e a D . C . b i a s when o p e r a t i n g w i t h A . C . power. The b a t t e r y o f r e c t i f i e r s i s f e d by the secondary c i r c u i t of the t r a n s f o r m e r w i t h a v a r i a b l e r e s i s t o r ( f i g u r e 15). resistor.  (0 to 10 mO) i n s e r i e s  D . C . c u r r e n t i s p r o p o r t i o n a l to the v a l u e o f the v a r i a b l e  - 19 11.12  -  Controls (D.C.)  When o p e r a t i n g w i t h D . C . power, the c o n t r o l s i n c l u d e - o n - o f f power s w i t c h o p e r a t i n g b o t h w e l d e r s - s i n g l e power r h e o s t a t t i o n of the r e a c t o r s  (resistor)  v a r y i n g the degree of  satura-  on b o t h w e l d e r s  - D . C . ammeter connected a c r o s s  the shunt and r e a d i n g 0 to 2000  amps (0-50 mV) - D . C . v o l t m e t e r connected between the b a s e p l a t e electrode  (0-50  and the  V)  - coulometer:  the s i g n a l from the shunt i s a m p l i f i e d t h r o u g h a  K e i t h l e y 153 m i c r o v o l t - a m m e t e r and i n t e g r a t e d by a L e c t r o c o u n t coulometer  (Royson E n g i n e e r i n g ) w i t h d i g i t a l r e a d i n g  - An a u t o m a t i c c o n t r o l d e v i c e m a i n t a i n i n g a c o n s t a n t the p r o c e s s  i s also a v a i l a b l e .  s i g n a l (shunt + source) circuit  current  during  The s i g n a l from the shunt i s opposed by  a mV source c o r r e s p o n d i n g to the d e s i r e d s i g n a l .  detector  II  The r e s u l t i n g )  i s fed i n t o a continuous balance or n u l l  (Brown I n s t r u m e n t s  I n c . , Minneapolis - Honeywell). ,  The o u t p u t of the c o n t i n u o u s b a l a n c e u n i t i s f e d to a r e v e r s i b l e D . C . motor which o p e r a t e s  a m i c r o s w i t c h t h r o u g h a cam arrangement.  on and o f f the e l e c t r o d e (figure  d r i v e to compensate  for current v a r i a t i o n s  5).  - Mains A . C . c u r r e n t  11.13  (0 to 250 amps).  Controls  This  (A.C.)  When o p e r a t i n g w i t h A . C . power, the c o n t r o l s i n c l u d e :  cuts  - 20 -  - on-off  power s w i t c h  - f i n e adjustment of v o l t a g e through the V a r i a c knob ( f i g u r e - RMS p r o c e s s v o l t a g e  (EICO 250  - RMS p r o c e s s c u r r e n t :  - Mains p r i m a r y c u r r e n t  11.14  voltmeter)  the shunt s i g n a l s i s r e c t i f i e d b e f o r e  f e d to the D . C . Ammeter and the  15)  being  coulometer  (0 to 250  Controls  amps).  (common)  The f o l l o w i n g c o n t r o l s are common to b o t h D . C . and A . C :  the  - d i g i t a l c o u n t e r showing the t o t a l e l e c t r o d e  t r a v e l i n mm  - c o n t i n u o u s r e c o r d i n g of the p r o c e s s  (in p a r a l l e l with  current  ammeter) - measurement of the mold p o t e n t i a l r e l a t i v e to the  or baseplate  electrode  potential  - measurement of b o t h the i n l e t and o u t l e t temperature of mold c o o l i n g w a t e r ( T h e r m i s t o r probe and Y . S . I . 42SC  the  telethermometer)  - t w i n c h a n n e l o s c i l l o s c o p e w h i c h d e t e c t s the a m p l i t u d e and the phase a n g l e of b o t h p r o c e s s c u r r e n t  (shunt)  and p r o c e s s v o l t a g e .  The  o s c i l l o s c o p e i s a l s o used f o r measuring the r a t e of decay of  electrode  p o l a r i z a t i o n when a r u n i s stopped ( T e k t r o n i c I n c . ,  storage  type 564  oscilloscope).  11.15  P a r t i c u l a r i t i e s of the d e s i g n  Several c h a r a c t e r i s t i c s  of t h i s u n i t f i t the needs of  r a t h e r than i n d u s t r i a l p r o d u c t i o n of i n g o t s . the emphasis p l a c e d on p o l a r i t y of o p e r a t i o n .  research  The most o b v i o u s one i s  - 21 Most i n d u s t r i a l u n i t s o p e r a t e on A . C . power as t h i s mode has been found to be e c o n o m i c a l b o t h i n i n v e s t m e n t and power c o n s u m p t i o n .  It  i s c u r r e n t l y not known however i f any amount of r e c t i f i c a t i o n  occurs  d u r i n g the p r o c e s s :  above  The s l a g i s an i o n i c c o n d u c t o r used w e l l  i t s decomposition ( e l e c t r o l y s i s )  p o t e n t i a l and a v e r y s m a l l amount pf  imbalance between the k i n e t i c s of a n o d i c and c a t h o d i c r e a c t i o n c o u l d have i m p o r t a n t consequences on the f i n a l l e v e l of a l l o y i n g o r i m p u r i t y elements later  (sulphur, oxygen...).  T h i s i s examined i n more d e t a i l i n a  section. From a p u r e l y m e c h a n i s t i c p o i n t of v i e w , i t i s t h e r e f o r e  important  to i n v e s t i g a t e the r o l e p l a y e d by p o l a r i t y o r the use of A . C . c u r r e n t on the b e h a v i o u r of the r e m e l t e d m a t e r i a l . of a s m a l l D . C . component current  S i m i l a r l y , the i m p o s i t i o n  ( D . C . b i a s ) when o p e r a t i n g w i t h a l t e r n a t i n g  w i l l a l l o w an i n v e s t i g a t i o n of t h i s mode of o p e r a t i o n where  a s p e c i f i c e l e c t r o l y t i c e f f e c t i s sought, f o r instance  i n enhancing  deoxidation. The c h o i c e o f a c o n t i n u o u s c h a i n d r i v e f o r the e l e c t r o d e  is in  k e e p i n g w i t h the problems i n h e r e n t i n a s m a l l u n i t ; the w e i g h t of electrode  i s not always s u f f i c i e n t to break s l a g p a r t i c l e s or  b r i d g e s w h i c h i n t e r f e r e w i p h the downward m o t i o n . on the e l e c t r o d e  is required.  the  solid  A p o s i t i v e pressure  T h i s system a l s o ensures  a smooth and  c o n t i n u o u s m o t i o n when the c h a i n i s kept under s l i g h t t e n s i o n . , The same f u n c t i o n i s b e s t performed i n i n d u s t r i a l i n s t a l l a t i o n by a h y d r a u l i c d r i v e system w i t h the added advantage t h a t h y d r a u l i c d r i v e s can u s u a l l y .move v e r y q u i c k l y at the time of e l e c t r o d e i n i t i a l run p r e p a r a t i o n .  s e t - u p and .  - 22  -  F o r a s m a l l u n i t , d e a l i n g w i t h s m a l l e l e c t r o d e w e i g h t s and n o t bound by t i g h t p r o d u c t i o n s c h e d u l e s , found  the c o n t i n u o u s c h a i n d r i v e has been  satisfactory.  11.16  Starting  procedure  S t a r t i n g of the u n i t i s done o n l y w i t h D . C . power. to A . C , when r e q u i r e d , i s  Switchover  cfone a f t e r a p o o l of l i q u i d s l a g i s formed.  At the s t a r t of the p r o c e s s ,  the o v e r a l l e l e c t r i c a l  r e s i s t a n c e can v a r y  w i d e l y and u n p r e d i c t a b l y making t h e use of an A . C . t r a n s f o r m e r source of power i m p r a c t i c a l because of i t s istics  (see  power s u p p l i e s 1 1 . 9 , 1 0 ) :  constant voltage  as  the  character-  A sudden drop i n the o v e r a l l u n i t  r e s i s t a n c e Cor s h o r t c i r c u i t ) would cause the maximum a v a i l a b l e power to be exceeded hence the n e c e s s i t y f o r the s u p p l y to have a  significant  reactance. Only a c o l d s l a g s t a r t i s p r a c t i c a l w i t h s m a l l u n i t s as  liquid  s l a g poured i n s i d e the -mold would i m m e d i a t e l y f r e e z e a s k i n on the b a s e p l a t e and break the c u r r e n t  path.  S t a r t i n g c o n f i g u r a t i o n i s represented The s t a r t i n g compact c o n s i s t s  on f i g u r e  of a m i x t u r e of m e t a l  h a v i n g a m e l t i n g p o i n t near t h a t of the e l e c t r o d e ,  s i z e and r e s i s t i v i t y of the t u r n i n g s .  i.e.  i s adjusted  turnings  and c a l c i u m f l u o r i d e  powder to the r a t i o of 15 to 25 g per cm of compact,  c o l d compacts  2.  The e l e c t r i c a l  a c c o r d i n g to  the  r e s i s t a n c e of  the  to be comparable to t h a t of the r u n n i n g u n i t ,  8 to 50 -m^. The compacts  are u s u a l l y 37 mm i n d i a m e t e r and the t o t a l  v a r i e s between 25 mm 60 mm  height  Cpositive e l e c t r o d e of 25 mm i n diameter) and  Cnegative e l e c t r o d e of 50 to 60 -mm i n d i a m e t e r ) .  The c o m p o s i t i o n  -  of  the  turnings  important The size  to  solid  a n d 15  melting units).  do  to  is  adjusted  23  to  -  that  of  the  remelted  material  if  into  granules  less  been  found  favour  rapid  problem with  small  it  is  so. slag 30%  s h o u l d be powder as  crushed this  while preventing electrode  has  hangups  (a  to  than  6 mm i n  CHAPTER  III  REMELTING OF INGOTS III.l  P r o c e d u r e and p r e c a u t i o n s  The s t a r t i n g p r o c e d u r e has been d e s c r i b e d ( 1 1 . 1 6 ) .  The c o o l i n g  water i s t u r n e d on at the same time as the power to a v o i d c o n d e n s a t i o n i n the m e l t zone and on the e l e c t r o d e h o l d e r .  This i s a useful  p r e c a u t i o n as the r e a c t i o n between m o l t e n c a l c i u m f l u o r i d e and w a t e r (hydrolysis) i s usually explosive.  Warm w a t e r c o o l i n g (30°C) i s used  f o r the mold j a c k e t . The s t a r t i n g compacts are manufactured w i t h degreased  turnings  of the r e m e l t e d m e t a l c o m p o s i t i o n or of pure i r o n ( F e r r o v a c E)  to  a v o i d c h e m i c a l i n t e r f e r e n c e w i t h the m e l t . . They are k e p t i n a d r y i n g oven u n t i l needed.  The s l a g (lumps and powder) i s a l s o k e p t i n a d r y  atmosphere. The e l e c t r o d e s  are degreased and d e s c a l e d b e f o r e r e m e l t i n g .  Metal  o x i d e s , p a r t i c u l a r l y i r o n o x i d e , d i s s o l v e i n the m e l t and a c t as a s o u r c e of oxygen i n the s y s t e m .  I t i s i m p o r t a n t to remove the  q u a n t i t y of o x i d e from the e l e c t r o d e ,  s i n c e t h i s w i l l a l l o w us to  measure the amount by w h i c h the e l e c t r o d e o x i d i z e s as i t e n t e r s process.  initial  the  - 25 I n s u l a t e d mold o p e r a t i o n was chosen f o r a l l t h e e x p e r i m e n t s performed.  Both l i v e and i n s u l a t e d molds a r e used i n d u s t r i a l l y , t h e  l a t t e r b e i n g more common.  From an e l e c t r i c a l p o i n t o f v i e w , t h e mode  chosen w i l l determine the c u r r e n t p a t h through t h e s l a g t o a g r e a t e x t e n t as a g r e a t p a r t  (80% a c c o r d i n g t o ( 9 )) of the c u r r e n t f l o w s  through a l i v e mold w a l l . I n l i v e mold o p e r a t i o n , the mold w a l l and the s l a g s k i n a r e reaction interfaces  for electrochemical reactions  and a r e known t o  have an i n f l u e n c e on d e s u l p h u r i z a t i o n ( 1 4 ) I n i n s u l a t e d mold o p e r a t i o n , t h e mold w a l l s t i l l p l a y s a p a r t i n t h e t r a n s p o r t of c u r r e n t , a l t h o u g h t h i s p a r t i s s m a l l . have an i n f l u e n c e on the c o n t e n t of t r a c e elements if  I t could  i n the metal  (oxygen,  t h e system i s s h i e l d e d ) b u t would p r o b a b l y n o t be an i m p o r t a n t  r e a c t i o n s i t e f o r the v a r i o u s o x i d a t i o n p r o c e s s e s which we c o n s i d e r later. The e l e c t r i c a l r o l e of the mold has been t h e s u b j e c t o f a separate i n v e s t i g a t i o n (67).  III.2  C o m p o s i t i o n of t h e a l l o y s s t u d i e d ( i n wt %)  Austenitic stainless  steel:  321 g r a d e .  A i r melted.  (Supplied  by A t l a s S t e e l s Company, W e l l a n d , O n t a r i o ) . Fe  Cr  Ni  Bal  17.78  10.60  Ti  Si  .58  .56  Mn  C  1.86  .05  P .031  S .018  O  .  .0009  - 26 -  Maraging S t e e l :  300 g r a d e .  by R e p u b l i c S t e e l C o r p . , Fe  Ni  Co  Vacuum a r c r e m e l t e d .  (Supplied  Cleveland).  Mo T i  Mn S i  Al  Ca  B  Zr  C  P  S  S  B a l 18.52 8.80 4.95 .79 .09 .09 .12 .05 .003 .02 .025 .005 .005 0.05 Oxygen:  10 ppm.  Chromium Aluminium s t e e l :  1409 A l g r a d e .  A i r melted  ( S u p p l i e d by  Universal Cyclops, B r i d g e v i l l e , Pa.) Fe  Cr  Al  Ti  Mn  Si  Mo  Cu  Ni  C  P  S  0  B a l 12.86 3.74 0.49 0.58 0.58 0.10 0.05 0.28 0.11 0.018 0.008 0.0095  F e r r o v a c E : : Vacuum m e l t e d .  ( S u p p l i e d by C r u c i b l e S t e e l Company,  S o r e l Quebec). Fe  C  Mn  P  S  Si  Ni  Cr  B a l 0.010 0.001 0.002 0.004 0.006 0.01 <.01  Co .006  Cu  V  .006 <.002  1  Mo  0  H  .001 .0002 .00092 .000018  W  .02  III.3  Slags  C a l c i u m f l u o r i d e i s the b a s i c c o n s t i t u e n t . (flotation  N  P a r t of t h i s  material  c o n c e n t r a t e ) i s used as a d r y powder t o make up f o r the  r e q u i r e d p r o p o r t i o n o f powder and g r a n u l e s  (11.16).  Most o f the c a l c i u m f l u o r i d e used i s p r e f u s e d i n a g r a p h i t e crucible.  Induction heating  and an argon b l a n k e t  are used.  The  - 27  -  m a t e r i a l i s k e p t a t h i g h temperature o n l y f o r the time r e q u i r e d f o r fusion.  T h i s p r e c a u t i o n m i n i m i z e s carbon p i c k u p from the c r u c i b l e  The main i m p u r i t y i n the c a l c i u m f l u o r i d e i s c a l c i u m o x i d e ,  (3).  the  amount v a r y i n g s l i g h t l y from r u n t o r u n , depending m a i n l y upon t h e moisture content  of the i n i t i a l m a t e r i a l .  o x i d e have a l s o been d e t e c t e d .  T r a c e s of s i l i c a and i r o n  However, as the amount of i r o n o x i d e  s u b s t a n t i a l l y i n c r e a s e s d u r i n g most r u n s , the p r e s e n c e of a s m a l l q u a n t i t y of r e d u c i b l e oxides i n the o r i g i n a l s l a g has l i t t l e  influence  on the D . C . p r o c e s s but i s of s i g n i f i c a n c e i n the s p e c i a l case of A . C . m e l t i n g i n pure CaF^ (Chapter IV and S l a g c o m p o s i t i o n s to  III.14). B o t h CaO and s i l i c a t e s  and Masson (14)  d e p r e s s the m e l t i n g p o i n t of C a F ^ .  have r e c e n t l y p u b l i s h e d t h e i r experiments  s u b j e c t and r e v i e w e d  the a v a i l a b l e l i t e r a t u r e .  of the f r e e z i n g temperature  Their  determination  of CaF^ i s 1423°C, depressed by °^2.3°C  f r a c t i o n of CaO + s i l i c a t e s when s i l i c a t e s measured a f r e e z i n g p o i n t of  Kojima  on the  per 0.01 mole f r a c t i o n of CaO and depressed by ^3°C per 0.01  of  III.11  are p r e s e n t .  1405°C and t h e r e f o r e  mole  We have  have up to 5 wt %  such i m p u r i t i e s . Alumina i s used e i t h e r  g r a n u l e s of e l e c t r o f u s e d  as powder ( A l c a n , 99.9% p u r i t y ) or  a l u m i n a (Norton C o . ) of e q u i v a l e n t p u r i t y .  C a l c i u m monoalumihate ( C a O ' A ^ O ^ ) i s p r e f u s e d i n a g r a p h i t e c r u c i b l e u s i n g the same t e c h n i q u e as f o r c a l c i u m f l u o r i d e . m a t e r i a l s are l i m e p r e p a r e d from c a l c i u m c a r b o n a t e ( t e c h n i c a l f i r e d 12 h r s a t 1100°C and metric  quantities.  Starting grade)  a l u m i n a powder (see above) i n s t o i c h i o -  - 28 -  Calcium monotitanate  (CaO'TiO^) i s c o m m e r c i a l l y a v a i l a b l e as a  powder (Cerac C o r p . ) n o r m a l l y used f o r s p r a y It  coating.  i s c o l d p r e s s e d and s i n t e r e d i n a i r b e f o r e u s e .  This  procedure  has the advantage of p r o d u c i n g a d r y m a t e r i a l i n lump f o r m . As i s the case of the c a l c i u m f l u o r i d e , a t r a c e of i r o n o x i d e c o u l d be detected i n the c a l c i u m t i t a n a t e . reveals  the c h a r a c t e r i s t i c  strongest  l i n e s of T i O ^ .  the t i t a n i u m c o n t e n t  X-Ray e x a m i n a t i o n of the powder  p a t t e r n o f CaTiO^ a l o n g w i t h the  three  C h e m i c a l a n a l y s i s however i n d i c a t e s  that  i s s t o i c h i o m e t r i c w i t h i n the p r e c i s i o n of  method (appendix IV) - i . e .  the  35.10 wt % v e r s u s a t h e o r e t i c a l v a l u e of  35.25 %.  III.4  R e m e l t i n g c o n d i t i o n s f o r 321 S t a i n l e s s  Steel  The c o n d i t i o n s f o r r e m e l t i n g are k e p t w i t h i n the s t a b l e of ESR - i . e .  power i n p u t and melt r a t e are s e t  range  at v a l u e s l e a d i n g to  smooth s u r f a c e q u a l i t y and l o n g i t u d i n a l d e n d r i t i c  structure.  T a b l e s 1 to 19 r e v i e w the power c o n d i t i o n s i n w h i c h the v a r i o u s i n g o t s have been r e m e l t e d and the m e l t r a t e w h i c h has been a c h i e v e d . Only those i n g o t s f o r w h i c h s t a b l e c o n d i t i o n s have been a c h i e v e d reported.  are  V a l u e s are averaged o v e r a p e r i o d c o v e r i n g 80 to 130  seconds. Mold diameter i s 5.84 cm p r o d u c i n g an average when the s l a g s k i n has been removed.  i n g o t d i a m e t e r of 5.60 cm  The e l e c t r o d e  d i a m e t e r i s 2.54 cm  w h i c h corresponds to a c r o s s s e c t i o n r a t i o of 4.87 between the and the e l e c t r o d e .  i  The amount ,' of e l e c t r o d e m e l t i n g i s t h e r e f o r e \ to the downward t r a v e l (L) augnifented of the r a t e of i n g o t r i s e : v  £  IP  i I  ingot equal  - 29 -  L(  - —  i  ± _  ) = 1.26 L  4.87 L i s measured d i r e c t l y from the d i g i t a l c o u n t e r  (11.14). -3  The d e n s i t y o f the s t e e l was measured t o be 7.916 g cm  a t room  temperature. The i n i t i a l q u a n t i t y o f s l a g was 380 g o f w h i c h 80 g was i n the starting  compacts.  The v a r i a b l e s l i s t e d below a r e the i n s t a n t a n e o u s w e i g h t of  the dW * m r e m e l t e d m e t a l ( i n g o t ) W , the s l a g w e i g h t W , the m e l t r a t e W = - — , rn. s m dt the o p e r a t i n g c u r r e n t and v o l t a g e .  These v a r i a b l e s a r e l i s t e d f o r  p o i n t s i n the p r o c e s s which c o r r e s p o n d to the m e l t i n g o f the p o r t i o n o f the i n g o t from w h i c h samples were t a k e n f o r c h e m i c a l a n a l y s i s . samples extend f o r a h e i g h t o f about 6 mm o r 120 g of i n g o t . W v a r i a t i o n o f _m W s  The  over a sample i s a p p r o x i m a t e l y 0.3 w h i l e the time needed  to r e m e l t 100 g ; i s  o40 to 80 s e c o n d s .  The margin: .of random e r r o r o v e r the v a r i o u s measurements to:  These  corresponds  : i 30 g on ( r e p o r t e d to the n e a r e s t 10 g) ± 3% .. on W m ± 10:amp on the o p e r a t i n g c u r r e n t when u s i n g D . C . ± 30 amp on the o p e r a t i n g c u r r e n t when u s i n g A . C . ± 0 . 1 V on the o p e r a t i n g v o l t a g e . A few i n g o t s have not been c u t i n t o samples.  Ingots  been used to measure e l e c t r o d e temperature g r a d i e n t s  and  ( I I I . 1 7 ) and S  have Q  l o  and S^g t o s t u d y c o n v e c t i o n p a t t e r n s i n the s l a g ( I I I . 1 6 ) .  - 30 Table 1.  Ingot S l  Electrode negative Argon B l a n k e t S l a g ( i n wt %): 75% C a F , 25% C a A l ^ 2  Sample number  Ingot weight  Ingot-slag weight r a t i o  W  m W~ s  W  IS 1C 2S 2C 3S 3C 4S 4C  T a b l e 2.  Melt rate  550 750 1260 1470 1770 1980 2600 2800  Oper. voltage  Amp  Volt  W g sec  1.6 2.2 3.7 4.3 5.2 5.8 7.6 8.2  I n g o t S2  Oper. current  1  1.62 1.84 1.57 1.62 1.56 1.66 1.68 1.70  615 625 635 630 630 630 630 620  23.7 23.8 23.7 23.8 23.8 23.8 23.6 23.5  Electrode negative In a i r S l a g : 75% C a F  Sample  wm', g 6  W  m  W  2 >  25% C a A l 0  W , g sec m  2  -1  Amp  Volt  660 650 650 650 640 650  23.7 23.6 23.4 23.7 23.7 23.7  s IS 1C 2S 2C 3S 3C  650 850 1650 1850 2600 2800  1.9 2.5 4.8 5.4 7.6 8.2  0.85 1.27 1.52 1.45 1.52 1.50  - 31 T a b l e 3.  Ingot S3  Electrode negative Argon B l a n k e t Slag:  Sample  IS IC 2S 2C 3S 3C  T a b l e 4.  75% C a F  2>  25%  W , g m  W m — w s  " -1 W , g sec m' °  610 820 1460 1670 2020 2210  1.8 2.4 4.3 4.9 6.1 6.5  1.78 1.93 1.97 1.98 2.03 2.08  Ingot S4  CaAl^ Amp  720 720 720 715 710  Volt  24.5 24.4 24.4 24.3 24.4  Electrode negative Argon B l a n k e t Slag:  Sample  Table 5.  W m  W , g m  77-  400 900 1350  1.1 2.6 3.9  W  s  75% C a F , 25% 2  CaAl^  * -1 W , g sec m  Amp  1.61 1.49 1.37  670 670 660  Volt  23.7 23.5 23.8  Ingot S 5 E l e c t r o d e p o s i t i v e Argon B l a n k e t Slag: ...  Sample  W , g  600 1200 1760  W m W s 1.8 3.6 5.3  75% CaF„, 25% CaAl^O . . . . 2 ...  .  " -1 W , g sec  1.52 1.81 1.68  .z  Amp  560 575 570  Volt  22.8 22.8 22.8  - 32 T a b l e 6.  Ingot S6  Electrode negative Argon B l a n k e t Slag:  Sample  IS 1C 2S 2C 3S 3C  T a b l e 7.  W, g  __m  700 920 1740 1950 3000 3200  2.2 2.6 5.4 6.0 9.4 10.0  "  Ingot S7  100% C a F  2  W , g sec  1.37 1.37 1.48 1.69 1.56 1.51  Amp  Volt  680 670 700 700 710  22.8 23.1 22.7 22.5 22.5  Electrode negative Argon B l a n k e t Slag:  Sample r  IS 1C 2S 2C 3S 3C  W, g m  560 780 1060 1270 2130 2330  W m  r— W  s  1.6 2.2 3.0 3.6 6.0 6.6  84.4% C a F , 15.6% C a T i 0 2  ••* -1 W , g sec m  2.00 2.00 1.97 1.97 1.99 1.95  Amp  680 680 670 675 670 670  3  Volt  23.2 23.3 23.6 23.6 23.7 23.7  - 33 -  T a b l e 8.  Ingot S8  Electrode negative Argon B l a n k e t Slag:  Sample  W  W,.g  _m  68.4% C a F  2 >  31.6% C a T i 0  W , g sec - 1 m  Amp  2.05 2.33 2.35 2.35 2.52 2.54  680 670 660 660 640 640  3  Volt  W IS 1C 2S 2C 3S 3C  Table 9.  590 810 1080 1290 2000 2200  Ingot S9  1.7 2.3 3.1 3.7 5.7 6.3  23.4 23.6 23.3 23.4 23.7 23.7  Electrode p o s i t i v e Argon B l a n k e t Slag:  Sample  W , g m  W m — W s  IS 1C 2S 2C 3S 3C  810 .1010 1630 1830 2050 2250  2.3 2.9 4.6 5.2 5.8 6.4  100% C a F  2  W , g sec - 1 m  1.52 1.59 1.32 1.35 1.34 1.36  Amp  630 600 630 650 650 650  Volt  22.3 22. 22. 22. 22. 22.  - 34 -  T a b l e 10.  Ingot S10  Electrode p o s i t i v e Argon B l a n k e t Slag:  W m W s  Sample  W, g m °  77-  IS IC 2S 2C 3S 3C  620 820 1450 1650 2040 2240  2.0 2.6 4.7 5.3 6.8 7.2  r  Table 11.  Ingot S l l  92.1% C a F , 7.9% C a T i 0 2  • -1 W , g sec m 0  2.60 2.18 1.57 1.67 1.72 1.62  3  Amp  Volt  r  630 630 640 640 660 650  22.6 22.4 21.7 21.7 21,6 21.7  Electrode p o s i t i v e Argon B l a n k e t Slag:  Sample  W, g m'  IS IC 2S 2C 3S 3C  720 920 1450 1650 2010 2200  v  ;  0  W m — w s . . 2.1 2.6 4.1 4.7 5.7 6.3  68.4% C a F , 31.6% C a T i 0 2  * -1 W , e sec m' &  1.72 1.92 1.82 1.82 1.77 1.82  Amp r  620 650 650 650 630 640  3  Volt  22.4 22.2 22.2 22.2 22.3 22.1  - 35 T a b l e 12.  Ingot S l 2  Alternating  current  Argon B l a n k e t Slag: Sample  IS 1C 2S 2C 3S 3C  Table 13.  W, g m °  W m —  W  540 740 1180 1380 2410 2550  s  100% C a F  * -1 W , e sec m  1.8 2.4 3.9 4.5 8.0 8.5  Ingot S13  0  °  1.99 2.03 2.08 2.20 2.25 2.25  Alternating  Amp  Volt  590 600 580 570 570 580  27.2 27.2 27.3 27.2 27.3 27.3  current  Argon B l a n k e t Slag:  Sample  W, g m  W m ~— W s  IS 1C 2S 2C 3S 3C  1000 1200 1760 1960 2670 2730  3.2 3.8 5.7 6.3 8.3 8.8  r  64.7% C a F , 35.3% C a T i 0 2  ' -1 W , g sec m •  2.08 2.13 2.09 2.10 2.07 2.07  Amp  630 600 580 580 580 580  3  Volt  26.7 26.7 26.8 26.8 26.8 26.8  - 36 T a b l e 14.  Ingot S l 4  Electrode negative Argon B l a n k e t Slag:  68.4% C a F ~ , 31.6% CaTiO  Aluminium a d d i t i o n s t a r t s 1 g mm Sample  W, g m °  -IS  360 520 730 1260 1440 1740 1940 2170 2370  IC 2S 2C 3S 3C 4S 4C  T a b l e 15.  Ingot S l 5  W m — W s  " -1 W , g sec m  1.1 1.6 2.2 3.8 4.4 5.3 5.9 6.6 7.2  1.45 1.90 2.08 2.18 1.94 2.45 2.41 2.35 2.12  W at - ~ = 1.6: W  Amp  Volts .  ,  670 690 690 680 670 670 680 670 670  rate  .  . 23.8 23.7 23.6 23.0 23.2 23.2 23.1 23.2 23.2  Electrode p o s i t i v e Argon B l a n k e t Slag:  64.7% CaF , 35.3% CaTiO W Aluminium a d d i t i o n s t a r t s a t ^ = w  2.5;  rate  1 g mm Sample  IS IC 2S 2C 3S 3C  m> 8  W m W  W , g sec m  640 820 1250 1630 1810 2560 2730  2.5 3.1 5.0 6.5 7.1 10.3 10.9  1.72 1.86 2.03 2.19 2.21 2.23 2.18  W  -1  Amp  Volts  600 590 610 615 620 620 620  22.2 22.0 22. ,4 22. ,3 22. ,2 22. 3 22. ,2  - 37 Ingot S16  T a b l e 16.  W E l e c t r o d e n e g a t i v e — below 3.6 w s w  A l t e r n a t i n g c u r r e n t —m above 3.6 C l o s e d Argon Cap Slag:  100% C a F - w i t h 2 g aluminium 2  i n i t i a l deoxidation Sample  w  ,  m  W m W s  g  °  W  m  ,  g  sec  -  1  Amp  Volts  D.C./A.C.  D.C./A.C.  IS 1C 2S 2C  300 450 810 960  1.0 1.5 2.7 3.2  1.29 1.27 1.24 1.20  660 670 700 680  22.0 22.1 22.0 22.1  3S 3C 4S 4C  1290 1470 1890 2070  4.3 4.9 6.3 6.9  1.60 1.67 1.66 1.66  620 620 620 620  25.6 25.6 25.6 25.6  T a b l e 17.  I n g o t S17  W E l e c t r o d e n e g a t i v e — below 5.9 W s .W A l t e r n a t i n g c u r r e n t r ^ — above 5 . 9 W s C l o s e d Argon Cap Slag:  64.7% C a F , 35.3% C a T i 0 - w i t h 2  3  2 g aluminium f o r i n i t i a l d e o x i d a t i o n Sample  W . g • m'  6  W m W  W , g sec m  Amp  Volts  IS 1C 2S 2C  900 1080 1490 . 1670  2.9 3.5 4.8 5.4  2.02 2.26 2.33 2.29  660 650 660 670  22.5 22.6 22.5 22.4  3S 3C 4S 4C  2050 2230 2720 2820  6.6 7.2 8.8 9.1  2.58 2.54 2.66 2.54  650 650 650 650  27.0 27.2 27.1 27.1  - 38 T a b l e 18.  I n g o t S18  W E l e c t r o d e n e g a t i v e — b e l o w 3.7 s W A l t e r n a t i n g c u r r e n t — above 3.7  w  s  In A i r Slag: Sample  _  _ —  Table  19.  m  W m vT s  610 1190  2.0 3.6  1360 2310  4.5 7.6  Ingot S19  91.0% C a F , 9.0% C a T i 0 2  W , g sec m  -1  3  Amp  Volts  1.93 1.81  725 700  22.5 22.0  2.19 2.27  575 575  22.0 22.0  Electrode positive In A i r Slag:  Sample  -  wm > g  1000 1750  W m W s 3.1 5.5  91.0% C a F , 2  W , g sec m  1.97 2.15  -1  9.0% C a T i O  3  Amp  Volts  725 600  21.0 22.5  - 39 III.5  R e m e l t i n g c o n d i t i o n s f o r Maraging 300 S t e e l  The c o n d i t i o n s d i f f e r l i t t l e from those used w i t h 321 S . S . same v a r i a b l e s have been measured w i t h t h e same m a r g i n of e r r o r .  The The  q u a n t i t y o f s l a g , the e l e c t r o d e and mold diameter a r e a l s o the same and the r a t i o of the l e n g t h o f e l e c t r o d e m e l t e d to the downward t r a v e l i s therefore  1.26.  The d e n s i t y of t h e s t e e l was measured t o be 8.039 a t room temperature. T a b l e 20.  Ingot M l E l e c t r o d e n e g a t i v e Argon B l a n k e t Slag:  Sample  IS IC 2S 2C 3S 3C  W, g  _m W s  770 .950 1610 1790 2630 2800  2.2 2.7 4.6 5.1 7.5 8.0  100 wt % C a F .  2  _  W , g sec  1.63 1.74 2.15 2.15 2.58 2.46  Amp  Volt  690 690 680 680 690 690  23.8 24.1 24.1 24.0 24.0 24.0  - 40 Table 21.  Ingot M2 E l e c t r o d e n e g a t i v e In A i r Slag:  Sample  w , g  m  IS 1C 2S 2C 3S 3C  T a b l e 22.  690 850 2140 2290 3640 3790  W  m  W  100% C a F »  W , g sec  m  -1  Amp  Volt  690 700 680 690 660 670  22.5 22.5 22.6 22.9 22.1 21.9  s  2.1 2.6 7.0 7.5 11.9 12.4  Ingot M3  2  1.59 1.55 2.10 2.18 2.20 2.14  Electrode negative  >  Argon B l a n k e t Slag:  Sample  IS 1C 2S 2C 3S 3C  w , gB  m'  440 660 1100 1280 2020 2200  W W  m  82.4% C a F ^ , 17.6% C a T i 0  W^,  g sec •1  3  Amp  Volt  720 700 680 680 680 670  23.2 23.2 23.3 23.3 23.4 23.3  s  1.2 1.8 3.0 3.5 5.5 6.0  .93 1.67 2.10 2.11 2.19 2.16  - 41 T a b l e 23.  Ingot M4 E l e c t r o d e n e g a t i v e Argon B l a n k e t Slag:  Sample  IS 1C 2S 2C 3S 3G  T a b l e 24.  W , g m'  m — W s  660 840 1330 1500 2330 2500  2.0 2.5 4.0 4.5 7.0 7.5  68.4% C a F , 31.6% CaTiO 2  W , g sec m  Amp  Volt  660 660 670 680 680 680  23.8 24.0 23.7 23.3 23.4 23.4  Amp  Volt  620 640 630 640 630 630  22.6 22.6 22.8 22.7 22.4 22.4  r  2.81 2.58 2.48 2.54 2.29 2.29  Ingot M5 E l e c t r o d e p o s i t i v e Argon B l a n k e t Slag:  Sample  W , g m  W m 7— W s  IS 1C 2S 2C 3S 3C  540 700 1340 1500 2040 2200  1.7 2.2 4.2 4.7 6.4 6.9  100% CaF 1  W , g sec m  2.32 2.16 2.27 2.29 2.21 2.29  -1  r  - 42 T a b l e 25.  I n g o t M6 E l e c t r o d e p o s i t i v e Argon B l a n k e t Slag:  68.4% C a F , 31.6% CaTiO 2  Sample  W, g  _m_ W s  W , g sec  Amp  Volt  IS IC 2S 2C 3S 3C  390 570 1330 1490 2200 2380  1.1 1.6 3.7 4.2 6.2 6.7  1.55 2.16 2.30 2.26 2.18 2.19  620 620 630 630 630 630  22.3 22.3 22.2 22.1 22.3 22.3  T a b l e 26.  W Ingot M7 E l e c t r o d e n e g a t i v e -™- below 5 . 6 w  s W A l t e r n a t i n g c u r r e n t — above 5 . 6 C l o s e d Argon Cap Slag:  w  s  100% C a F + 2 g A l f o r i n i t i a l 2  deoxidation W m W s  Sample  W , g sec m  Amp  Volt  IS IC 2S 2C  650 760 1210 1320  2.3 2.7 4.3 4.7  1.26 ' 1.33 1.34 1.44  680 680 670 680  22.5 22.4 22.3 22.2  3S 3C 4S 4C  1770 1880 2360 2460  6.3 6.7 8.4 8.8  1.80 1.80 1.66 1.72  580 590 580 590  27.5 27.5 27.5 27.6  - 43 III.6  R e m e l t i n g c o n d i t i o n s f o r 1409 A l S t e e l  The c o n d i t i o n s a r e s i m i l a r t o t h o s e o u t l i n e d i n I I I . 4 e x c e p t f o r the f o l l o w i n g : The e l e c t r o d e s  used a r e h o t r o l l e d square s e c t i o n b a r s  2 a p p r o x i m a t e l y 3.76 x 3.76 cm . The c r o s s - s e c t i o n a l  ("gothic")  2  a r e a i s 1 3 . 6 cm .  Mold d i a m e t e r i s 7.75 cm  p r o d u c i n g an average i n g o t s i z e o f 7.35 cm. The c r o s s s e c t i o n r a t i o i s t h e r e f o r e  3 . 9 8 , t h e amount of m e l t e d  e l e c t r o d e b e i n g c a l c u l a t e d from the downward t r a v e l , L , u s i n g t h e relation L(  1  -  — 3.98  > = 1.34 L  -3 The d e n s i t y o f the s t e e l  i s 7.385 g cm  at room t e m p e r a t u r e .  740 g of s l a g was used o f w h i c h 80 g was i n t h e s t a r t i n g compacts. I n g o t s of t h i s c o m p o s i t i o n have been r e m e l t e d w i t h s l a g s i n the CaF2«Al202 s y s t e m .  This i s a simple e u t e c t i c  s o l u b i l i t y , the e u t e c t i c AlyOy  system w i t h no s o l i d  c o m p o s i t i o n i s at a p p r o x i m a t e l y 10 wt %  (15) and the l i q u i d u s  temperature  i n c r e a s e s s h a r p l y above  this  c o n c e n t r a t i o n of A l ^ O ^ . D u r i n g a g i v e n m e l t , the o x i d a t i o n o f aluminium from t h e i n g o t r a i s e d the a l u m i n a c o n t e n t increase  i n the l i q u i d u s  10% A l ^ O ^ .  o f the s l a g r e s u l t i n g i n a temperature of s l a g s  considerable  c o n t a i n i n g more than  T h i s produced a t h i c k e n i n g o f t h e s k i n w i t h i m p o r t a n t  e f f e c t s on the t h e r m a l b a l a n c e and a r a p i d d e c r e a s e of the s l a g p o o l  - 44 volume.  The e f f e c t on t h e i n g o t s t r u c t u r e was d e t r i m e n t a l and w i l l  be d i s c u s s e d .  T a b l e 27.  Ingot A l E l e c t r o d e n e g a t i v e Argon B l a n k e t Slag:  Sample  IS 1C 2S 2C 3S 3C  T a b l e 28.  W  100% C a F  0  m' 8  W m W  W , g sec m °  Amp  Volt  900 1250 2400 2750 3610 3960  1.4 2.0 4.8 5.5 8.1 8.8  2.03 2.03 2.41 2.59 2.78 2.81  1210 1220 1210 1180 1200 1200  22.9 22.6 22.7 22.9 22.5 22.5  Ingot A2  Electrode negative Argon B l a n k e t Slag:  95% C a F  2 >  5% A 1 0 2  Three a d d i t i o n s of 100 g CaF^ have been made a t W = 1070 g , 1490 g and 2900 g . Sample  W ,. g  _m  IS 1C 2S 2C 3S 3C  890 1240 1790 2130 3380 3740  -  W , g sec  2.31 2.26 2.21 2.26 2.21 2.23  Amp  Volt  1150 1160 1140 1120 1140 1130  21.3 21.4 21.5 21.3 21.3 21.4  - 45 T a b l e 29.  Ingot A3  Electrode negative Argon B l a n k e t Slag:  Sample IS IC 2S 2C 2S 3C  T a b l e 30.  W, g  _m  670 1020 1430 1780 2390 2740  82.5% C a F  2 >  17.5% A l ^  W , g sec  Amp  Volt  1010 1040 1030 1030 1020 1000  22.8 23.2 23.4 23.0 23.5 23.5  1.2 14.0  Ingot A4  3.22 3.41 3.72 3.63 3.89 3.85  Electrode p o s i t i v e Argon B l a n k e t Slag:  100% C a F  2  Aluminiun a d d i t i o n s t a r t s a t W = 2400 g ; m  rate: _  Sample  IS IC 2S 2C 3C 4C  W, g  880 1230 1860 2210 3030 3970  1 g min  _  _  _m  W , g sec  1.5 (2.1) (4.5) (5.6) (10.2) 17.6  _  3.22 3.32 3.60 3.56 4.15 4.06  Amp  Volt  1080 1080 1050 1050 1050 1030  22.0 22.8 21.9 21.8 22.2 22.0  - 46 III.7  -  R e m e l t i n g of F e r r o v a c E and m i l d  steel  I n g o t s of F e r r o v a c E have been r e m e l t e d i n o r d e r to compare the o x i d a t i o n r a t e of i r o n w i t h o t h e r a l l o y s . 3.28  Ferrovac E electrodes  cm i n d i a m e t e r and have been r e m e l t e d i n the 5.84 The amount of e l e c t r o d e m e l t e d i s t h e r e f o r e The d e n s i t y of the m e t a l i s ^ 7.8  c o n d i t i o n s are reported i n s e c t i o n  1.46  are  cm m o l d . L.  at room t e m p e r a t u r e .  Other  III.4.  As s a m p l i n g of the i n g o t has r e v e a l e d no t i t a n i u m o r aluminium p i c k u p from s l a g s c o n t a i n i n g T i O ^ o r A ^ O ^ , o n l y the average c o n d i t i o n s are r e p o r t e d .  These are t a k e n a f t e r  the i n i t i a l  period.  T a b l e 31.  Ingot F l  Ferrovac E Electrode negative Slag:  . _ W , g sec .m 2.70  T a b l e 32.  Ingot F2  68.6% C a F , 2  Amp  Volts  930  24.2  r  Ferrovac E Electrode Slag:  W , g sec  1.45  31.4% C a T i 0  1  negative  76.3% C a F , 23.7% A l ^ 2  Amp  1010  Volts  22.3  3  running starting  - 47 T a b l e 33.  I n g o t F3  Mild Steel  1018  Electrode negative Slag: W , g sec m  X  1.37  Table  34.  I n g o t F4  81% C a F , 19% C a T i 0 2  Amp  Volts  690  23.7  Mild Steel  3  1018  Electrode negative Slag: W  m  , g sec  -1  1.90  T a b l e 35.  Ingot F5  65.5% C a F , 34.5% CaTiO, 0  Amp  Volts  670  23.5  Mild Steel  1018  Electrode negative Slag:  W  m  , g sec 1.92  -1  48% C a F , 52% C a T i 0 2  Amp  Volts  680  23.2  3  - 48  III.8  -  A n a l y s i s and s a m p l i n g of  ingots  The c h e m i c a l a n a l y s i s of the r e m e l t e d i n g o t s has been performed i n two ways f o r those elements w h i c h are the s u b j e c t of t h i s s t u d y : T i t a n i u m i n 321 s t a i n l e s s  s t e e l and Maraging 300, aluminium i n 1409 A l .  a) The m a t r i x content has been measured u s i n g the  electron  m i c r o p r o b e f o l l o w i n g a t e c h n i q u e d e s c r i b e d i n appendix I V , b) The t o t a l c h e m i c a l c o n t e n t has been determined e i t h e r by s p e c t r o g r a p h i c a n a l y s i s o r by a wet c h e m i c a l method (appendix I V ) . The d i f f e r i e n t i a t i o n between m a t r i x and t o t a l c o n t e n t of t i t a n i u m i n 321 S . S .  and Mar 300 has p r o v e d v e r y u s e f u l i n d e t e r m i n i n g whether  any c o m p o s i t i o n change was the r e s u l t of d e s i r a b l e i n c l u s i o n removal o r of s t r a i g h t o x i d a t i o n . The c o n c e n t r a t i o n of the major a l l o y i n g element S.S.  and 1409 A l , n i c k e l i n M a r a g i n g 300)  e l e c t r o n probe.  (Chromium i n  321  has a l s o been measured on the  T h i s was found to p r o v i d e a needed check on the o p e r a -  t i o n o f the probe and the p r e p a r a t i o n of the s p e c i m e n .  The r e s u l t  is  n o r m a l l y found to be c l o s e to the i n i t i a l c o m p o s i t i o n , f l u c t u a t i n g by l e s s than 0.5% around the average v a l u e ( i . e . 321 S . S .  1 8 + 0 . 5 % f o r Cr i n  etc...).  The oxygen c o n t e n t of the i n g o t i s a l s o r e p o r t e d (see  analysis  method i n appendix I V ) . Sampling of the i n g o t s was done a c c o r d i n g t o f i g u r e 17. i s f i r s t cut l o n g i t u d i n a l l y i n two h a l v e s .  The i n g o t  One h a l f i s p o l i s h e d and  e t c h e d f o r s t u d y of the m a c r o g r a p h i c s t r u c t u r e .  Horizontal  slices,  a p p r o x i m a t e l y 10 mm i n t h i c k n e s s are taken from v a r i o u s l e v e l s of  the  - 49 -  ingots. marked  Specimens marked ' S ' a r e c u t near the s u r f a c e and specimens !  C ' a r e c u t from the c e n t e r .  M a r e k e p t f o r oxygen a n a l y s i s .  The i n t e r m e d i a t e specimens marked  Spectrographic analysis i s u s u a l l y  performed on t h e top f a c e o f t h e r e m a i n i n g s l i c e  (except f o r the top  slice). Because of the c u r v e d shape o f the l i q u i d m e t a l p o o l , the s i d e , specimen corresponds to a l o w e r m e t a l / s l a g r a t i o than t h e c e n t e r s p e c i men.  T h i s i s taken i n t o account i n the t a b l e s  the d i f f e r e n c e b e i n g about 0.5 f o r most i n g o t  III.9  I n c l u s i o n count and r a t i n g  1 t o 26 ( I I I . 4 to I I I . 6 ) , structures.  (16,17,19,20)  I n c l u s i o n types have been determined w i t h t h e h e l p of t h e e l e c t r o n probe.  Counts c o r r e s p o n d i n g to v a r i o u s c o m p o s i t i o n s and s i z e s o f  i n c l u s i o n s a r e g i v e n i n most c a s e s . R e d u c t i o n o f i n c l u s i o n s i z e s and counts i s one purpose o f s p e c i a l remelting processes.  As e l e c t r o s l a g r e m e l t i n g s u c c e s s f u l l y  eliminates  l a r g e i n c l u s i o n s , t h e a t t e n t i o n s h o u l d be f o c u s s e d on t h e m i c r o c l e a n l i n e s s of t h e m a t e r i a l . The most c u r r e n t r e f e r e n c e f o r d e t e r m i n i n g i n c l u s i o n c o n t e n t s the ASTM s t a n d a r d n ° E 4 5 - 6 3  (16).  I t distinguishes four general  types of i n c l u s i o n s : A - sulphides type B - a l u m i n a type C - silicates  type  D - g l o b u l a r oxides  type  A l l f o u r types a r e s e p a r a t e d i n t o t h i n and h e a v y .  is  - 50  -  The most common type found i n 321 S . S . Mar 300 i s t i t a n i u m c a r b o n i t r i d e .  a n d , to a l e s s e r  extent i n  T h i s type i s d i s t i n c t from those  r e p o r t e d above and may a d v a n t a g e o u s l y be c o n s i d e r e d a s e p a r a t e t y p e . I n the same s t e e l s , light stringers.  t i t a n i u m s u l p h i d e s are a l s o p r e s e n t  i n the form of  G l o b u l a r o x i d e s are p r e s e n t i n v a r i a b l e amounts,  r e l a t e d to the oxygen  content.  The i n c l u s i o n s observed i n the r e m e l t e d m a t e r i a l are e x c l u s i v e l y l i g h t w i t h a t h i c k n e s s l e s s than 8u (4y  almost  for sulphide  stringers). The i n t e n t i o n of t h i s s t u d y however was not to produce a c o m m e r c i a l l y acceptable  material.  A d i f f e r e n t method f o r c o u n t i n g i n c l u s i o n s has  been p r e f e r r e d as i t r e l a t e d b e t t e r to the oxygen c o n t e n t and o t h e r chemical c h a r a c t e r i s t i c s counted s e p a r a t e l y ,  of the p r o c e s s :  as T i ( C , N ) ,  separate s i z e ranges:  1.5  The main t y p e s c o u l d be  " T i S " , globular oxides  (D) i n  to 5 u c o r r e s p o n d i n g to the l o w e s t  three sizes  i d e n t i f i a b l e w i t h the e l e c t r o n p r o b e , g e n e r a l l y s m a l l e r than any i n c l u s i o n counted under the ASTM r a t i n g .  5 to 10 u c o r r e s p o n d i n g to  l i g h t type of the ASTM s t a n d a r d and b i g g e r than 10 y when they are  the  present.  The u n i t used i s number of i n c l u s i o n s p e r square m i l l i m e t e r as averaged o v e r . a number of 100 o r more i n c l u s i o n s .  Only T i ( C , N ) . a n d  D type i n c l u s i o n s have been found to be r e l e v a n t to the p r e s e n t and t h e s e are the types r e p o r t e d .  study  Because of the r a t h e r l i m i t e d s i z e  of the examined s p e c i m e n , the numbers r e p o r t e d here s h o u l d o n l y be , c o n s i d e r e d as a q u a l i t a t i v e i n d i c a t i o n o f the type (D and c a r b o n i t r i d e s ) and s i z e range r e p r e s e n t e d i n a random c r o s s s e c t i o n of the specimen.  - 51 III.10  -  A n a l y s i s and s a m p l i n g of the  slag  A s e c t o r w e i g h i n g about 10% of the s l a g cap i s a c c o r d i n g t o f i g u r e 17 and t h e n powdered.  separated  S l a g s k i n s are  sampled  longitudinally. The s l a g i s a n a l y z e d f o r the r e a c t i v e  element c o r r e s p o n d i n g  the m e t a l s t u d i e d ( t i t a n i u m o r aluminium) by one o f the methods i n appendix I V .  The c o n t e n t of the elements most l i k e l y to  These are chromium f o r 321 S . S . ,  described  control  the oxygen p o t e n t i a l i n the absence of the p r e v i o u s element i s measured.  to  also  and i r o n f o r Mar 300.  Because of the e x i s t e n c e of v o l a t i l e f l u o r i d e s of i r o n and t i t a n i u m the f l u o r i n e content of a few caps has a l s o been a n a l y z e d . A n a l y s i s of m e t a l l i c elements i n the s l a g s r e q u i r e s an o x i d i z i n g f u s i o n w i t h s u l f u r i c a c i d or p y r o s u l f a t e . i m p o s s i b l e to determine s e p a r a t e l y of i r o n and t i t a n i u m .  For t h i s r e a s o n i t has been  the v a r i o u s l e v e l s of o x i d a t i o n .  Q u a l i t a t i v e i n d i c a t i o n o f the presence of  v a r i o u s l e v e l s of o x i d a t i o n c o u l d be o b t a i n e d i n most X - r a y i n v e s t i g a t i o n of the s l a g s t r u c t u r e s method l e a d s to few r e s u l t s  as o n l y the p a t t e r n s  by the  cases. Debye-Scherrer  corresponding  to  CaF2 and CaTiO^ c o u l d be i d e n t i f i e d . A few t y p i c a l r e s u l t s s l a g caps are p r e s e n t e d . CaTiO^ i n i t i a l l y , constituents  r e g a r d i n g the s t a t e of o x i d a t i o n of  the  They concern s l a g s c o n t a i n i n g at l e a s t  as the X - r a y method i s n o t s e n s i t i v e  i n s m a l l amounts.  enough t o d e t e c t  The dominant p a t t e r n on the Debye  photograph i s always found to be CaF„.  10%  Sherrer  - 52  Table Ingot  -  36. D e t e c t e d by X - r a y CaTi0 Other (unknown)  Detected  X X X X X X X  S7 S8 Sll S13 S14 S15 S17  X X X X X X X  The c h e m i c a l d e t e c t i o n of T i  chemically  Ti  3  3 +  _  X X X >50% >50% X  can be done i n two ways  (appendix  IV): a) A f t e r a s h o r t f u s i o n i n fuming H^SO^, the specimen i s  copied  3+ down and d i l u t e d r a p i d l y .  Ti  i s p r e s e n t i f methylene b l u e  reduced when added to the s o l u t i o n .  is  The f u s i o n has to be l i m i t e d to a  few seconds as compared w i t h the normal time of 30 minutes when T i i s t o be d i s s o l v e d q u a n t i t a t i v e l y . b) The f u s i o n i s performed i n fuming H^SO^ ' presence of an 3+ 2+ excess o f ¥e^0^ f o r two m i n u t e s . Ti reduces the i r o n o x i d e . Fe ± n  t  i e  can be t i t r a t e d w i t h an o x i d a n t  ( e e r i e s u l f a t e ) i n the p r e s e n c e of  orthophenanthroline i n d i c a t o r .  The method i s not t r u l y  quantitative  2+ as p a r t of the Fe  i s o x i d i z e d by s u l p h u r i c a c i d .  The c o l o r of the cap and the f u s i o n l i q u i d d u r i n g the e a r l y s t a g e s -3+ i s also a very sensitive indicator for T i ( b l u e to v i o l e t  coloration).  Sampling of the s l a g f o r i n g o t M2 has been done a t r e g u l a r d u r i n g the m e l t .  intervals  At a p p r o x i m a t e l y two minute i n t e r v a l s , a copper r o d  was immersed i n the m o l t e n cap f o r a few s e c o n d s .  The s k i n formed, on  - 53 copper c o n s t i t u t e d a sample w e i g h i n g about 1 g . The method chosen was d i c t a t e d by the n e c e s s i t y  for collecting  samples at s h o r t i n t e r v a l s , making the use of a s u c t i o n method d i f f i c u l t and cumbersome.  The c o m p o s i t i o n of the sample c o l l e c t e d i s  extremely  c l o s e to t h a t of the cap as i s p r o v e n by the a n a l y s i s of the f i n a l I t s h o u l d be n o t e d t h a t the s k i n c h i l l e d on a c o l d copper s u r f a c e  sample. is  l i t t l e i n f l u e n c e d by the s o l i d u s c o m p o s i t i o n so l o n g as no r e d i s s o l u t i o n i s a l l o w e d to o c c u r .  The s i t u a t i o n i s t h e r e f o r e  d i f f e r e n t from t h e  mechanism of e q u i l i b r i u m s l a g s o l i d i f i c a t i o n , as o u t l i n e d i n r e f e r e n c e (10)  f o r the mold s k i n .  The c h i l l sample p r o b a b l y  represents  more c l o s e l y the b u l k l i q u i d c o m p o s i t i o n .  III.11  C o m p o s i t i o n and m a t e r i a l b a l a n c e of 321 S . S .  I n i t i a l electrode  contents:  M a t r i x [ T i ] : 0.45% Total [Ti] [0]  :  ingots  (% stands f o r wt %)  0.58%  : 9 ppm  I n c l u s i o n s T i ( C , N ) <5 u : 160 mm 5 -* 10 u :  -2  32 mm"  2  - 54 T a b l e 37.  Ingot S l ( f i g .  Final slag:  330 g , 1.75%  -  18) T i , 0.36%  T i t a n i u m g a i n e d by s l a g c o r r e s p o n d s  Cr. to a 0.20%  l o s s i n the  ingot.  A n a l y s i s of the i n g o t shows a l o s s of a p p r o x i m a t e l y 0.30%. F l u o r i n e shows a s m a l l d e c r e a s e i n the s l a g c o r r e s p o n d i n g 72.2%  CaF^ i n the f i n a l c o m p o s i t i o n ( i n i t i a l l y  to  75%).  I n c l u s i o n s mm D+Ti(C,N) D Ti(C,N) < 5 p 5 10 y 5 -> ,10 y 2  Sample  IS IM 1C 2S 2M 2C 3S 3M 3C 4S 4M 4C  [Ti] % matrix 0.14 0.16 0.21  0.22  [0] ppm  0.25  105  -  0.23 0.20 0.23 0.24  [Ti] % total  ''•  102  0.33  104  0.31  103  [Cr] % 17.51  615  55  12  17.84 17.81  725 560  72 80  26 28  17.64 17.63  670  92  31  17.70 17.76  785  64  37  18.01  560  32  22  - 55 T a b l e 38.  Ingot S2 ( f i g . 18)  F i n a l s l a g : 340 g , 1.62% T i , 0.28% C r . T i t a n i u m g a i n e d by s l a g c o r r e s p o n d s to 0.20% l o s s from i n g o t . A n a l y s i s of t h e i n g o t shows a l o s s bf a p p r o x i m a t e l y 0.30%. The F l u o r i n e c o n t e n t 74.1%  o f the s l a g shows a d e c r e a s e from 75.0 t o  (equivalent)  Sample  [Ti] % matrix  is  0.16  IM IC 2S 2M 2C 3S 3M 3C  CaF  0.14 0.18 0.22 0.22 0.21  T a b l e 39.  2<  [Ti] % total  [0] ppm  0.23  103  0.34  109  0.30  103  [Cr] %  17.78 17.91 18.11 18.16  . Ingot S3 ( f i g . 18)  F i n a l s l a g : 340 g , 1.37% T i , 0.32% C r . T i t a n i u m g a i n e d by t h e s l a g corresponds to a 0.21% l o s s of t h e i n g o t . A n a l y s i s of the i n g o t shows a l o s s of about 0.30%. Decrease i n f l u o r i n e c o r r e s p o n d s to a l o s s o f 2.4% CaF  Sample  IS IM IC 2S 2M 2C 3S 3M 3C  (initial:  2  [Ti] % Matrix 0.17 0.14 0.22 0.25 0.28 ".  75%).  [Ti] % Total  [0] ppm  0.24  80  0.28  110  0.31  99  :  0.24  (equivalent)  [Cr] %  I n c l u s i o n s mm D+Ti(C,N) T i ( C , N ) < 5 y 5+10 y  D 5+10  18.35  460  15  70  17.12 18.14  590 715  25 25  88 72  17.80 17.61  680 575  32 48  120 94  17.92  410  12  34  y  - 56 T a b l e 40.  Ingot S6 ( f i g .  F i n a l s l a g : 320 g , 1.96%  -  19) T i , 0.91% C r .  T i t a n i u m g a i n e d by the s l a g c o r r e s p o n d s to a 0.20%  l o s s i n the  i n g o t w h i l e the i n g o t a n a l y s i s i n d i c a t e s a l o s s of a p p r o x i m a t e l y 0.31%. The l o s s of f l u o r i n e c o r r e s p o n d s to 5.7% CaF^, s l i g h t l y h i g h e r than the d i l u t i o n e x p e c t e d from t h e b u i l d up of T i and; Cr oxides. I n c l u s i o n s mm Ti(C,N)+D D < 5 p 5 •+ 10 u 2  Sample  IS IM 1C 2S 2M 2C 3S 3M 3C  [Ti] % [Ti] % Matrix : Total 0.22 0.24 0.20 0.26 0.21 ' 0.28  [0] ppm  0.25  219  0.27  159  Q.27  144  [Cr] .% 18.10  665  144  17.75 18.19  730 595  135 160  17.54 18.22 17.50  780 765  170 65  1240  45  - 57 Table 41.  Ingot S7 ( f i g .  Final slag:  -  20)  350 g ; 5.11% T i , 0.91% C r .  T i t a n i u m g a i n e d i n s l a g : 5 . 1 1 - 4 . 4 4 = 0.57% l o s s of 0.09%  c o r r e s p o n d s to a  i n ingot.  A n a l y s i s of the i n g o t shows a l o s s g r e a t e r  than 0.15%.  F l u o r i n e l o s s c o r r e s p o n d s to a d e c r e a s e i n CaF^ c o n t e n t (from 84.4 Sample  IS IM IC 2S 2M 2C 3S 3M 3C  to 79.4%).  [Ti] % Matrix  ,:  '  0.28 0.32 0.25 0.31 0.30 0.24  [Ti] % Total  [0] ppm  0.37  151  0.37  148  0.35  132  [Cr] % 17.34 18.02 18.34 17.20 17.87 17.47  of  5.0%  - 58 T a b l e 42.  I n g o t S8 ( f i g .  Final slag:  -  20)  355 g ; 10-30% T i , 0.77% C r .  Titanium gained i n s l a g : l o s s of 0.22%  1 0 . 3 0 - 8 . 9 2 = 1.38%  c o r r e s p o n d s to a  i n the i n g o t , a g r e e i n g w i t h the f i n a l  of the m e t a l .  analysis  The b a l a n c e between m a t r i x and t o t a l t i t a n i u m  corresponds t o the i n c l u s i o n removal i n d i c a t e d on the g r a p h . F l u o r i n e l o s s i s e q u a l to a d e c r e a s e i n CaT?^ ° f  Sample  IS IM 1C 2S 2M 2C 3S 3M 3C  [Ti] % Matrix 0.23  0.26 0.25 0.27 0.30 0.31  ..;  [Ti] % Total  [0] ppm  0.35  109  0.38  100  0.34  103  [Cr] %  4.95%.  I n c l u s i o n s mm D+Ti(N,C) D < 5 p 5->-10y  2  Ti(C,N) 5 -> 10 u  17.38  840  175  12  18.60 17.44  675 725  145 190  8 12  17.20 18.03  490 525  130 95  8 4  17.43  350  48  2  -  T a b l e 43.  Ingot S9 ( f i g .  Final slag:  5 9  -  21)  347 g , 1.61% T i , 0.54% C r .  The t i t a n i u m c o n t e n t of  the s l a g c o r r e s p o n d s  the i n g o t w h i l e the  to a 0.25% l o s s  in  a n a l y s i s of the m e t a l i n d i c a t e s a 0.44%  loss.  Sample  IS IM  ic  2S 2M 2C 3S 3M 3C  [Ti] % Matrix 0.09 0.15 0.07 0.12 0.07 0.09  ;..  [Ti] % Total  [0] ppm  0.15  26  0.09  24  0.13  18  [Cr]  %  I n c l u s i o n s mm D+Ti(C,N) D < 5 y 5 + 10 y  2 Ti(C,N) 5 + 10 y  17.71 17.37 17.63  360 510  92 85  17.43  310  125  18.03  180  24  -  -  25 8 65  -  2  - 60 Table'44.  I n g o t S10 ( f i g . 21)  Final slag:  312 g , 2.91% T i , 0.40% C r .  The t i t a n i u m g a i n e d by t h e s l a g 2 . 9 1 - 2 . 2 2 = 0.69% c o r r e s p o n d s to a 0.10% l o s s i n t h e i n g o t w h i l e t h e a n a l y s i s o f t h e m e t a l i n d i c a t e s a 0.42% l o s s o v e r the l a s t  2/3 o f the i n g o t .  A r c i n g and sharp i r r e g u l a r i t i e s i n t h e m e l t r a t e c r e a t e to the s i g n i f i c a n c e o f t h e f i r s t samples  (n°l).  doubts as  P a r t of  the s l a g was l e f t unmelted a t t h e bottom o f the i n g o t .  Sample  [Ti] % Matrix  IS IM 1C 2S 2M 2C 3S 3M 3C  (0.73) (0.32) 0.07 0.12 0.12 0.10  [Ti] % Total  [0] ppm  0.52  84  0.14  26  0.16  21  [Cr] % 17.42 18.07 17.94 17.21 17.67 18.03  - 61 Table 45.  Ingot S l l ( f i g . 21)  Final slag:  357 g , 10.60% T i , . 0.57% C r .  The t i t a n i u m g a i n e d by the s l a g :  1 0 . 6 0 - 8 . 9 2 = 1.68%  corresponds  to a 0.27% l o s s i n t h e i n g o t w h i l e the a n a l y s i s o f the m e t a l i n d i c a t e s a 0.41% l o s s .  Sample  IS IM IC 2S 2M 2C 3S 3M 3C  [Ti] % Matrix 0.10 0.17 0.10 0.18 0.08 0.09  :  [Ti] % Total  [0] ppm  0.18  40  0.17  43  0.15  43  [Cr] %  I n c l u s i o n s mm D+Ti(C,N) D < 5 p 5+10 y  D >10y  18.13  285  92  6  17.86 17.67  465 305  145 64  9 12  17.49 17.75  390 485  100 62  22 0  17.75  325  42  2  :  - 62 T a b l e 46.  Ingot S12  Final slag:  (fig.  22)  286 g ; 2.10% T i , 0.09% C r .  The t i t a n i u m c o n t e n t of the s l a g c o r r e s p o n d s to a 0.24%  loss  i n the i n g o t w h i c h agrees w i t h the a n a l y s i s of the m e t a l (0.25% l o s s ) . The b a l a n c e of m a t r i x and t o t a l t i t a n i u m i n d i c a t e s an i n c l u s i o n removal account f o r 0.07%[Ti]  Sample  IS IM 1C 2S 2M 2C —  3S 3M 3C  [Ti] % Matrix 0.19 0.26 0.28 0.32 0.34 0.35 0.35 0.37  [Ti] % Total  [0] ppm  0.30  41  0.39  0 0  0.42  0  [Cr] %  (see  graph).  I n c l u s i o n s mm D+Ti(C,N) Ti(C,N) < 5 u 5 -> 10 y  Ti(C,N) > 10 y  18.11  1510  25  0  18.39 17.53  600 350  20 42  0 3  17.59  505  85  4  17.39  230  70  2  17.17  840  18  0  - 63 T a b l e 47.  Ingot S13 ( f i g .  Final slag:  23)  282 g ; 12.40% T i , 0.22% C r .  The t i t a n i u m g a i n e d by the s l a g 1 2 . 4 0 - 9 . 7 6 =2.64% c o r r e s p o n d s  to  a 0.30% l o s s i n the i n g o t w h i c h agrees w i t h the a n a l y s i s  of  the m e t a l (0.32% l o s s ) . The b a l a n c e of t o t a l and m a t r i x t i t a n i u m i n d i c a t e s an i n c l u s i o n removal a c c o u n t i n g f o r 0.03  Sample  IS IM ic 2S 2M 2C 3S 3M 3C  [Ti] % Matrix 0.15 0.15  [Ti] % Total  [0] ppm  : 0.26  0.30 :  -2 I n c l u s i o n s mm D+Ti(N,C) Ti(C,N) < 5 p 5 -»- 10 u  18.09  320  6  18.18 18.12  570 230  15 8  17.38 17.75  400 575  28 10  17.60  550  2  1  0.25 0.22 ,• 0.18  [Cr] %  [Ti].  7  0.18 ,: 0.21 0.31  t o 0.05%  5  - 64  T a b l e 48.  Ingot S14  F i n a l s l a g : 294  g;  -  ( f i g . 24) 7.60%  T i , 0.16%  Cr.  Aluminium a d d i t i o n s have been made at the r a t e o f 1 g/min, s t a r t i n g W at — =1.6. w s Over the e n t i r e p r o c e s s , the s l a g l o s e s 8.92-7.60 = 1.32% T i or 3.88  g which can be  g a i n i n the  c a l c u l a t e d to be the r e s u l t of a 1.56 g W f i r s t p e r i o d ( — < 1.6) and a 4.44 g l o s s d u r i n g w  s the aluminium a d d i t i o n (see The  b u i l d up  graph).  of t i t a n i u m i n t o the i n g o t i s p r o g r e s s i v e  s t a r t of the aluminium a d d i t i o n . between the t o t a l and p r o b a b l y due  A strong discrepancy  m a t r i x content of the m e t a l .  to the method of a n a l y s i s  have c o n s i d e r e d  only  those m a t r i x p o i n t  i n g o t showed a much h i g h e r  the occurs  This i s  (appendix I V ) .  We  counts which do  exceed the average by more than 4 times w h i l e the  The  after  s c a t t e r of measured  not  present  concentrations.  o p t i c a l count f o r i n c l u s i o n s remains normal.  A b a l a n c e of m a t r i x and t h a t 0.27  t o t a l t i t a n i u m i s p o s s i b l e and  indicates  to 0.29%[Ti] i s added to the m e t a l i n the form of  i n c l u s i o n s o r , more p r o b a b l y , of l o c a l t i t a n i u m r i c h these c o u l d be  the r e s u l t of the r e d u c t i o n  reaction  regions, leading  to p a r t i c u l a t e t i t a n i u m at the s l a g m e t a l i n t e r f a c e . P a r t of the aluminium i s v a p o r i z e d t h i s i s evidenced by  i t can reduce the  a p r e c i p i t a t i o n of y A ^ O ^  e l e c t r o d e , the mold and Sherrer  before  X-ray p a t t e r n ) .  the fumehood  on  ( d e t e c t e d by  slag,  the Debye-  - 65 T a b l e 48  -  (Continued)  We r e p o r t i n the t a b l e b e l o w , the t h e o r e t i c a l l i m i t f o r  the  t i t a n i u m c o n t e n t of the i n g o t , assuming a q u a n t i t a t i v e r e d u c t i o n of T i C ^ i n t o [ T i ] by the a l u m i n i u m p l u s the r e t e n t i o n of the o r i g i n a l amount i n the  imple [ T i ] % [ T i ] % [Ti] % [0] T h e o r e t i c a l Matrix Total ppm maximum _ IS IM 1C 2S 2M 2C 3S 3M 3C 4S 4M 4C  0.18 0.24 0.23 0.21 0.30 0.44 0.54 0.7.8 1.00  0.45 0.70  0.58 0.58 1.69 1.66  101 89  0.81  1.50  81  1.16  1.58  53  electrode.  I n c l u s i o n s mm [Cr] T i ( C , N ) + D T i ( C , N ) + D D % < 5 y 5 •+ 10 y > 10  -  840  28  0  17.60  -  860 590  155 54  0 0  17.91  810  95  1  18.03 17.84  335  325  8  17.62  250  285  32  -  66  -  T a b l e 49.  Ingot S15  Final slag:  (fig.  -  25)  251 g ; 5.25% T i , 1.12%  Cr.  Aluminium a d d i t i o n has been made a t the r a t e of 1 g / m i n s t a r t i n g W a t ^ = 2.5. s Over the e n t i r e p r o c e s s , o r 11.8  the s l a g has l o s t 9 . 9 6 - 5 . 2 5 = 4.71% T i  g ; t h i s amount can be c a l c u l a t e d to be the  o f a 3.9  result  g g a i n from the f i r s t p a r t of the i n g o t and a 15.-7 g  l o s s d u r i n g the a l u m i n i u m a d d i t i o n . The b u i l d up of t i t a n i u m i n t o the i n g o t i s s l i g h t l y more r a p i d t h a n i n the e l e c t r o d e n e g a t i v e case (S14) W p l a t e a u above — =7. s  and reaches a  The d i f f e r e n c e between m a t r i x and t o t a l c o n c e n t r a t i o n s  is  also  lower a l t h o u g h the apparent i n c l u s i o n g a i n remains h i g h and is  p r o b a b l y due t o l o c a l t i t a n i u m r i c h r e g i o n s , as i n the  former c a s e . ppm  The h i g h oxygen c o n t e n t  of a p p r o x i m a t e l y 200  can o n l y account f o r the t i e up of 0.04%  form of  T l  2°3  i  n  c  l  u  s  l  o  n  s  [ T i ] under the  '  The t h e o r e t i c a l  [ T i ] l i m i t i s o b t a i n e d as e x p l a i n e d above ( S 1 4 ) . I n c l u s i o n s mm 2 Sample [ T i ] % [ T i ] % [Ti] % [0] [Cr] T i ( C , N ) + D D+Ti(C,N) D % M a t r i x T o t a l T h e o r e t i c a l ppm < 5 y 5 -* 10 y > 10 y maximum -  IS IM IC  -  2S 2M 2C 3S 3M 3C  0.14 0.16 0.68 1.24 1.10 1.42 0.99  0.50  1.82  37  1.53  1.59  192  1.49  1.59  221  —  18.41  430  35  17.58  390  230  17.32  530  565  58  17.51 17.23  480 670  390 155  65 2  18.04 .  560  490  10  i  :6  - 67  T a b l e 50.  Ingot S16  Final slag:  (fig.  -  26)  303 g ; 0.75% T i , 0.24%  Cr.  The t i t a n i u m c o n t e n t of the s l a g c o r r e s p o n d s to a 0.11%  loss  i n the i n g o t t o be compared w i t h the c h e m i c a l a n a l y s i s o f the m e t a l (0.12% l o s s ) . The b a l a n c e of m a t r i x and t o t a l c o n t e n t  shows a s m a l l i n c l u s i o n  removal.  Sample  [Ti] % Matrix  IS IM 1C 2S 2M 2C 3S 3M 3C 4S 4M 4C  0.19 0.30 0.38 0.35 .:  0.36 0.36 0.36 0.33  [Ti] % Total  [0] ppm  0.34  124  0.42  30  0.41  1  Q.42  3  [Cr] % 17.91 17.27 18.06 17.84 17.61 17.38 18.21 17.53  - 68 Table 51.  Ingot S17 ( f i g . 27)  Final slag:  309 g ; 10.30% T i , 0.07% C r .  The t i t a n i u m g a i n i n t h e s l a g :  1 0 . 3 0 - 9 . 2 6 = 1.04% c o r r e s p o n d s t o  a 0.11% l o s s i n t h e i n g o t i n agreement w i t h t h e a n a l y s i s of the i n g o t ( 0 . 1 1 t o 0.13%). The b a l a n c e of m a t r i x and t o t a l [ T i ] c o n t e n t  shows a s i g n i f i c a n t  i n c l u s i o n removal (see g r a p h ) . The i n i t i a l aluminium d e o x i d a t i o n appears t o have titanium  content; i n the lower l e v e l s of the  increased.the ingot.  I n c l u s i o n s mm Ti(C,N)+D Ti(C,N) < 5 y 5 •*• 10 y 2  Sample _ IS IM IC 2S 2M 2C 3S 3M 3C 4S 4M 4C  [Ti]% Matrix 0.49 0.34 0.37 0.365 0.39 0.33 0.39 0.42 0.38  [Ti] % Total  [0] ppm  0.44  73  0.44  100  0.46  5  0.47  5  [Cr] %  low  D 5- •+ 10 y  17.67  1330  (290)  17.62 17.94  1040 250  (105) (220)  17.83  540  (180)  18.11  705  52  17.82 17.72  725 575  55 32  17.61  505  12  2  - 69 -  III.12  Composition  and m a t e r i a l b a l a n c e o f Maraging 300 i n g o t s  I n i t i a l electrode contents:  Matrix[Ti]  0.69%  Total  0.79%  [Ti]  Oxygen [0] [Mo]  10 ppm 4.95% -2  Inclusions  T a b l e 52.  Ingot Ml  Final slag: The  (fig.  350 g, 3.12%  T i ( C , N ) 5 ->• 10 y: 76  mm  Ti(C,N)  mm"  >10  y  : 7  28) T i ; 0.23%  Fe  ,  t i t a n i u m content o f the s l a g corresponds  to a 0.39%  loss  from the i n g o t , a p p r o x i m a t e l y equal to the m a t r i x l o s s showing t h e r e f o r e l i t t l e  imple  [Mo]  % IS IM 1C 2S 2M 2C 3S 3M 3C  [Ti] % Matrix  [Ti] % Total  [0] ppm  0.36  88  0.23 4.93  4-94  0.26 0'. 33 0.40  0.41 0.29  inclusion  [Ni]  %  and  removal.  Inclusions Ti(C,N)+D D ' < 5 y 5  -y  mm > 10 y  D 10  18.23  580  345  34  18.61 18.41  940 180  375 65  28 11  18.58 18.96  295 165  180 58  18 12  18.37  175  44  9  26  0.24 0.31 4.88  2  37  - 70 T a b l e 53.  I n g o t M2 ( f i g .  Final slag:  328 g ; 4.06%  -  29) T i , 0.05%  Fe.  The t i t a n i u m c o n t e n t of the s l a g i s seen to i n c r e a s e  linearly  a f t e r an i n i t i a l p e r i o d where the g a i n o c c u r s at a h i g h e r rate. The m e t a l c o m p o s i t i o n c a l c u l a t e d  from the s l a g c o m p o s i t i o n  can  be compared w i t h the a c t u a l a n a l y s i s of the i n g o t as shown on f i g .  29.  The i r o n c o n t e n t of the s l a g remains low throughout ment,  the  experi-  f l u c t u a t i n g between 0.01 and 0.05% w i t h o n l y the W specimen (— = 1) h a v i n g a h i g h e r c o n t e n t of 0.24%. •. s Sample  IM 2M ->- 3S 3M  [Ti] % Total  [Mo] % Total  0.48 0.41 0.52 0.47  4.90 4.92 4.91 4.85  first  - 71 T a b l e 54.  I n g o t M3 ( f i g .  Final slag:  370 g ; 7.12%  28) T i , 0.07%  The t i t a n i u m g a i n i n the s l a g : to a 0.45%  thereby  IS IM IC 2S 2M 2C 3S 3M 3C  4.98 4.88 4.90  [Ti] % Matrix 0.33 0.24 0.24 0.25 0.35 0.32  corresponds the  i n d i c a t i n g some i n c l u s i o n removal  ( a p p r o x i m a t e l y 0.05%  [Mo] %  7 . 1 2 - 4 . 4 4 = 2.68%  average l o s s i n the i n g o t , s l i g h t l y above  matrix loss,  Sample  Fe.  [Ti]).  [Ti] % Total  [0] ppm  0.33  81  0.35  88  0.42  79  [Ni] %  I n c l u s i o n s mm Ti(C,N)+D Ti(C,N)+D < 5 p 5+10 u  19.02  380  9  18.33 18.41  700 560  22 12  18.88 18.64  635 400  19 11  18.96  490  14  - 72 -  T a b l e 55.  Ingot M4 ( f i g . 28)  Final slag:  350 g ; 10.55% T i , 0.04% F e .  The t i t a n i u m g a i n i n the s l a g :  10.55-8.92  = 1.63%  corresponds  to an average i n g o t l o s s o f 0.23%, lower t h a n i s by the i n g o t  indicated  content.  I n c l u s i o n s mm Ti(C,N)+D Ti(C,N) D < 5 y 5 -> 10 y 5 + 10 y 2  Sample  IS IM IC 2S 2M 2C 3S 3M 3C  [Mo] %  4.83 4.90 4.89  T a b l e 56.  [Ti] % Matrix 0.30 0.31 0.29 0.32 0.38 0.39  [Ti] % Total  [0] ppm  0.39  81  0.42  10  0.57  5  [Ni] %  :  18.27  350  8  112  18.74 18.39  270 285  6 18  27 13  18.81 18.65  150 265  23 18  8 4  18.60  435  25  i  2  I n g o t M5 ( f i g . 30)  Final slag:  321 g ; 1.02% T i , 1.89% F e .  The t i t a n i u m content of the s l a g corresponds 0.15% from the i n g o t ,  to an average l o s s of  lower than i s i n d i c a t e d by the  analysis  of t h e m e t a l .  ample IS IM IC 2S 2M 2C 3S 3M 3C  [Ti] % . Matrix 0.46 0.53 0.38 0.40 0.42 0.37  [Ti] % Total  [0] ppm  0.62  375  0.77!  484  0.59  272  [Ni] %  I n c l u s i o n s mm Ti(C,N)+D D < 5 y 5 -> 10 u  2  D > 10 y  18.61  46  42  2  18.87 18.73  165 340  105 58  32 14  18.96 18.48  88 220  55 42  20 16  18.89  180  39  4  - 73 T a b l e 57.  Ingot M6 ( f i g . 30)  Final slag:  336 g ; 8.75% T i , 0.98% F e .  The s l a g shows an a c t u a l l o s s of t i t a n i u m :  8 . 9 2 - 8 . 7 5 = 0.17%;  t h i s i s i n disagreement w i t h t h e l o s s observed i n t h e i n g o t and may be due to the f a c t t h a t p a r t of t h e i n i t i a l s l a g was l e f t unmelted a t t h e bottom of t h e i n g o t .  I n c l u s i o n s mm D+Ti(C,N) D < 5 y 5+10 2  Sample  IS IM 1C 2S 2M 2C 3S 3M 3C  [Ti] % Matrix 0.46  .  0.45 0.47 0.46 0.42 0.45  .  [Ti] % Total  [0] ppm  0.58  110  0.60 0.56  '  108 80  [Ni] % 18.86  280  38  18.47 18.71  245 210  35 46  18.30 18.65  490 380  28 12  18.50  320  16  y  - 74 Table 58.  Ingot M7 ( f i g .  Final slag:  -  31)  282 g ; 0.73% T i , 0.18%  Fe.  The t i t a n i u m c o n t e n t of the s l a g corresponds from the i n g o t .  IS IM 1C 2S 2M 2C 3S 3M 3C 4S 4M 4C  T h i s i s i n agreement w i t h the  0.08%  composition  of the m e t a l .  Some i n c l u s i o n removal (0.03  i s observed as  the t o t a l m a t e r i a l b a l a n c e c l o s e s w i t h i n  0.02%  Sample  to a l o s s of  [Ko] %  4.88 4.90  4.92  to 0.05%  [Ti])  [Ti].  [Ti] % Matrix 0.60 0.67 0.67 0.61 0.74 0.67 0.68 0.61  -  [Ti] % Total  [0] ppm  0.74  6  0.65  4  0.74  21  0.63  27  [Ni] %  I n c l u s i o n s mm Ti(C,N)+D T i ( C , N ) < 5 y 5 -»• 10 y  2  5  D  -y  18.50  495  22  36  18.45 18.62 .  690 385  65 20  0 0  18.38 18.44  315 105  32 12  0 0  18.97 18.71  215 240  4 18  0 0  18.48  110  6  0  10  y  - 75 III.13  C o m p o s i t i o n of 1409 A l s t e e l  I n i t i a l electrode  contents: Total  ingots  [Al]  3.74%  [Ti]  0.49%  Matrix [Ti]  0.06%  Oxygen  95 ppm  Inclusions  -2 T i ( C , N ) > 10 y : 25 mm -2 T i ( C , N ) 5 + 10 y : 127 mm -2 T i ( C , N ) < 5 y : 171 mm  A l l the specimens of the r e m e l t e d s t e e l a r e found t o c o n t a i n o n l y a few ppm o f oxygen.  S i n c e n o n - m e t a l l i c a l u m i n i u m s h o u l d be i n t h e  :  form of a l u m i n a i n c l u s i o n s , o n l y one a n a l y s i s f o r t h i s element was • considered necessary.  The d i f f e r e n c e between m a t r i x and t o t a l [ A l ]  i s not measurable by our method and t h e e l e c t r o n probe r e v e a l e d no i  aluminium r i c h i n c l u s i o n s i n t h e r e m e l t e d m a t e r i a l . The a n a l y s i s of the s l a g f o r aluminium does n o t l e a d to s i g n i f i c a n t r e s u l t s f o r two main r e a s o n s : a)  The c o m p o s i t i o n o f t h e s k i n i s r e l a t i v e l y v a r i a b l e w i t h a  l a r g e r a l u m i n a content above the e u t e c t i c b)  than t h e s l a g cap when the average c o n t e n t  (15,18).  P a r t of the aluminium l o s t by the i n g o t i s swept away t h r o u g h  the atmosphere as a f i n e c l o u d of Al^O^ p a r t i c l e s . the m o l d , t h e e l e c t r o d e  Dust c o l l e c t e d on  and t h e fumehood r e v e a l s the presence of  Y a l u m i n a and a s m a l l q u a n t i t y o f a a l u m i n a (Debye-Sherrer pattern).  is  X-ray  The presence of t h e low temperature phase i n d i c a t e s  the o x i d a t i o n o f aluminium has t a k e n p l a c e i n a r e l a t i v e l y low temperature zone o f the atmosphere.  .  that  ;  - 76 -  I r o n and chromium c o u l d n o t be measured i n the f i n a l s l a g s (Fe < 0.02%, Cr < 0.05%). Manganese and s i l i c o n were a l s o a n a l y z e d i n t h e s t e e l and showed o n l y a s m a l l v a r i a t i o n ( - 0 . 0 3 t o +0.05% f o r [ S i ] and - 0 . 0 7 t o -0.03% f o r [Mn]) w h i c h was n o t c o n s i d e r e d to be s i g n i f i c a n t .  T a b l e 59.  Ingot A l  A r e l a t i v e l y t h i c k s k i n (up t o 2 mm) was f o r m e d , w e i g h i n g a p p r o x i m a t e l y 350 g , w h i l e t h e r e m a i n i n g cap was o n l y 300 g (initial total:  660 g ) .  Aluminium c o n c e n t r a t i o n 0.31%  i n the s l a g :  average l o s s from t h e i n g o t o r somewhat l o w e r than the  a n a l y s i s o f the m e t a l  Sample  [Al] %  IS IM IC 2S 2M 2C 3S 3M 3C  (3.28) 3.28 (3.35) (3.40) 3.42 (3.61) (3.51) 3.32 (3.44)  1.9% c o r r e s p o n d i n g t o a .  indicates  [ T i ] % [ T i ] % [0] Matrix Total ppm 0.06 0.05 0.04 0.06 0.07 0.13  0.52  4  0.50  0  0.50  1  _2 Inclusions T i ( C , N ) mm < 5 p 5 + 10 y > 10 u 345  125  10  425 400  142 60  8 4  550 365  170 98  11 12  260  85  6  - I l l-able 6 0 .  Ingot A2  To overcome d i f f i c u l t i e s encountered w i t h o t h e r i n g o t s s k i n and e x c e s s i v e  (thick  m e l t r a t e ) , a d d i t i o n s o f CaF^ have  been done d u r i n g t h e m e l t i n g of t h i s i n g o t  (see I I I . 6 ) ,  l e a d i n g t o a t o t a l s l a g w e i g h t o f a p p r o x i m a t e l y 1050 g .  This  m a i n t a i n e d the s l a g c o m p o s i t i o n below t h e 10% a l u m i n a ( e u t e c t i c ) l e v e l throughout t h e m e l t , b u t i t a l s o the s u p p l y o f i m p u r i t i e s ( o x i d a n t )  Specimen  IS IM 1C 2S 2M 2C 3S 3M 3C  [Al] %  0.06  2.97 3.30 3.18  Table 6 1 .  [Ti] % Matrix  0.08 0.04 0.09 0.05 0.05  [Ti] % Total  increased  t o the m e l t .  [0] ppm  Inclusions  Mostly Ti(C,N) 0.51  2  0.51  1  0.50  1  Ingot A3  A t h i c k s k i n formed on the s u r f a c e : of t h e i n g o t . s l a g cap to d e c r e a s e i n  T h i s caused the  s i z e t o the p o i n t where the  t i p was no l o n g e r immersed.  electrode  A r c i n g o c c u r r e d and t h e p r o c e s s  had to be s t o p p e d . The f i n a l  s l a g cap weighed o n l y 195 g .  Sample  [Al] %  [Ti] % Matrix  IS IM 1C 2S 2M 2C 3S 3M 3C  (3.05) 3.08 (3.02) (3.38) 3.40 (3.53) (3.42) 3.38 (3.48)  0.07 0.03 0.04 0.03 0.08 0.05  [Ti] % Total  [0] ppm  0.43  2  0.47  4  0.49  5  Inclusions Mostly Ti(C,N)  - 78 T a b l e 62.  Ingot A4  A t h i c k s k i n was formed throughout the m e l t c a u s i n g the  final  s l a g cap to be o n l y 225 g . Aluminium a d d i t i o n was used d u r i n g the l a s t p a r t of the i n g o t l i m i t i n g the s t a t e o f o x i d a t i o n o f the s l a g . f u l l y d e o x i d i z e d by the s t e e l , be c o l l e c t e d i n the i n g o t  If  the s l a g  is  the a l u m i n i u m a d d i t i o n s s h o u l d  (excluding vaporization losses)  t h e r e b y i n c r e a s i n g the c o n c e n t r a t i o n by 0.39%.  In t h i s  case,  the t o t a l l o s s of a l u m i n i u m , r e p o r t e d i n the t h i r d c o l u m n , would be c o n s t a n t .  Sample  IS IM IC 2S 2M 2C 3M 3C 4M 4C  .  IA1] %  [Al] % [Ti] % Loss Matrix  3.21  0.53  3.28 3.35 3.57 :  0.46  ''  0.78 0.56  0.02 0.06  0.04  0.07 0.05 0.09  [Ti] % Total  [0] ppm  0.49  6  0.47  1  0.47  5  0.49  6  Inclusions  Mostly Ti(C,N)  Mostly Ti(C,N)  - 79 III.14  Rate of o x i d a t i o n o f i r o n  The m a t r i x t i t a n i u m c o n t e n t  of i n g o t s F l , F 3 , F 4 , F5 has been  found to be l e s s t h a n 0.06% [ T i ] o r o n l y 1.5 times t h e background n o i s e f o r t i t a n i u m on pure i r o n w i t h t h e e l e c t r o n Titanium bearing i n c l u s i o n s are present, F5 where they r e a c h up t o 25 u i n s i z e .  probe.  especially i n ingot  The s l a g used f o r t h i s m e l t  (52% CaTiO^) produced an e x c e e d i n g l y d i r t y s t e e l w h i c h c o n t a i n e d m o s t l y heavy c a l c i u m t i t a n a t e s i n c l u s i o n s .  A c c o r d i n g l y , t h i s slag composition  has n o t been used f o r t h e o t h e r experiments of s t a i n l e s s  i n v o l v i n g the r e m e l t i n g  and Maraging s t e e l s .  The o x i d a t i o n r a t e r e p o r t e d i n the t a b l e i s c a l c u l a t e d from the i r o n c o n t e n t o f the s l a g .  Table 6 3 .  Ingot  Ingot w e i g h t g  F l (FVE) F2 (FVE) F3 (1018) F4 (1018) F5 (1018)  2150 1785 2225 2260 2245  Slag weight g 350 380 370 375 370  % Fe Slag  Oxidation rate Wt % m e t a l  4.12 2.30 1.63 1.89 1.97  0.67 0.49 0.27 0.31 0.33  - 80 -  III.15  S o l i d i f i c a t i o n patterns  I t has been r e p o r t e d (18) t h a t an a c c e p t a b l e i n g o t s u r f a c e can o n l y be produced i f the l i q u i d p o o l o f m e t a l i s i n permanent w i t h the s l a g s k i n i . e . , i f the s o l i d i f i c a t i o n i n t e r f a c e reaches t h e s k i n ( f i g . 1 ) .  In contrast,  when the p r o c e s s  contact  o f the m e t a l i s run  w i t h i n s u f f i c i e n t heat i n p u t , l a p p i n g o c c u r s , g i v i n g r i s e t o a v e r y poor s u r f a c e .  Our experiments  confirm this point  entirely.  Most a l l o y s , i n c l u d i n g those used i n t h i s w o r k , produce a d e n d r i t i c s t r u c t u r e when r e m e l t e d by e l e c t r o s l a g .  Among the f a c t o r s w h i c h  f a v o u r t h e f o r m a t i o n of d e n d r i t e s i n e l e c t r o s l a g cation rate,  a r e t h e slow s o l i d i f i -  the d i r e c t i o n a l s t a b i l i t y of the heat f l o w and moderate  c o n v e c t i v e motions i n the m e t a l p o o l . The d e n d r i t e o r i e n t a t i o n i s p a r a l l e l to t h e d i r e c t i o n o f the maximum temperature g r a d i e n t .  Both t h i s d i r e c t i o n and the p o o l  depth a r e v e r y dependent upon t h e t o t a l heat i n p u t s u p p l i e d to the process.  ;  The geometry o f the system i s a l s o i m p o r t a n t , t h e c u r v a t u r e  of the p o o l i n c r e a s e s w i t h b o t h t h e m e l t r a t e (power i n p u t ) and a d e c r e a s i n g r a t i o of e l e c t r o d e  t o i n g o t diameter  (10,21,22).  A sudden change i n the t h e r m a l c o n d i t i o n s w i l l be r e f l e c t e d i n the i n g o t s t r u c t u r e .  An example i s g i v e n i n f i g u r e 32 ( i n g o t  S17)  where the power was i n t e r r u p t e d f o r a few seconds when s w i t c h i n g from D.C. to A . C .  A change i n d e n d r i t e o r i e n t a t i o n and s i z e i s e v i d e n t  at about 60% of the i n g o t In contrast,  height.  f i g u r e 33 r e p r e s e n t s  the macrostructure  i n g o t r e m e l t e d w i t h D . C . c u r r e n t throughout i n t h e r m a l c o n d i t i o n s has o c c u r r e d .  (S14).  of an  No sudden change  The d e n d r i t e s a r e v e r t i c a l  - 81 i n i t i a l l y as most of the heat f l o w i s absorbed by the b a s e p l a t e and the o r i e n t a t i o n changes p r o g r e s s i v e l y when the w a l l s of the mold p l a y an i n c r e a s e d p a r t i n the c o o l i n g of the i n g o t . d e l i n e a t e d by d a r k e r bands i n the A l l of our s t a i n l e s s  The p o o l shape  is  etching.  s t e e l and Maraging s t e e l i n g o t s  exhibit  n e a r - v e r t i c a l d e n d r i t e growth and have been produced w i t h a f l a t p o o l profile. Maraging 300 produces c o a s e r d e n d r i t e s than s t a i n l e s s the same power i n p u t (M3, f i g . 34 and M7, f i g . 3 5 ) .  steel  I n f i g u r e 35, power  has a l s o been s w i t c h e d from D . C . to A . C ; new d e n d r i t e s have on the w a l l s when power was cut o f f .  at  nucleated  The c e n t r a l d e n d r i t e s c o n t i n u e  to  grow i n the same d i r e c t i o n a l t h o u g h i t i s o b v i o u s t h a t they do not f o l l o w the heat f l o w i n t h i s case (see demonstrates  f i g . 32 f o r c o m p a r i s o n ) .  This  t h a t the p o o l p r o f i l e cannot be d e r i v e d from a l i n e ;  drawn o r t h o g o n a l to the d e n d r i t e growth d i r e c t i o n i n m a t e r i a l s  with  a h i g h growth a n i s o t r o p y . F i g u r e 36 i s the macrograph of a 1409 A l s t e e l  ingot.  The p o o l  has been deep throughout the m e l t , r e s u l t i n g i n the observed Because of the poor c o n t a c t w i t h the b a s e p l a t e , extracted  structure.  too l i t t l e heat was  through the bottom of the i n g o t and hence the d e n d r i t e growth  d i r e c t i o n i s f a r from the v e r t i c a l . e f f e c t on the f o r g e a b i l i t y .  T h i s would have a d e t r i m e n t a l }  T h i s i n g o t was a l s o r e m e l t e d w i t h a,  h i g h m e l t r a t e p a r t i a l l y because of o x i d a t i o n of the a l u m i n i u m component o f the. a l l o y . It  i s observed  that  t h i s a l l o y w i l l nucleate  f o r a v e l o c i t y of the s o l i d i f i c a t i o n i n t e r f a c e , dendritic structure i n stainless  an e q u i a x e d  structure  which r e s u l t e d i n a  and Maraging s t e e l .  When power was  - 82  -  shut o f f at the end of the m e l t , most of the r e m a i n i n g p o o l s o l i d i f i e d r a p i d l y i n t o an equiaxed s t r u c t u r e .  A dendritic structure  p r o b a b l y have been p r e s e r v e d by " h o t t o p p i n g " the  III.16  O b s e r v a t i o n of c o n v e c t i v e  could  ingot.  motions  The d r i v i n g f o r c e f o r n a t u r a l t h e r m a l c o n v e c t i o n i n the m e t a l p o o l i s s m a l l , the h i g h e s t pool.  temperature  r e g i o n b e i n g the top of  Thermal c o n v e c t i o n can be i n c r e a s e d however when the  the  isotherms  i n the m e t a l have a pronounced c u r v a t u r e i . e . , when the p o o l p r o f i l e i s deeply curved. The most l i k e l y causes f o r s t i r r i n g of the p o o l a r e momentum transfer effects.  from the s l a g and the f a l l i n g d r o p , and  electromagnetic  The combined r e s u l t i s l a r g e l y unknown.  V i s u a l e x a m i n a t i o n of the s l a g and the atmosphere above the m e l t sheds some l i g h t on m o t i o n p a t t e r n s  i n t h e s e two p h a s e s .  Films  have been taken d u r i n g the f a b r i c a t i o n of i n g o t s S18 and S19. camera was aimed down at the s l a g s u r f a c e , melt and the atmosphere above i t . the s l a g s u r f a c e  The  c o v e r i n g one h a l f of  the  G r a p h i t e p a r t i c l e s were dropped onto  and were v i s i b l e a g a i n s t  the b r i g h t background when  c o n v e c t i o n i n the s l a g c a r r i e d them a c r o s s the s u r f a c e .  The  and the s l a g e m i t t e d enough smoke to o u t l i n e the movements of  electrode the  atmosphere. The c o n v e c t i o n p a t t e r n s  seem to d i f f e r l i t t l e when d i r e c t  of e i t h e r p o l a r i t y o r a l t e r n a t i n g c u r r e n t i s u s e d . i n t r o d u c i n g the powder (of the o r d e r of 0.05 the e l e c t r o d e  sec)  i s c l e a r e d of g r a p h i t e p a r t i c l e s .  current  A s h o r t time a f t e r an annulus around  At t h i s time, a l a r g e r  - 83 -  c o n c e n t r i c annulus c o l l e c t s most of the g r a p h i t e .  This region i s  s h a r p l y d e f i n e d near i t s i n n e r boundary and more d i f f u s e on the Some g r a p h i t e o c c a s i o n a l l y l e a v e s mold w a l l  (fig.  the annulus to p i l e up a g a i n s t  outside. the  37).  The fumes i n d i c a t e some t u r b u l e n c e i n the atmosphere, b e i n g g e n e r a l l y upward near the e l e c t r o d e  currents  and downward near the w a l l s ,  i n k e e p i n g w i t h normal t h e r m a l c o n v e c t i o n .  The same p a t t e r n i s  c o u r s e l i k e l y i n the s l a g b a t h where heat g e n e r a t i o n i s  of  concentrated  i n the c e n t e r and c o o l i n g on the o u t s i d e .  The e x i s t e n c e of an a n n u l a r  r e g i o n of s t a b i l i t y f o r f l o a t i n g p a r t i c l e s  c o u l d r e s u l t from the  o p p o s i n g motions of c o n v e c t i o n i n the s l a g and the atmosphere  i  (fig.  38). The outward m o t i o n of the g r a p h i t e was r a p i d near the (5 to 10 c m / s e c ) ,  electrode  s h a r p l y d e c r e a s i n g near the g r a p h i t e r i c h a n n u l u s .  The range of v e l o c i t i e s  seemed to v a r y l i t t l e when changing the  type  of power used a l t h o u g h t h i s c o u l d n o t be a s s e s s e d w i t h a p r e c i s i o n b e t t e r than ±50%.  III.17  E l e c t r o d e temperature  profiles  I n g o t s S4 and S5 have been m e l t e d i n o r d e r to e s t a b l i s h g r a d i e n t s i n the e l e c t r o d e  temperature  under e l e c t r o d e p o s i t i v e and n e g a t i v e  conditions. The s t e e p n e s s of the temperature  g r a d i e n t near the s l a g  e x p l a i n e d by the poor t h e r m a l c o n d u c t i v i t y of s t a i n l e s s 1000°C and the s i g n i f i c a n t downward m o t i o n of the  steel  electrode.  is below  - 84  -  The p o s i t i o n o f the s l a g was sensed u s i n g two W/W-26% Re thermoc o u p l e s which a l s o r e c o r d e d the a t m o s p h e r i c temperature above the s l a g . Chromel A l u m e l thermocouples were used to measure the  electrode  temperature at p o i n t s 1 mm below the s u r f a c e and a t the c e n t e r ( f i g .  39).  F i g u r e 40 r e p o r t s t y p i c a l temperature g r a d i e n t s o b t a i n e d d u r i n g these e x p e r i m e n t s .  The r e s u l t s are used to compute the r a t e of  a t m o s p h e r i c o x i d a t i o n o f the e l e c t r o d e .  They a l s o g i v e an i n d i c a t i o n  of the amount o f p r e c i p i t a t e r e d i s s o l u t i o n t o be e x p e c t e d i n the electrode. T i t a n i u m c a r b i d e i s s o l u b l e i n i r o n a t h i g h temperatures a l t h o u g h the d i s s o l u t i o n r a t e i s s l o w below 1200°C; s o l u t i o n treatment t y p i c a l l y 3 h r s at 1150°C (19)  i n an a u s t e n i t i c s t e e l .  The  is, electrode  m e t a l spends o n l y a few seconds above 1200°C as i n d i c a t e d by the temperat u r e p r o f i l e and the v e l o c i t y of e l e c t r o d e t r a v e l (0.5  t o 1 mm/sec).  A s e c t i o n c u t t h r o u g h an e l e c t r o d e t i p has shown t h a t a n g u l a r T i C p r e c i p i t a t e s r e a c h the s u r f a c e of the m o l t e n f i l m on the (321 s t a i n l e s s  steel) without s i g n i f i c a n t d i s s o l u t i o n .  surface  CHAPTER IV DISCUSSION  The  r e s u l t s reported  i n sections  a l o s s o f r e a c t i v e elements has ments, over and  above what we  I I I . 1 1 to I I I . 1 4 i n d i c a t e  been observed, i n a l l of our  that  experi-  would have expected from i n c l u s i o n  removal. In a number of c a s e s , the m a t e r i a l ingot  and As  two  the s l a g c l o s e d w i t h i n  the r e s u l t s by  the  we  the  a narrow margin.  themselves c o n s t i t u t e an answer to the  q u e s t i o n s of s e c t i o n 1.2,  questions regarding  b a l a n c e w r i t t e n between  s h a l l now  t r y to answer the  i n f l u e n c e of the atmosphere, of the  c o m p o s i t i o n , of e l e c t r o c h e m i c a l  reactions  r e a c t i o n s i t e s on mass t r a n s f e r  rates.  and  first other  slag  of the l o c a t i o n of  - 86  IV.1  -  Q u a n t i t a t i v e e v a l u a t i o n of o x i d a t i o n causes  I n the i n t r o d u c t i o n  ( 1 . 4 ) ,  m e n t i o n has been made of the two p o s s i b l e  s o u r c e s of o x i d a n t namely the atmosphere and the  slag.  For the purpose of the f o l l o w i n g d i s c u s s i o n , we s h a l l c o n s i d e r as s i g n i f i c a n t a l o s s of 0 . 1 % of T i o r A l s i n c e t h i s v a l u e i s c o n s i s t e n t w i t h the m e c h a n i c a l p r o p e r t i e s and c h e m i c a l  specifications  of the r e m e l t e d a l l o y s . IV.1.1 Atmospheric oxygen can e n t e r the system i n two w a y s : t h r o u g h the slag-gas  interface  o r by o x i d i z i n g the e l e c t r o d e above the m e l t .  A s t r a i g h t f o r w a r d m a t e r i a l b a l a n c e l e a d s to the v a l u e s r e p o r t e d i n the f o l l o w i n g  Table  table:  64.  Causes f o r l o s s  Melting conditions  To cause 0 . 1 % l o s s of  0 £ from a i r  2 g sec  3.5 m l sec  Argon b l a n k e t 1% o  2 g sec  70 ml sec  Flow of O2 t h r o u g h slag interface  I n g o t diam^ 5 . 3 cm 2 g sec  0.04  of 1  [Ti]  air  of argon  2  Oxide s k i n on round electrode  3 -2 -1 cm cm sec  FeO t h i c k n e s s = 10 diameter  W h i l e a s m a l l f l o w of a i r i s s u f f i c i e n t to cause the l o s s , a s u b s t a n t i a l f l o w of argon i s n e c e s s a r y .  -3  aforementioned  The f l o w of argon  that  - 87  -  we used i n the fumehood was about 80 ml sec  1  although the a c t u a l f l o w  i n t o the mold may have been i n c r e a s e d by t u r b u l e n t m o t i o n s around the electrode. It  seems t h e r e f o r e  t h a t t h e argon b l a n k e t may have c a r r i e d j u s t  enough oxygen t o cause t h e r e p o r t e d l o s s e s  (around 0.3%  [Ti]),  assuming  t h a t the oxygen r e a c t s q u a n t i t a t i v e l y . The r a t e o f a b s o r p t i o n r e q u i r e d i s f a i r l y h i g h . temperatures,  A t s t e e l making  exchange r e a c t i o n s between s l a g and atmosphere  are  c o n t r o l l e d by d i f f u s i o n i n the s l a g , as i s the case i n the open hearth furnace  (23).  Most s t e e l making  processes,  :  however, use  w h i c h a r e complex s i l i c a t e s , where the d i f f u s i o n c o e f f i c i e n t s  are  s e v e r a l o r d e r s o f magnitude l o w e r t h a n i s the case i n h a l i d e s . and Chipman (24)  report d i f f u s i o n c o e f f i c i e n t s  system w h i c h are i n the range 3 x 10 coefficients  —7  t o 10  i n the  —8  10~  2  2  A  Diffusion  i n m o l t e n h a l i d e s are s u b s t a n t i a l l y h i g h e r , a l t h o u g h most B u r e l (15)  cm sec"  1  has found 8.5  x  f o r A l ^ d i f f u s i n g i n CaF ~20% A l ^ a t 1518°C w h i l e  D e l i m a r s k i i and P a v l i n o v (25) c r y o l i t e at 1000°C.  g i v e 1.0  x 10  -5  2 -1 3+ cm sec f o r Fe in  When a p p r o p r i a t e l y combined w i t h the  other  p h y s i c a l p r o p e r t i e s of the system ( k i n e m a t i c v i s c o s i t y . . . ) , h i g h e r v a l u e s w i l l acount f o r the h i g h r a t e o f exchange w i t h atmosphere  Towers  CaO*Si02• ^2°3  2 —1 cm sec .  a v a i l a b l e data concern s e l f - d i f f u s i o n . 5  slags  (see  these the  IV.2.2).  IV.1.2 O x i d a t i o n o f the e l e c t r o d e above the m e l t i s e v i d e n t when the c l o s e d argon cap i s n o t u s e d .  - 88 Because o f t h e i r s m a l l diameter and poor e l e c t r i c a l c o n d u c t i v i t y , s t a i n l e s s s t e e l e l e c t r o d e s a r e u s u a l l y t a r n i s h e d throughout t h e i r length.  I n a l l c a s e s , a c o n t i n u o u s and t h i c k e r l a y e r o f o x i d e o c c u r s  o n l y c l o s e t o the m o l t e n s l a g .  T h i s l a t t e r i s t h i n (< 1 u ) and a d h e r e n t .  Among the m e t a l s s t u d i e d h e r e , i r o n e x h i b i t s the h i g h e s t r a t e pf o x i d a t i o n at h i g h t e m p e r a t u r e , i t i s a l s o the m e t a l w h i c h  experiences  h i g h temperature f o r t h e l o n g e s t t i m e as i s c l e a r l y shown by the e l e c t r o d e temperature g r a d i e n t s ( f i g . 40 and 4 1 ) . Kubashewski and Hopkins (26)  g i v e the f o l l o w i n g e x p r e s s i o n s f o r the  p a r a b o l i c t h i c k e n i n g of the v a r i o u s c o a t s of o x i d e :  FeO: Fe 0.: 3 4 Fe„0 •. 2 3 o  k' P k' P k' P  1.05  x IO"  5.4 x I O "  4  2  x i 0  x i 0  4 0  4 0  '  '  5 0  5 0  °/  °/  R T  R T  cm sec2  cm sec2  1  1  As FeO i s by f a r the f a s t e s t growing o x i d e , the p a r t i a l of oxygen i n the g a s , p r o v i d e d i t s t a y s above 7 mm Hg (0.01  pressure atm.),  has  v e r y l i t t l e i n f l u e n c e of the t o t a l amount of o x i d a t i o n . The r u l i n g e q u a t i o n o f the o x i d a t i o n o f Fe i s t h e r e f o r e  approxi-  mated by  *  6  2  =  r /  T f  600  c -40,500/RT 5.75 x e  .dt. (—) '  dT  . _„ (eq I V . 1)  where T^ i s the. m e t a l temperature at the s l a g - a t m o s p h e r e i n t e r f a c e , T the a b s o l u t e temperature and t ,  the t i m e .  O x i d a t i o n a t low  t e m p e r a t u r e s , below the range o f s t a b i l i t y of FeO, can be n e g l e c t e d .  - 89 -  The r a t e o f temperature i n c r e a s e i s g i v e n by e l e c t r o d e temperat u r e measurements  ( f i g . 41 E l e c t r o d e n e g a t i v e ) .  For the purpose o f c a l c u l a t i o n , the above e q u a t i o n has been s i m p l i f i e d by d i v i d i n g the time of exposure i n t o i n t e r v a l s of 1 second o r about 7.77°K d u r i n g the l a s t s t a g e s of o x i d a t i o n - . „  6 =  ±  n E  0  i - 4 0 , 5 0 0 / R ( 6 0 0 + EATi) 5.75 x e °  (eq  where n i s the number of time i n t e r v a l s between 600°K and  IV.2)  T. f R  For e l e c t r o d e n e g a t i v e c o n d i t i o n s t h i s g i v e s a p o s s i b l e c o a t i n g -3 o f 6 = 1.73 x 10  cm. -3  We have seen above t h a t an o x i d e c o a t i n g o f 10 d i a m e t e r o f the e l e c t r o d e would g i v e a 0.1% [ T i ] l o s s . For the 2.5 cm e l e c t r o d e , 6 -3 s h o u l d t h e r e f o r e be 2.5 10  cm.  E l e c t r o d e o x i d a t i o n w i l l undoubtably  p l a y a s m a l l r o l e i n s u p p l y i n g oxygen t o t h e s l a g when i r o n i s r e m e l t e d . I n the case o f . s t a i n l e s s s t e e l , Maraging 300 and 1409 A l however, the resistance  t o o x i d a t i o n i s much h i g h e r .  The temperature g r a d i e n t i n the  e l e c t r o d e i s a l s o s t e e p e r up to 1 0 0 0 ° C , l i m i t i n g o x i d a t i o n .  ;  I n the case of s t a i n l e s s s t e e l , the p a r a b o l i c law of o x i d a t i o n i s no l o n g e r v a l i d ;  Kubashewski and Hopkins (26)  r e p o r t t h a t the rate; o f  o x i d a t i o n of F e - 1 8 C r ' 8 N i s h o u l d be a t l e a s t 100 times s l o w e r than f o r iron. We have observed t h a t a s t a i n l e s s s t e e l e l e c t r o d e e n t e r s the s l a g at a temperature a p p r o x i m a t e l y 200°C l o w e r t h a n m i l d (fig.  40);  steel  i f i r o n was exposed to t h i s temperature c y c l e , the same , -4 c a l c u l a t i o n s as above would g i v e cS = 2.25 x 10 cm, t h i s l a y e r of  - 90  -  o x i d e would have no measurable e f f e c t on the m e t a l c o m p o s i t i o n (equivalent:  0.01% T i ) .  a hundred times l o w e r  I n the case of s t a i n l e s s  steel,  the e f f e c t  is  still.  Chromium a l u m i n i u m s t e e l s are a l s o o x i d a t i o n r e s i s t a n t temperature and the same c o n c l u s i o n can be drawn:  at h i g h  The o x i d a t i o n pf  e l e c t r o d e p l a y s no s i g n i f i c a n t r o l e i n s u p p l y i n g oxygen to the Maraging s t e e l does n o t have o u t s t a n d i n g o x i d a t i o n  the  system.  resistance.  I n the absence of f u r t h e r d a t a , i t can be assumed t h a t i t s b e h a v i o u r i s b e t t e r than or e q u a l to t h a t of  iron.  IV.1.3 Electrolysis.  S i n c e d i r e c t c u r r e n t has been used f o r many of  the  r e m e l t e d i n g o t s , i t i s l i k e l y t h a t e l e c t r o l y s i s w i l l p l a y a major p a r t i n the  o x i d a t i v e processes  i n t h e s e cases  (27).  When u s i n g D . C . c u r r e n t , t y p i c a l m e l t i n g c o n d i t i o n s can be d e s c r i b e d as f o l l o w s : Diameter of i n g o t :  5 . 3 cm  Melting rate:  2.5 g sec  Current:  700 Amp. =7.10  -3  x  gram-equivalent  sec  -1  Assuming t h a t the above c u r r e n t i s i n v o l v e d i n a two e l e c t r o n r e a c t i o n , about.0.17 each s e c o n d .  g of m e t a l o f atomic w e i g h t 48 c o u l d r e a c t  This represents  6.7% of the t o t a l melt  at  rate.  _ 1409 A l , w i t h 12.86% Cr and 3.74% A l s h o u l d have an o x i d a t i o n r e s i s t a n c e e q u i v a l e n t to s t a i n l e s s s t e e l as i t forms Cr^)^ on the surface (26).  - 91 It  s h o u l d of course be p o i n t e d out t h a t m e t a l w h i c h has been  subjected  to c a t h o d i c r e a c t i o n s w h i l s t m e l t i n g on the e l e c t r o d e  be s u b j e c t e d  will  to the i n v e r s e a n o d i c r e a c t i o n s when i t reaches the  ingot.  N e v e r t h e l e s s , a d i f f e r e n c e i n o v e r a l l c u r r e n t e f f i c i e n c y of t h e s e s t e p s of 1.5% would be s u f f i c i e n t to cause the 0.1% l o s s t h a t we, mentioned. Big  ;  u n i t s would be f a v o u r e d i n t h i s r e s p e c t  as they r e q u i r e a  lower c u r r e n t d e n s i t y f o r the same f l o w of m e t a l and work u s u a l l y w i t h d i a m e t e r r a t i o s between the i n g o t and the e l e c t r o d e w h i c h  are  c l o s e r to one. The c o n d i t i o n s f o r mass t r a n s f e r of t i t a n i u m and a l u m i n i u m i n the m e t a l and the s l a g w i l l p l a y an i m p o r t a n t r o l e i n the e f f i c i e n c y of o x i d a t i o n and r e d u c t i o n .  It  relative  i s apparent t h a t  the  e f f i c i e n c y of the r e d u c t i o n r e a c t i o n of t i t a n i u m or aluminium w i l l low when these elements a r e at a low c o n c e n t r a t i o n i n the s l a g ,  be  the  l i m i t i n g c u r r e n t f o r d i f f u s i o n towards the cathode b e i n g the c o n t r o l l i n g factor i n this  case.  The presence of i r o n and chromium o x i d e s i n the s l a g s of 321 and Maraging 300 i s a l s o an i n d i c a t i o n t h a t the l i m i t i n g c u r r e n t  S>S. for  the combined d i f f u s i o n of oxygen i n t o , and of r e a c t i v e elements out of the m e t a l has been  reached.  IV.1.4 Thermochemical d a t a .  Mass t r a n s f e r  c a l c u l a t i o n s r e q u i r e a knowledge  of the c h e m i c a l p o t e n t i a l of the r e a c t i n g An attempt has been made (Appendix m )  species. to measure the a c t i v i t y of  -  92 -  t i t a n i u m o x i d e s i n the C a O ' C a F  2  system.  The s h o r t c o m i n g o f  r e s u l t s are e x p l a i n e d i n the same appendix I I I ,  namely t h a t  these the  measurements have been made on a carbon s a t u r a t e d system where the oxygen p a r t i a l p r e s s u r e was imposed by the C/CO e q u i l i b r i u m . CaF  base s l a g s u s u a l l y c o n t a i n a s u b s t a n t i a l amount o f CaO .  2  ;  The  p r i n c i p a l r e a s o n f o r t h i s i s the h y d r o l y s i s o f CaF^ w h i c h o c c u r s r e a d i l y a c c o r d i n g to the r e a c t i o n (3) :  CaF  +  2  H 0 (g)  •  2  CaO  +  2HF (g)  D i s c r e p a n c i e s i n the r e p o r t e d f r e e z i n g p o i n t of C a F p r o b a b l y due to the presence of CaO i n the m e l t . c r y o s c o p i c s t u d i e s of K o j i m a and Masson ( 1 4 ) , pure C a F  2  2  are  also  A c c o r d i n g to  the  the m e l t i n g p o i n t of  i s depressed by ^ 2.3°C f o r every 1% CaO.  Since T i Q  2  and A l 0^ have a s t r o n g a f f i n i t y f o r CaO, i t has been  considered reasonable  t o c a l c u l a t e the a c t i v i t y c o e f f i c i e n t s o f  these  o x i d e s by assuming t h a t they r e a c t t o form the complex o x i d e s CaTiO^ and C a A ^ O ^ . CaF . 2  These o x i d e s are then assumed to d i s s o l v e i d e a l l y i n  A s i m i l a r b e h a v i o u r has been observed w i t h s i l i c a t e s CaF ~Al 0 2  2  3  (15)  is a single eutectic  (28).  system w i t h no s o l i d  s o l u b i l i t y , w h i l e C a F ^ T i O ^ C a O has no i m m i s c i b i l i t y r e a c t i o n a c c o r d i n g t o Evseeb ( 2 9 ) .  CaF ~Ti0 2  2  (30)  has a m i s c i b i l i t y gap c o v e r i n g  a p p r o x i m a t e l y 5 to 55% T i 0 « 2  We have t h e r e f o r e chosen the f o l l o w i n g b a s i s f o r a c t i v i t y c a l c u l a t i o n s i n the s l a g a t 1800°K.  Standard f r e e e n t h a l p i e s  f o r m a t i o n (AF°) a r e taken f o r E l l i o t t and G l e i s e r  (31).  of  - 93 -  S l a g base  at 1800°K  a t 1800°K at  CaF  0.56 (Appendix I I I )  2  CaF„ + CaTiO,. 1 3 CaF + C a A l 0 . 2 2 4 0  o  • CaTiOj AF° = - 2 2 , 1 5 0 c a l / m >• C a T i O  T°K  Raonltian act. c o e f . (32) Y  o [A1]  1923  1  >- CaAl„0, 2 4 AF° = - 1 1 , 9 1 0 c a l / m • CaAl„0, 2 4  3  1  the f o l l o w i n g d a t a a r e a v a i l a b l e ( 3 1 , 3 2 ) .  I n t e r a c t i o n parameters  e  Ti 0  1  £  Ti Ti  e  Al O  e  Al A l  Henryan a c t . c o e f . (321 S . S . ) (32) .. l  o  g  C r , .. Ni Ti Ni £  f  £  +  l  o  g  f  0.020  1823 1873  o ^[Ti]  1  n  I n the m e t a l l i c p h a s e s ,  1800°K  0.063  0.033 0.060  -0.187  0.034  -0.94  0.048  -0.70  - 94 -  IV.1.5 The e q u i l i b r i u m o f d e o x i d a t i o n by t i t a n i u m . Stainless steel,  I n i r o n and 321  t h i s e q u i l i b r i u m can be s t u d i e d i n t h e t h r e e  following cases: 1.  The p r o d u c t o f d e o x i d a t i o n i s T i O ^ ( h y p o t h e t i c a l ) .  2.  The p r o d u c t of d e o x i d a t i o n i s T i ^ O ^ .  observed when homogeneous stainless  steel,  This i s the oxidei  d e o x i d a t i o n of s t e e l i s p e r f o r m e d .  In  T i ^ O ^ w i l l form i n p r e f e r e n c e t o chromium o x i d e  (Cr^O^) when t h e t i t a n i u m c o n t e n t i s more than 0 . 1 to 0.2% ( 3 2 ) . 3.  The p r o d u c t o f d e o x i d a t i o n i s C a T i O ^ , one of t h e major  slag  components used i n t h i s work. The f o l l o w i n g v a l u e s a r e r e p o r t e d by E l l i o t t and G l e i s e r f o r the r e a c t i o n s  of i n t e r e s t h e r e :  ( T i i s a s o l i d at  (31)  1800°K).  F o r m a t i o n of o x i d e s Ti  +  0  o- T i 0  2Ti  +  3/20  CaO  +  Ti0  2  AF° = - 2 2 3 , 5 0 0 + 41.55 T c a l .  2  • Ti 0  2  2  y CaO-Ti0  2  AF° = - 3 5 4 , 0 0 0 + 58.36 T c a l .  3  2  AF° = - 2 2 , 1 5 0 c a l . (1800 °K)  D i s s o l u t i o n of T i i n i r o n Ti(solid)  y Ti(liquid)  Ti(liquid)  >• [ T i ] (% d i s s o l v e d )  A F = AH - TAS = 3,700 - 1.91 T C a l . AF = RT I n a ^ j f  f  " where y°  i s 0.016 a t 1800°K  (see above)  For t h e whole r e a c t i o n T i ( s o l i d ) —> [ T i ]  R T  l  f  n  * T i [ l % Ti] N  A F = -14,753 - 8.83 T c a l . d  AF° = - 1 1 , 0 5 3 - 10.74 T. c a l .  D i s s o l u t i o n o f 0£ i n i r o n l/20  2  •  [0]  AF° = - 2 8 , 0 0 0 - 0.69 T c a l .  - 95 Overall deoxidation reactions:  Reaction constants  can be c a l c u l a t e d  d i r e c t l y using  AF° = -RT l n K  P  [ T i ] + 2[0] — T i O . : 2  AF° = AF° - AF° - 2AF° 6 1 4 5 K , = 1.96 x 1 0 a t 1800°K 6 7  2 [ T i ] + 3[0]  >-  T i  2  °3  :  A F  7 K  [ T i ] + 2[0] + CaO  2 "  A F  ?  = 1.76 x 1 0  • CaTiO.: j  AF° = AF° o g  3 A F  5  1 2  + AF°  3  b  K  These r e a c t i o n c o n s t a n t s  2 A F  4 ~  =  = 9.22 x 1 0  9  a r e w r i t t e n f o r d i l u t e s o l u t i o n s i n the  m e t a l phase and they must s t i l l be c o r r e c t e d f o r i n t e r a c t i o n coefficients.  K  = 6  where l o g f  and l o g f  F o r example:  Q  T i  (TiO ) f [0] 2  2  f  T i  [Ti]  = e ° x (%[0j) + e j  1  (%[Ti])  = e ^ x (%[Ti]) + e ° . (% [0]) + l o g ff in stainless  The e q u i l i b r i u m c o n c e n t r a t i o n s and s t a i n l e s s  ±  + log  f^  steel.  f o r t i t a n i u m and oxygen i n i r o n  s t e e l at 1800°K a r e r e p o r t e d i n f i g u r e s 43 and 42  -  96  -  r e s p e c t i v e l y w h i l e f i g u r e 44, shows the same e q u i l i b r i u m i n terms oxygen p a r t i a l  pressures.  An i n t e r e s t i n g f e a t u r e of t h e s e graphs i s to demonstrate greater  of  effectiveness  containing slag.  of d e o x i d a t i o n i n the presence  The g r e a t e r  the  of the CaO-  s t a b i l i t y of CaTiO^ a l s o s t a b i l i s e s  the  4+ s t a t e of o x i d a t i o n T i  i n the s l a g .  Ranges of c o m p o s i t i o n f o r r e m e l t e d i n g o t s are r e p r e s e n t e d on the same g r a p h s .  The f i g u r e s p l o t t e d are m a t r i x t i t a n i u m and t o t a l oxygen.  F i g . 43 r e a d i l y shows t h a t the excess c o n c e n t r a t i o n of oxygen i n stainless  s t e e l i s g r e a t e r when the e l e c t r o d e  i s negative.  Alternating  c u r r e n t on the o t h e r hand produces a l e v e l of d e o x i d a t i o n c l o s e to ; t h a t i d e a l l y produced by the  slag.  The a d d i t i o n of aluminium does not seem t o a f f e c t the oxygen c o n c e n t r a t i o n when the e l e c t r o d e i s n e g a t i v e w h i l e the e f f e c t  is  important w i t h reverse p o l a r i t y . The o p p o s i t e e f f e c t of m e l t p o l a r i t y i s seen w i t h Mar 300. 42 shows zones of c o n c e n t r a t i o n where the oxygen c o n t e n t  Fig.  depends sharpy  upon the t i t a n i u m c o n c e n t r a t i o n . The above c o n s i d e r a t i o n s have been developed f o r an e q u i l i b r i u m temperature of 1800°K.  T h i s temperature has been a r b i t r a r i l y chosen  as b e i n g s l i g h t l y above the m e l t i n g p o i n t of most a l l o y - s t e e l s .  It  s h o u l d c o r r e s p o n d a p p r o x i m a t e l y to the temperature of the m e t a l on the electrode  t i p and near the s o l i d i f i c a t i o n i n t e r f a c e where d e o x i d a t i o n  p r o d u c t s can s t i l l be removed by f l o t a t i o n .  I n the h i g h e r  temperature  r e g i o n s the t r e n d w i l l be f o r a l e s s e f f e c t i v e d e o x i d a t i o n e q u i l i b r i u m (see  IV.2.1).  ..  - 97  -  IV.1.6 The e q u i l i b r i u m of d e o x i d a t i o n w i t h aluminium can be c a l c u l a t e d  in  a s i m i l a r f a s h i o n , the p r o d u c t of d e o x i d a t i o n b e i n g 1.  2.  A  2®3  L  (homogeneous)  CaO'Al 0  Elliott  (slag)  (31)  reports  the f o l l o w i n g v a l u e s f o r the r e a c t i o n s  of  interest: F o r m a t i o n of  oxides  2A1  +  3/20  • A1 0  2  2  o  AF  3  = -262,900 c a l .  (1800°K)  O  CaO  +  A1 0 2  •  3  Ca0'Al 0 2  3  AF = -11,910 c a l . 1Q  (1800°K)  D i s s o l u t i o n of A l i n i r o n For a s t a n d a r d s t a t e of pure l i q u i d a l u m i n i u m , the coefficient  for  [ A l ] i s g i v e n as a f u n c t i o n of the atomic  log Y A l -> Deoxidation  activity  = -1.20  + 2.25X  (1873°K);  A1  [ A l ] (% d i s s o l v e d )  AF  X  <  A 1  fraction:.  0.2  = -23,750 cal/m  (1800°K)  reactions  2[A1]  +  y A1 0  3[0]  2  3  AF K  2[A1]  +  3[0]  +  CaO  12  g  =  3  '  7 7  5  X  1 0 + 1 5  >- C a O - A l ^ AF K  The e q u i l i b r i u m c o n c e n t r a t i o n s  = A F - 3 A F - 2AF  1 2  O  O  O  = AF + AF 13 12 10 1 3  = 2.46  x  10  1 8  f o r aluminium and oxygen i n i r o n  can a l s o be a p p l i e d to 1409 A l i n the absence of d a t a on the of chromium.  interaction  - 98 Al It  s h o u l d be noted t h a t the r e p o r t e d i n t e r a c t i o n c o e f f i c i e n t  i s o n l y v a l i d f o r v e r y d i l u t e s o l u t i o n s as i t l e a d s to the result  t h a t the oxygen c o n c e n t r a t i o n  [0]  e^  inconsistent  i n c r e a s e s w i t h the a l u m i n i u m  c o n c e n t r a t i o n when t h e r e i s more t h a n a few t e n t h of one p e r c e n t [Al] present. The r e s u l t of the c a l c u l a t i o n l e a d s t o e x t r e m e l y low v a l u e s the oxygen c o n c e n t r a t i o n ,  b e i n g l e s s than 1 ppm when [ A l ] i s  for  greater  than 0.01%. The oxygen p a r t i a l p r e s s u r e imposed by a l u m i n i u m i s  correspondingly  lower than t h a t imposed by t i t a n i u m as an e x a m i n a t i o n of the enthalpies  of the f o l l o w i n g r e a c t i o n s w i l l 4/3[Al]  +  Ti0  •  2  O  A F ^ = -25,800 c a l . IC. = 14  and  4/3[Al]  [Ti]  +  free  demonstrate: 2/3Al 0 2  3  (1800°K)  1200  '+  CaTi0  3  • [Ti]  +  2/3CaAl 0 2  4  +  l/3CaO  O  AF K  1 5  = -11,600 c a l .  15 =  6 1  T i t a n i u m i n the s l a g s h o u l d t h e r e f o r e  be reduced by aluminium  d i s s o l v e d i n the m e t a l even when the t i t a n i u m c o n c e n t r a t i o n exceeds  that  of aluminium by more than an o r d e r of magnitude. H o l f e r t and a l .  (33)  r e p o r t a 10% l o s s of aluminium (from 0.87%)  when r e m e l t i n g DIN 35CrA16 s t e e l w i t h a s l a g c o m p o s i t i o n  - 99 -  70CaF  2  - 30A1 0 2  3  +  10% T i C y  The l o s s i s i n c r e a s e d to 15% when  u s i n g 15% TiO„.  IV.1.7 S t a t e o f o x i d a t i o n of t i t a n i u m i n the s l a g . i n ( [ I I . 10) show t h a t CaO*TiG"  The r e s u l t s  reported  can s t i l l be d e t e c t e d i n a D e b y e - S h e r r e r  2  p a t t e r n of the s l a g when a l u m i n i u m i s c o n t i n u o u s l y added to i t the m e l t .  during  Aluminium imposes a p a r t i a l p r e s s u r e of oxygen s e v e r a l  o r d e r s of magnitude lower than the s t e e l r e m e l t e d i n t h i s c a s e ,  thus  i n d i c a t i n g a s t r o n g n e g a t i v e d e v i a t i o n of the a c t i v i t y c o e f f i c i e n t s  of  t i t a n i u m oxides i n d i l u t e s o l u t i o n . We have not c o n s i d e r e d the o x i d a t i o n s t a t e T i r e s u l t s of R o s s o k h i n and coworkers ( 3 4 ) , p o t e n t i a l of the system T i / T i  2+  2+  following  the  who r e p o r t t h a t the redox  i s n e g a t i v e w i t h r e s p e c t to  Ti/Ti  3+  3+ above 1100°K i n KC1 m e l t s .  Ti  i s thus the s t a b l e s p e c i e s i n  e q u i l i b r i u m w i t h m e t a l l i c t i t a n i u m (see  a l s o Appendix I I I ) .  The thermodynamic d a t a f o r the f o r m a t i o n of complex o x i d e s . CaO'xTi  2  n  3  are not a v a i l a b l e .  I f we assume t h a t t h e i r s t a b i l i t y i s  the  same as C a O ' T i O , , , the e f f i c i e n c y of a d e o x i d a t i o n r e a c t i o n l e a d i n g to complex o x i d e s of T i 0  would be even h i g h e r than t h a t of CaO'TiO .  - 100  IV.2  -  P h y s i c a l d e s c r i p t i o n of the  process  IV.2.1 Temperature g r a d i e n t s and n a t u r a l c o n v e c t i o n i n the s l a g . The c o n v e c t i v e f l o w i n the s l a g has been o b s e r v e d and a p a t t e r n is  suggested on f i g . 38 w h i c h can be j u s t i f i e d by u s i n g the t h e r m a l  c o n d i t i o n s e x i s t i n g i n the s l a g ( S e c t i o n  III.16).  Campbell (35) mentions the p o s s i b i l i t y t h a t a L o r e n t z f o r c e i n the f l u i d ,  analogous t o the " p i n c h e f f e c t " w i l l  f o r c e the  slag i n a  downward d i r e c t i o n i n the r e g i o n below the e l e c t r o d e , w h i c h would oppose t h e r m a l c o n v e c t i o n .  This electromagnetic  f o r c e i s due t o  the  spread o f the c u r r e n t l i n e s between two e l e c t r o d e s o f u n e q u a l s i z e s . The geometry of the p r e s e n t e l e c t r o s l a g system l e a d s t o a r a p i d spread of the c u r r e n t l i n e s around the e l e c t r o d e t i p , a c c o r d i n g t o c a l c u l a t i o n s of J o s h i  (36),  the  l i m i t i n g the p o s s i b l e e f f e c t t o a s h a l l o w  c r o s s s e c t i o n around the t i p .  The p r o p o r t i o n of the c u r r e n t b e i n g  t r a n s f e r r e d t o the mold remains s m a l l when the mold i s i n s u l a t e d as is  the case here  (37).  In an e l e c t r o s l a g u n i t u s i n g l a r g e e l e c t r o d e s , would be m o d i f i e d by s k i n e f f e c t s  any such c o n c l u s i o n  ( a l t e r n a t i n g current) which w i l l  m o d i f y c u r r e n t l i n e s i n the r e g i o n o f the e l e c t r o d e  tip.  An e l e c t r o m a g n e t i c s t i r r i n g p a t t e r n would a l s o i n c l u d e e f f e c t s a r i s i n g from s t r a y m a g n e t i c f i e l d s produced by the power l e a d s P a n i n and coworkers (38)  have i n v e s t i g a t e d the f l o w of m e t a l i n  ESR u s i n g a r a d i o g r a p h i c method. free f a l l i n g , drop i s  etc...  They r e p o r t t h a t the drop seems  i m p l y i n g t h a t the e x t e n t of downward c u r r e n t s near  limited.  the  - 101 -  The temperature e x i s t i n g a t the v a r i o u s r e a c t i o n s i t e s i s an i m p o r t a n t v a r i a b l e a f f e c t i n g e q u i l i b r i u m and d i f f u s i o n c a l c u l a t i o n s . At the e l e c t r o d e s u r f a c e , t h e temperature can be e s t i m a t e d from the heat  f l u x being t r a n s f e r r e d along the e l e c t r o d e .  Immediately  above the s l a g we see from f i g . 40 t h a t the temperature g r a d i e n t i s a p p r o x i m a t e l y 200°C cm \ (fig.  41)  S i m i l a r experiments on m i l d s t e e l  l e a d t o a p p r o x i m a t e l y the same v a l u e .  electrodes  I n the l i q u i d  film  the temperature g r a d i e n t w i l l then depend upon the t h e r m a l c h a r a c t e r i s t i c s of the m o l t e n m e t a l f l o w .  A c c o r d i n g to the c a l c u l a t i o n s  of s e c t i o n I V . 2 . 4 , we can n e g l e c t the c o n t r i b u t i o n of c o n v e c t i o n p e r p e n d i c u l a r t o the i n t e r f a c e and o n l y take i n t o account the t h e r m a l c o n d u c t i v i t y o f the l i q u i d . is  u s u a l l y 1/2  can  Schuhmann (39)  r e p o r t s t h a t t h i s parameter  to 2/3 of the v a l u e i n the s o l i d f o r p u r e m e t a l s .  then e s t i m a t e the temperature d i f f e r e n c e a c r o s s a 0.015  We  cm l i q u i d  f i l m ( I V . 2 . 4 ) t o be o n l y 4 . 5 to 6°C (300 t o 400°C c m " ) . 1  H a v i n g chosen an e q u i l i b r i u m temperature of 1800°K w i l l  then be  l e g i t i m a t e when pure i r o n i s r e m e l t e d (m.p. 1808°K) b u t w i l l l e a d t o some e r r o r w i t h h i g h a l l o y s t e e l s .  Maraging s t e e l o f a s l i g h t l y ;  d i f f e r e n t c o m p o s i t i o n (17% N i , 10% C o , 4.6% Mo a p p r o x i m . ) i s r e p o r t e d to have a s o l i d u s temperature (T^) of 1705°K and a l i q u i d u s (T ) 2  of 1727°K ( 4 0 ) .  The m e l t i n g range of 321 S . S .  temperature  i s 1672 t o 1700°K  (41). The temperature a t the i n g o t - s l a g i n t e r f a c e depends upon the melting conditions.  Sun (22)  has shown t h a t the c a l c u l a t i o n s of p o o l  p r o f i l e s c o u l d be a c c u r a t e l y made by assuming a c o n s t a n t a c r o s s the s l a g - i n g o t i n t e r f a c e .  temperature  S i n c e h i s experiments were conducted  - 102 w i t h H a s t e l l o y X (T  ±  -  = 1533°K; T  2  = 1 6 2 7 ° K ) , he f i n d s an average m e t a l  p o o l temperature w h i c h i s 31°K above  f o r a 14 cm d i a m e t e r i n g o t .  The i n t e r f a c e temperature ( i n g o t - s l a g cap) w h i c h b e s t f i t t e d the c a l c u l a t i o n s t o the e x p e r i m e n t a l p o o l depth was 1772°K, o r 145°K above T^ f o r a wide v a r i a t i o n i n the m e l t r a t e s ( A . C . p o w e r ) . If  t h i s r a t i o of temperatures i s a l s o v a l i d f o r our e x p e r i m e n t s ,  the s l a g - i n g o t i n t e r f a c e temperature would be above 1800°K i n a l l o f our e x p e r i m e n t s , v a r y i n g from 1840°K w i t h 321 S . S .  t o 1950°K w i t h  iron. The s l a g temperature i s s i g n i f i c a n t l y h i g h e r .  U s i n g a W.3% Re -  W.25%Re thermocouple l i n e d i n a boron n i t r i d e s h e a t h , s l a g temperat u r e s o f 1950 t o 2050°K have been measured i n the U . B . C . u n i t when r e m e l t i n g i r o n (42).  I n h i s e x p e r i m e n t s w i t h H a s t e l l o y X ; Sun  (22)  r e p o r t s an average temperature o f 1911°K i n the s l a g c a p . The problem i s f u r t h e r c o m p l i c a t e d by the f a c t t h a t an i n c r e a s e i n the temperature g r a d i e n t p r o b a b l y o c c u r s a t the s l a g - m e t a l i n t e r faces.  T h i s s h o u l d be e s p e c i a l l y t r u e a t the p o s i t i v e s i d e of a  D . C . p r o c e s s , w h i c h i s a l s o the s i t e of the o x i d a t i o n r e a c t i o n s .  IV.2.2 D i f f u s i o n at the s l a g - a t m o s p h e r e  interface  I f o x i d a t i o n o f the e l e c t r o d e can be n e g l e c t e d as a means of s u p p l y i n g oxygen to the system ( I V . 1 . 2 ) ,  the r a t e of a b s o r p t i o n by the  s l a g can be approached by c o n s i d e r i n g t h a t i t i s dependent upon the limiting diffusion rate, systems Fe  -H-  /Fe  t o and from the s u r f a c e , o f i o n s i n the  | | | 3+ and T i / T i  4+ .  The l i k e l y r e a c t i o n s i n v o l v e complex  - 103  -  i o n s of the s p e c i e s h a v i n g the h i g h e s t o x i d a t i o n s t a t e , such as Ti0„  and F e 0 ~ : o  3+ 2Ti  +  50  2Fe  +  30  + l/20  2  =  2Ti0 "  + l/20  2  =  2Fe0 "  3  2  The r e s i d e n c e time o f a volume element o f s l a g a t t h e s l a g atmosphere i n t e r f a c e w i l l depend upon the v e l o c i t y a t t h e s u r f a c e as observed i n I I I . 1 6 . Assuming t h a t the f l o w i s l a m i n a r and r a d i a l over most of  the  s u r f a c e , the outward v e l o c i t y v ^ i s o n l y a f u n c t i o n of r i n the c y l i n d r i c a l c o o r d i n a t e system r e p r e s e n t e d on f i g . 45.  The c o n s e r v a -  t i o n of f l o w t h r o u g h any c y l i n d r i c a l s u r f a c e ( r = c o n s t a n t ) near  the  free surface requires  2irrv  where v  r  r  = k  (k = c o n s t a n t )  = 10 cm/sec f o r r = 1.27  cm as measured i n I I I . 1 6 .  2 -1 k = 79.8 cm sec The o t h e r assumptions used can be t r a n s l a t e d i n t o  v the  z  = 0  interface)  (near the s u r f a c e as t h e r e i s no s l a g f l o w t h r o u g h  - 104  -  The time of exposure to the atmosphere of a volume element slag  of  is  t  5l  dr  R  r  w i t h R^. r a d i u s of the mold - 2 . 8 cm R  r a d i u s of the e l e c t r o d e = 1.27  t  =  > 2 , r f z— k R  , dr =  : k  cm.  R 2 )  . _ - 0.25 n  second  U s i n g a semi i n f i n i t e d i f f u s i o n model f o r the f l o w of o x i d i z e d i o n s from the i n t e r f a c e  i s c o n s i s t e n t w i t h the above mentioned  assumptions f o r the s l a g f l o w .  F u r t h e r m o r e , complex o x i d i z e d i o n s  are  a l s o l i k e l y t o have the l o w e r d i f f u s i o n c o e f f i c i e n t s when compared to „ ++ , 3+ Fe and T. . m  l  The problem can then be t r e a t e d i n the same manner as d i f f u s i o n i n a s e m i - i n f i n i t e medium where the s u r f a c e c o m p o s i t i o n i s h e l d constant,  the s o l u t i o n ( c o m p o s i t i o n p r o f i l e ) b e i n g  C(z,t) - C  = (C. - C ) [ 1 - e r f ( 1  C:  °  Z  )]  irw  (71):  (eq  IV.3)  c o n c e n t r a t i o n of d i f f u s i n g s p e c i e s  C : same i n the b u l k of the s l a g o &  G\: same a t the i n t e r f a c e  ( c o r r e s p o n d i n g to oxygen s a t u r a t i o n ) ;  D:  of same  diffusion coefficient  - 105 The mass f l u x  t h r o u g h a u n i t a r e a a t time t i s g i v e n by  n i£ ° ^  dt "  -  I ! -0  -  "  z  (  (r  C  M  i "  C  o  )  /  D  The t o t a l f l u x per u n i t time and over the whole a r e a S,  f- / t  JS=  e  e  o  (C. - c ) / ° O  1  TT  t"  1 / 2  being,  dt  or  JS = 2S (C. - C ) / ^ — X  O  (eq  IV.4)  ^  7ft  To a p p l y t h i s e q u a t i o n to the p r e s e n t  s y s t e m , i t i s u s e f u l to  c o n s i d e r the s i m p l e case of a s l a g c o n t a i n i n g no T i O ^ o r "FeO" as a major c o n s t i t u e n t . S6,  I n pure CaF^ s l a g s , used f o r r e m e l t i n g  ingots  S9, S16, M l and M5, we observed heavy element b u i l d up of 1% to  3% ( F e , T i and C r ) .  It i s therefore  u n l i k e l y t h a t C. - C o  1  _3 exceed 1% i n the s l a g , c o r r e s p o n d i n g to 0.026 g cm  would  in a liquid  of  _3 d e n s i t y 2.6 g cm (74). -3 -3 The observed r a t e of o x i d a t i o n i s 3 x 10 x 2.3 - 6 . 9 x 1 0 g s i n i n g o t M l , l e a d i n g f o r the t r i v a l e n t o x i d a t i o n of the d i f f u s i n g species  to 4DS (C. - C ) i o Tr(JSr 2  t  = 6  2  „ ^ 0.6  r  sec  2 2 i n the 58 mm mold where S = T T ( R ^ - R ) - 19.8  2 cm .  D has been chosen e q u a l to the d i f f u s i o n c o e f f i c i e n t i n C a F 2 A l 2 0 m e l t s r e p o r t e d by B u r e l , i . e . _  3  8.5 x 10  -5  f o r Al^O^  2 -1 cm sec  (15).  - 106 The main causes o f e r r o r i n t h i s c a l c u l a t i o n a r e i n t h e v a l u e s chosen f o r D and C. - C . 1  t  g  Since the value p r e v i o u s l y c a l c u l a t e d f o r f  O  j  a l s o depends upon t h e p r e c i s i o n on v ^ (+50%), i t c a n be s a i d  the two v a l u e s r e p o r t e d f o r t  are i n r e l a t i v e  that  agreement.  The i n f l u e n c e o f temperature on D i s a l s o w o r t h b e i n g n o t e d . A v a i l a b l e d a t a concern m a i n l y d i f f u s i o n i n m o l t e n c h l o r i d e s where the a c t i v a t i o n energy f o r t h e s e l f d i f f u s i o n o f the c a t i o n s i s u s u a l l y 7 to l l k c a l / i o n - g r a m . self  I n N a F , G r j o t h e i m and Zuca (43) r e p o r t t h e  diffusion coefficient  between 1022 and 1132°C.  o f N a t o be D = 3 . 0 8 x 10 +  3  e ,700/RT 8  An a c t i v a t i o n energy o f 9 k c a l p e r i o n - g r a m  would i n c r e a s e d i f f u s i o n c o e f f i c i e n t s  by a f a c t o r o f 1.27 between  1800 and 2000°K. A major assumption of t h e above c a l c u l a t i o n i s o f course the f a c t t h a t t h e f l o w s h o u l d be l a m i n a r near the s u r f a c e . A t u r b u l e n t model might be p r e f e r r e d , n o t because i t i s more v a l i d i n o u r case b u t because i t would remain a p p l i c a b l e t o l a r g e r size units.  We s h o u l d t h e n d e c i d e whether the p e n e t r a t i o n  the f i l m (45) t h e o r y f o r mass t r a n s f e r Toor and M a r c h e l l o (46) suggest  (44) o r  i s more adapted t o o u r c a s e .  t h a t the p e n e t r a t i o n  theory  holds f o r e  D  where L i s the d e p t h over w h i c h the f l o w i n an eddy r e a c h i n g the s u r f a c e may be c o n s i d e r e d t o be l a m i n a r and D i s t h e d i f f u s i o n coefficient.  - 107  -  The p e n e t r a t i o n t h e o r y l e a d s a l s o t o e q u a t i o n ( I V . 4 ) .  Our  assumption of a s i n g l e eddy i n the whole system i s t h e r e f o r e w i t h the p e n e t r a t i o n t h e o r y .  consistent  As f o r the d e p t h of p e n e t r a t i o n , L , we  haye:  t  e  x D - 0.25  x 8.5 x 1 0 ~  5  - 2.1 x 10~  5  cm  2  Hence, L s h o u l d t h e r e f o r e be s i g n i f i c a n t l y g r e a t e r 10~ ) 5  1 / 2  = 4.6 x 10~  3  than ( 2 . 1  x  cm w h i c h i s v e r y p r o b a b l y the case h e r e .  I n d i r e c t c u r r e n t o p e r a t i o n , a n o t h e r approach to the problem of o x i d a t i o n of the s l a g i s p o s s i b l e , namely the r e d u c t i o n and e v a p o r a t i o n of c a l c i u m .  R e d u c t i o n of m e t a l l i c i o n s at the cathode does not ,  e x c l u s i v e l y i n v o l v e the c a t i o n s produced by the r e m e l t e d m e t a l but I |  a l s o the major c a t i o n s of the s l a g .  When Ca  i s the o n l y major  c a t i o n i n the s l a g , i t w i l l be reduced a l o n g w i t h the o t h e r s the l i m i t i n g d i f f u s i o n c u r r e n t of i r o n , t i t a n i u m e t c . . Calcium metal w i l l  concentrate  reached.  i n the s l a g , as Ca and CaF^ are  c o m p l e t e l y m i s c i b l e above 1400°C (47) most f e r r o u s a l l o y s  is  because  and Ca i s n e a r l y i n s o l u b l e i n  (48).  The vapour p r e s s u r e of pure c a l c i u m i s one atmosphere a t w i t h a heat o f v a p o u r i z a t i o n AH^ = 36,390 c a l gram-atom can e x t r a p o l a t e  to a t y p i c a l s l a g temperature of 2000°K:  x  1756°K  (49). p  We =  18.7  atmosphere o r , i f Ca i s i n d i l u t e s o l u t i o n , one atmosphere f o r an a c t i v i t y of  0.053.  We can then p o s t u l a t e t h a t CaO i s a p o s s i b l e s u p p l y of oxygen f o r the o x i d a t i o n of t i t a n i u m i n d i r e c t c u r r e n t o p e r a t i o n by the f o l l o w i n g mechanism.  CaO i s e l e c t r o l y s e d i n t o i t s c o n s t i t u e n t s ,  the  - 108  -  oxygen r e a c t i n g w i t h the m e t a l w h i l e the c a l c i u m produced a t cathode i s c a r r i e d by the s l a g to the s u r f a c e . and l e a v e s the system or a l t e r n a t e l y , atmosphere and condenses  There, i t  evaporates,  the  evaporates  o x i d i z e s i n the  i n the s l a g as CaO.  A q u a n t i t a t i v e c a l c u l a t i o n o f the mechanism would r e q u i r e least  at  the knowledge of the c o n c e n t r a t i o n of c a l c i u m i n the s l a g . , We  have not been a b l e to d e t e c t c a l c i u m m e t a l i n the s o l i d s l a g , but t h i s does not p r e c l u d e the p o s s i b i l i t y t h a t i t e x i s t s ture during e l e c t r o l y s i s .  X-ray patterns  t a k e n on the  a t h i g h temperacondensates  c o v e r i n g the gas cap and the e l e c t r o d e r e v e a l the presence of CaF2, i r o n o x i d e (magnetite)  or a l u m i n i u m o x i d e , w h i l e the presence  of complex  o x i d e s such as 2Ca0'Fe0 i s p r o b a b l e .  IV.2.3 D i f f u s i o n i n an e l e c t r i c  field  The s i t u a t i o n p r e v a i l i n g i n the s l a g at the m e t a l p o o l presents  g e o m e t r i c a l s i m i l a r i t i e s w i t h the s l a g - a t m o s p h e r e  The e s s e n t i a l d i f f e r e n c e i s the f l o w of a l a r g e e l e c t r i c a l  interface interface. current.  When d i r e c t c u r r e n t i s u s e d , the e l e c t r i c a l f i e l d w i l l p l a y a r o l e i n the d i f f u s i o n of i o n i c Jost  (50)  species.  n o t e s the f a c t t h a t m o b i l i t i e s d e r i v e d from e l e c t r o l y t i c  c o n d u c t i o n and from c h e m i c a l d i f f u s i o n measurements agreement,  are g e n e r a l l y i n  and w r i t e s the f o l l o w i n g r e l a t i o n f o r the f l u x of  d i f f u s i o n i n a p o t e n t i a l and a c o n c e n t r a t i o n  J = -UIRT'H + F n '||] C  gradient:  (eq IV. 5)  - 109 -  where u i s the m o b i l i t y of the i o n , n i t s charge and F i s  Faraday's  constant. If  the i o n c o n s i d e r e d i s i n d i l u t e s o l u t i o n , i t can be assumed  that —  i s constant  i n the d i f f u s i o n p r o c e s s .  The second term can then be c a l c u l a t e d u s i n g the constants  at  following  1800°K.  C = 1 w e i g h t % - 5.4 x 10  -4  mole cm  -3  ( i o n i c w e i g h t 48, d e n s i t y 2.6)  slag  (36),  8$ „ V'-„ 23 _ , _ -1 — ~ T ~ o~7T ~ 11'5 v o l t cm 3z i Z.U 1(  where V i s the a p p l i e d v o l t a g e a c r o s s  the s l a g and 1 the  average  gap between the e l e c t r o d e and the i n g o t  D = u RT = 10 »• u = J = n(6.7  -4  2 -1 cm sec  10~ d oi / Venn 8.314 x 1800 4  x 10"  9  , , -9 - o.7 x 10  x 9.6 x 1 0  i n  4  2 -1 . . . -1 cm sec mole j o u l e  x 5.4 x 1 0 ~  4  x 11.5)  = n x  4.0  -6 , -2 -1 10 mole cm sec 1n  • Over the s u r f a c e a r e a of the i n t e r f a c e f l u x of n x 1.07. x 10 r a t e of 2.5 g sec  1  .  -4  mole sec  -1  2 ( 27 cm ) ,  o r n x 20 x 10  t h i s represents -2  a  % T i at a melt  T h i s c o n t r i b u t i o n i s comparable w i t h the  total  f l u x of o x i d a t i o n . The s i t u a t i o n p r e s e n t i n g most i n t e r e s t i o n s toward the c a t h o d e .  i s the d i f f u s i o n of  reducible  N e g a t i v e l y charged i o n s which are i n g e n e r a l  - 110  -  complexes of i o n s h a v i n g the h i g h e s t s t a t e of o x i d a t i o n (such as 3+ Fe  4+ and T i  ) w i l l be r e p e l l e d a c c o r d i n g to t h e i r c h a r g e . 3+ 2+  charged i o n s such as T i  and Fe  Positively  have t h e i r d i f f u s i o n r a t e  increased,  i n t h i s c a s e , i m p r o v i n g the c u r r e n t e f f i c i e n c y of the o x i d a t i o n reduction e l e c t r o l y s i s . The magnitude of the f l u x c a l c u l a t e d above i s p r o b a b l y o v e r e s t i m a t e d because of the h i g h v a l u e chosen f o r the d i f f u s i o n c o e f f i c i e n t and the f a c t t h a t the p o t e n t i a l g r a d i e n t i s assumed to be  constant  a c r o s s the u n i t . IV.2.4 Flow of m e t a l on the  electrode  S i n c e the e l e c t r o d e i s c o n i c a l , an attempt can be made to determine the f l o w p a t t e r n of m e t a l m e l t i n g on i t s  surface.  We s h a l l c o n s i d e r m e t a l f l o w i n g under g r a v i t y , n e g l e c t i n g e f f e c t of s u r f a c e t e n s i o n .  The model i s o n l y v a l i d o u t s i d e  the  the  r e g i o n of the e l e c t r o d e a f f e c t e d by the f o r m a t i o n of the d r o p s . geometry of the system i s r e p r e s e n t e d on f i g .  The  46.  I f momentum t r a n s f e r from the s l a g to the l i q u i d m e t a l f i l m  is  n e g l e c t e d , w h i c h w i l l be l e g i t i m a t e i f gas e v o l u t i o n o c c u r s on the electrode,  the system can be s o l v e d r e a d i l y as f o l l o w s .  B i r d and . a l . (51)  g i v e s the p r o f i l e of v e l o c i t i e s f o r a f i l m  f l o w i n g on an i n c l i n e d p l a n e .  This expression (Eq.  2 . 2 . 1 6 of  remains v a l i d i n our case and can be w r i t t e n i f we change to f i t our p a r t i c u l a r c a s e :  (51))  variables  - Ill  X  =  -  Z  y = -x + 6  0 = 90° - B  therefore v  where y  m  X  -  S i n  (2y  9  Z Hn  m  i s the v i s c o s i t y  g r a v i t y on the m e t a l . by w r i t i n g P = P  The e x p r e s s i o n s  (eq  2  s  IV.6)  of the l i q u i d m e t a l , pg the a c t i o n of  We s u b t r a c t  " P  m  - y )  6  the buoyancy p r e s s u r e of the s l a g  •  f o r the maximum and mean v e l o c i t i e s remain  unchanged as shown b e l o w :  v  x,max  v  = v  =  . = x,i  S ± n  I n the p r e s e n t  pg6  H &  2 . sin 0 „ 2u m  ,  IV.8)  c a s e , the v o l u m e t r i c f l o w of m e t a l c r o s s i n g a  produced by the annulus of e l e c t r o d e o u t s i d e x . x  T T 7  (eq  9  g i v e n a b s i s s a x i s not the e n t i r e melt r a t e but t h a t p a r t of  Q  v  (eq I V . 7)  it  I f we c a l l t h i s f l o w  (volumetric):  Q  = PS^  S ± n  9  2,r w i t h r = -xcos 0  , 2  u p  g6  3  3  s i n 0cos0  x  ^m  Since i t maintains a constant cone m e l t s at an e q u a l r a t e . rate  ;  geometry,  I f W' m  the whole s u r f a c e of  i s the t o t a l v o l u m e t r i c m e l t  the  -  \  • K  ( 1  112  -  -K  " 4>  ( 1  " %  R  ^  >  «  r  _  o / J / -75—7 \/ w  IV 10)  R  R e p l a c i n g eq I V . 1 0 i n eq I V . 9 and s o l v i n g f o r 6 l e a d s  o =  -  (e  W  3y  m  m \  2ir(p -p )gx  m  s  :—«  sm0  cos  2  , (1  0  x cos o 2 R  2  Q  to  A N  )  ,  ,  (eq IV.11) T  1  T  n  \ M .  N  . /  P r o f i l e s c a l c u l a t e d from t h i s e q u a t i o n a r e p l o t t e d on f i g . 4 7 . T y p i c a l c o n s t a n t s are u s e d :  0 - 5 to 5 0 ° u  = 0 . 0 5 poise  m . p =7.0 m p  g  = 2.6  R = 1.27  ( l i q u i d metal) (36)  cm  W = 2 . 5 g sec m  -1  —>  *  W' = 0 . 3 6 m  3 - 1  cm sec  The f i l m t h i c k n e s s i s seen to v a r y from 50 t o 200 u o v e r most of the s u r f a c e o f the cone i f the a n g l e i s about 4 0 ° as we u s u a l l y observe on 25 mm e l e c t r o d e s . A more e l a b o r a t e c a l c u l a t i o n would t a k e i n t o account momentum t r a n s f e r at the s l a g - m e t a l i n t e r f a c e .  I n the absence o f c o n c l u s i v e  e v i d e n c e r e g a r d i n g s l a g c o n v e c t i o n i n the v i c i n i t y of the  electrode,  we have l i m i t e d o u r s e l v e s to l a y i n g down the b a s i s f o r such a c a l c u l a t i o n (Appendix 1 ) . K l y u e v and M i r o n o v (52) have c a l c u l a t e d , by an unknown method, f i l m t h i c k n e s s e s v a r y i n g from 29 t o 2567 y .  These v a l u e s a r e determined  as a f u n c t i o n of the d i a m e t e r of the e l e c t r o d e , the p r o p e r t i e s o f  the  - 113  -  s l a g and of the m e t a l , the m e l t r a t e and the e l e c t r o m a g n e t i c  stirring.  The t h i c k n e s s was found to d e c r e a s e w i t h i n c r e a s i n g e l e c t r o d e  sizes.  A l t h o u g h i t i s d i f f i c u l t to compare t h i s c a l c u l a t i o n w i t h o u r s ,  it  seems to l e a d to h i g h e r v a l u e s than e q . I V . l l . I f i n t e r f a c i a l v e l o c i t i e s i n the f i l m at c o n s t a n t X  CO  radius  s 0  (  = r = constant) are p l o t t e d , v  9 (fig.  . i s seen t o d e c r e a s e w i t h  48).  L a r g e e l e c t r o s l a g f u r n a c e s are known to have r e l a t i v e l y electrode  t i p s where drops can form at various p o i n t s .  flat  K l y u e v and  Mironov c o n s i d e r o n l y s i n g l e drops f o r m i n g on e l e c t r o d e of  diameter.  20 to 200 mm. The geometry of a m u l t i p l e drop e l e c t r o d e  t i p i s not  presently  known w i t h s u f f i c i e n t a c c u r a c y to extend t h i s c a l c u l a t i o n . IV.2.5 D i f f u s i o n of oxygen i n the e l e c t r o d e  f i l m (reverse p o l a r i t y )  We may d e r i v e e q u a t i o n s w h i c h d e s c r i b e d i f f u s i o n i n the m e t a l f o r the f l o w regime d e s c r i b e d above.  In electrode p o s i t i v e conditions,  p o l a r i z a t i o n of the e l e c t r o d e w i l l cause the f o r m a t i o n of a l a y e r of oxygen o r of the o x i d e of the m e t a l w h i c h c o n t r o l s the oxygen p o t e n t i a l at the i n t e r f a c e .  The r e s u l t i s thus to e s t a b l i s h s a t u r a t i o n i n  oxygen at the s u r f a c e .  E v i d e n c e f o r the d e p o s i t i o n of a FeO l a y e r on  an i r o n e l e c t r o d e w o r k i n g i n v a r i o u s m i x t u r e s of Ca¥^ and A ^ O ^ o r CaO at c u r r e n t d e n s i t i e s comparable to ESR has been o b t a i n e d by M i t c h e l l (53).  He i n d i c a t e s t h a t 2% CaO i n pure C a F  c o n c e n t r a t i o n to p e r m i t the d i s c h a r g e of 0  2  2-  is a s u f f i c i e n t l y high from the m o l t e n s l a g . .  - 114  -  The s a t u r a t i o n c o n c e n t r a t i o n i s o x i d i z a b l e element,  [0].. l  I f we assume t h a t the most  t i t a n i u m or aluminium i s very d i l u t e ,  i n t e r f a c i a l concentration w i l l  its  tend to zero and the r a t e a t w h i c h  the o x i d a t i o n t a k e s p l a c e w i l l depend upon the r a t e of t r a n s p o r t oxygen i n t o the e l e c t r o d e m e t a l .  of  The subsequent p r e c i p i t a t i o n and  removal of T i ^ O ^ , A ^ O ^ e t c . . . w i l l have no i n f l u e n c e on the r a t e of oxygen s u p p l y to across  the system.  The c o m p o s i t i o n p r o f i l e a c h i e v e d  the f i l m i s q u a l i t a t i v e l y d e s c r i b e d by f i g . 49.  d i f f u s i v e f l u x of r e a c t i n s i d e the  ;  (Note t h a t  [ T i ] must not be c o n s i d e r e d because [0]  and  the  [Ti]  metal.)  The problem i s s i m p l i f i e d i f the boundary l a y e r f o r d i f f u s i o n is  s m a l l compared to the t h i c k n e s s of the f i l m , s i n c e the s i t u a t i o n  reduces t o d i f f u s i o n i n a s e m i - i n f i n i t e medium. The p r o f i l e at x i s dependent upon the exposure time t  g  of a  s u r f a c e element t r a v e l l i n g between the base of the cone (x cos 0 = - R ) and x .  10]  It  y  i s g i v e n by  = 10],  ±  (1 - e r f  ~ /Tn  C S  y )  )  f^uTm e ~  where Di s the d i f f u s i o n c o e f f i c i e n t 0,m  t  e  -7  (eq I V . 12) (equation 17.5-15 i n ref  (51))  of oxygen i n the m e t a l and J O  d x  _ R cos Q n  v  , x,i  As we are p r i m a r i l y concerned w i t h the t o t a l f l o w of oxygen i n t o the m e t a l , e q u a t i o n I V . 1 2 must be i n t e g r a t e d . oxygen  t r a n s f e r r e d per u n i t time a c r o s s  The t o t a l w e i g h t of  the s l a g - m e t a l  interface  - 115 is  t h e same as t h e f l o w r a t e of oxygen a t t h e system e x i t  tip),  (electrode  w h i c h may be c a l c u l a t e d by i n t e g r a t i o n o f the d i f f u s i v e f l o w  through the e n t i r e area o f t h e e l e c t r o d e c o n e ,  S.  The r e s u l t i s s i m i l a r t o e q u a t i o n I V . 4 .  Q  t  0  =  /  S o  2[0].  /  / D U  >  v  dS  m  e  (eq I V . 1 3 )  i s c a l c u l a t e d using the expression (IV.7)  for v  e  expressed by the e q u a t i o n ( I V . 1 1 ) . integral  . where 6 i s  X,1  T h i s l e a d s t o the E u l e r i a n  (see Appendix I ) :  =  e  27rcos 0 , 2 / 3 m ,1/3, R ^_ " , ^ psin 0 ^cos0 m  r  f  3  M  ;  ;  W i t h t h e c o n s t a n t s ' used above i n e q ( l V . l l ) , t  e  .5/3 '  T(5/6)r(l/3) r(7/6) (eq  comes t o :  t  IV.14)  -0.56  second  e 5/3 ' 2/3 and v a r i e s o n l y s l o w l y w i t h 0 ( f i g . 50) i f the r a t i o R /W' i s held m constant. I n e q u a t i o n (IV. 13) , t h e a r e a of exposure dS i s dS = 2TJ-— dx cos 0 r  and a f u r t h e r a p p r o x i m a t i o n has been made when c o n v e c t i o n i n the y direction i s neglected.  In fact,  t h e r e d u c t i o n o f a r e a toward t h e  t i p o f the c o n e . t e n d s t o make t h e f l o w d i v e r g e n t w h i l e t h e p r o g r e s s i v e i n c r e a s e i n v . has the r e v e r s e e f f e c t . x,i A good a p p r o x i m a t i o n can n e v e r t h e l e s s be o b t a i n e d by u s i n g t h e a r e a of the cone as an average v a l u e f o r S and t This w i l l  g  as c a l c u l a t e d above.  p r o b a b l y l e a d t o a s l i g h t u n d e r e s t i m a t e of the d i f f u s i v e /  f l u x because  t h e r i m o f the e l e c t r o d e g i v e s t h e h i g h e s t c o n t r i b u t i o n  to the f l u x , h a v i n g b o t h the h i g h e s t dS andthe h i g h e s t t .  The f l u x  - 116 -  of oxygen can then be r e w r i t t e n as  Q  = 2S[0]J  Q  -2iS_ e  V using S =  s  2.4 x 10  3  g/sec.  (eq I V . 1 5 )  - 5.13 c m .  — — — cos 9  2  D. = 3 . 10 0,m  cm sec  4  2  - 1  (value f o r D „ (54)) ^ 0,Fe rt  = 0.2 wt % ( s a t u r a t i o n o f Fe a t 1550°C)  [0]  ±  = 0.014 g c m "  3  -j—  The mass f l u x o f oxygen i n t r o d u c e s t h e r e f o r e  W  *  i n the m e t a l (W = 2.5 g sec m for  ).  o r 0.09% [o]  m  Converted i n t o a r a t e o f o x i d a t i o n  [ T i ] t o T i 0 , t h i s v a l u e c o r r e s p o n d s t o 0.18%[Ti] w h i c h i s 2  t o t h e observed l o s s . It  close  3  Q  Q  i s independent o f time ( W ^ / W ^ .  s h o u l d be noted t h a t the c o n t r i b u t i o n o f the s u r f a c e of  drop has not y e t been t a k e n i n t o a c c o u n t .  Its  the  e f f e c t s h o u l d be  s i g n i f i c a n t , but i t i s d i f f i c u l t to a s s e s s because the e x t e n t o f c o n v e c t i o n i n the hanging drop i s unknown. E l e c t r o d e p o s i t i v e c o n d i t i o n s produce b i g d r o p s , from 1.4 to 3.5 grams ( i n g o t  S15: 2.5 g ; i n g o t M5: 2.85 g ) .  If  d i a m e t e r r e a c h e s 0.7 to 1.0 cm when they l e a v e the  spherical, their electrode,  r e p r e s e n t i n g a t t h a t time a s i g n i f i c a n t p o r t i o n of the exposed area  *  surface  .  Our o b s e r v a t i o n s on drop s i z e c o n t r a d i c t the r e s u l t s r e p o r t e d by W h i t t a k e r (21) who has observed drop s i z e s w h i c h i n c r e a s e i n the o r d e r D . C . p o s i t i v e e l e c t r o d e , D . C . n e g a t i v e e l e c t r o d e and A . C . S i n c e the drop s i z e i s p r i n c i p a l l y dependent on the i n t e r f a c i a l t e n s i o n at the s l a g - m e t a l i n t e r f a c e , we would expect v a r i a t i o n s w i t h p o l a r i t y and m e t a l c o m p o s i t i o n .  - 117  -  I f we assume t h a t the s u r f a c e of the drop i s a e x t e n s i o n of  the  f l o w s i t u a t i o n i n the f i l m , i t s c o n t r i b u t i o n to the t o t a l f l u x of d i f f u s i o n s h o u l d c e r t a i n l y be s m a l l .  A drop of 1 cm d i a m e t e r hanging  from the t i p of the cone (0 = 4 0 ° ) w i l l  c o n t r i b u t e on the average  by  about 30% of the s u r f a c e a r e a of the cone w h i l e growing from z e r o  to  its final size.  It  s h o u l d be remembered however t h a t t h i s  o c c u r s f o r the h i g h e s t v a l u e s of t  , when the c o n c e n t r a t i o n  a t the s u r f a c e and the d i f f u s i o n f l u x are l o w e s t . b o t h the i n c r e a s e adding to t 1.4  sec),  i n surface  the average  a r e a and i n t  contribution gradient  I f we i n s e r t  i n t o e q u a t i o n I V . 1 3 by ,  time of f o r m a t i o n o f a drop  (approximately  we f i n d t h a t the t o t a l oxygen p i c k up s h o u l d a c t u a l l y be  lower than i f the drop was i n f i n i t e l y s m a l l .  T h i s r e s u l t i s of  course  a consequence of the assumptions made but i t shows t h a t the c o n t r i b u t i o n of the drop can o n l y be s i g n i f i c a n t i f i t i s homogenized by  turbulence.  I n e q u a t i o n I V . 1 3 , we see i m m e d i a t l e y t h a t S f o l l o w s a d i r e c t p r o p o r t i o n a l i t y r e l a t i o n s h i p w i t h [0]^ and D with 0 for 0 < 40°.  t  g  ^,  geometry:  « W•.. e m  . Q_ <* X)  . -1/2 t e  • 1/3 <* W m  The e f f e c t on the c o m p o s i t i o n of the m e t a l :  AI0] - £  W m  S varies only slowly  i s p r i n c i p a l l y dependent on W w h i c h w i l l  the f o l l o w i n g i n f l u e n c e at c o n s t a n t  t  Q  . iiJ ' 2  m  3  have  - 118  For an e l e c t r o d e  -  of a g i v e n s i z e , an i n c r e a s e i n m e l t r a t e s h o u l d  reduce the c o m p o s i t i o n  change.  IV.2.6 D i f f u s i o n of a l l o y i n g elements i n the e l e c t r o d e  film  R e a c t i v e a l l o y i n g elements can become s u f f i c i e n t l y d e p l e t e d i n the a n o d i c i n t e r f a c e ,  t h a t they no l o n g e r c o n t r o l the oxygen p o t e n t i a l .  T h i s o c c u r s when t h e o x i d a t i o n r a t e of r e a c t i v e the c o n d i t i o n s of s e c t i o n ( E V . 2 . 5 ) i s  elements r e q u i r e d by  i n excess of the l i m i t i n g d i f f u s i v e  flux. The excess oxygen can then d i f f u s e i n t o the m e t a l  (see  s e c t i o n ) o r r e a c t w i t h one of the m a t r i x elements a t the interface  preceding  slag-metal  to form an o x i d e (FeO, CT^>^ e t c . . ) w h i c h would d i f f u s e  i n t o the s l a g .  The o x i d e formed i s n o t i n thermodynamic e q u i l i b r i u m  w i t h the average c o m p o s i t i o n of the m e t a l w h i c h s t i l l  contains  some  d e o x i d a n t , but i t may n e v e r t h e l e s s c o n c e n t r a t e i n the s l a g to a l e v e l dependent upon the t r a n s p o r t of the c o r r e s p o n d i n g m e t a l l i c the c a t h o d i c  ions,to  interface.  T h i s e x p l a n a t i o n p r o b a b l y accounts f o r the p r e s e n c e of i r o n and chromium o x i d e s i n the s l a g s of the Mar 300 and 321 S . S , remelted w i t h D . C . p o s i t i v e  ingots  electrode.  When u s i n g A . C . on the o t h e r h a n d , chromium and i r o n o x i d e s i n a s i g n i f i c a n t l y lower c o n c e n t r a t i o n  (S12,  the e l e c t r o d e  i n oxygen and the k i n e t i c s  s u r f a c e i s not s a t u r a t e d  o x i d a t i o n are l e s s  favourable.  S13,  S16,  S17, M 7 ) .  are Here, of  - 119  -  The presence of a major a l l o y i n g element w h i c h has a h i g h a f f i n i t y f o r oxygen (Cr i n s t a i n l e s s  s t e e l ) w i l l l o w e r the i n t e r -  f a c i a l c o n c e n t r a t i o n of oxygen [0]^ as e x p r e s s e d paragraph.  If  i n the p r e v i o u s  the d e o x i d a n t i s d e p l e t e d , chromium w i l l be o x i d i z e d  r a p i d l y , p r e v e n t i n g the d i f f u s i o n o f oxygen i n t o the m e t a l .  Here  [0]^ w i l l be almost two o r d e r s of magnitude lower than w i t h pure Fe (fig.  42).  The c o m p o s i t i o n of the i n g o t w i l l not be g r e a t l y  as most of the chromium can be reduced a g a i n at the In c o n t r a s t ,  affected  cathode.  t i t a n i u m and a l u m i n i u m w i l l not be reduced at  cathode when another o x i d e i s p r e s e n t . r a t e of o x i d a t i o n at the anode of  T h i s e x p l a i n s why the  limiting  [ T i ] and [ A l ] w i l l approach the  r a t e of o x i d a t i o n observed i n most of the p r e s e n t  actual  experiments.  T h i s l i m i t i n g r a t e of o x i d a t i o n can be c a l c u l a t e d a c c o r d i n g the scheme proposed f o r the r a t e of d i f f u s i o n of oxygen i n t o metal  the  to  the  (IV.2.5). We have shown t h a t s i g n i f i c a n t d i f f u s i o n of oxygen o c c u r s when  J0]^  i s h i g h , t h a t i s when the m a t r i x c o n t a i n s no d e o x i d a n t i n h i g h  concentration.  If  10]^ i s depressed by a m a t r i x c o n s t i t u e n t ,  the f l u x  Q Q i s s m a l l and does not s i g n i f i c a n t l y supplement the r a t e of d i f f u s i o n of i T i ] from the m e t a l . I f we a p p l y the treatment  of  ( I V . 2 . 5 ) to the d i f f u s i o n of  t i t a n i u m from the m e t a l ( e l e c t r o d e very l i t t l e . initial  positive),  the e q u a t i o n s  i T i ] ^ i s n e g l i g i b l e a n d , i f we put [ T i ]  c o n c e n t r a t i o n of [ T i ]  [Ti] = ITi]  o  erf  0  change  e q u a l to  the  (or I A 1 ] e t c . . . ) , we o b t a i n the p r o f i l e :  6 -  (eq I V . 1 6 )  - 120 t  Is  -  the same, l e a d i n g to the approximate f l u x of  Q i T  =  -  2 S [ T i ]  [Ti]  o  (eqIV.17)  which i s independent of time o r W /W . m  s  The main problem h e r e i s the s e l e c t i o n  of an a p p r o p r i a t e d i f f u s i o n -4  coefficient  for titanium.  Jost  (50)  g i v e s D„. _ = 1 0 Si,Fe -4 2 -1 x 10 cm sec i n ref.  1560°C w h i l e D... „ i s g i v e n as 1.22 Al,Fe °  -4  1550°C i n i r o n s a t u r a t e d w i t h oxygen and 2 x 10 c o n d i t i o n s by r e f .  (55).  -4  We s h a l l t h e r e f o r e s e l e c t D . =10 Ti,m m  In s t a i n l e s s (p  s t e e l 321,  - 7 . 0 ) , Q l e a=d i3.0 ng T i  t 1o 0: "  g  3  [Ti] sec"  Q  2 -1 cm sec at  2  (30)  at  -1  cm sec  i n the same  2 -1 cm sec _3  = 0.45  wt % = 0.0315 g cm  1  or Q  T i  W  m  =0.12  weight %  T h i s i s somewhat l e s s t h a n the observed c o m p o s i t i o n  change,  a l t h o u g h the same r e s t r i c t i o n w h i c h has been mentioned i n applies here,  (IV.2.5)  namely i n the c o n t r i b u t i o n of the h a n g i n g d r o p .  Also,  the v a l u e chosen f o r D^,^ ^ i s an a p p r o x i m a t i o n . I n the case of Maraging 300 s t e e l , lost  ( f i g . 30,  i n g o t s M5, M 6 ) .  I il T  0  I  a p p r o x i m a t e l y 0.35% s  somewhat h i g h e r , at 0.68  -3 or 0.048 g cm , l e a d i n g t o the c a l c u l a t e d Q - r ^ * 0.18 wt % W ™ m T i  w  jTi]  value:  is wt %  - 121  -  I V . 2 . 7 D i f f u s i o n i n the m e t a l a t the s l a g - i n g o t i n t e r f a c e S i n c e c o n v e c t i o n p a t t e r n s i n the m e t a l p o o l K l y u e v (52)  (direct polarity)  a r e p r e s e n t l y unknown,  assumes t h a t each drop spreads a c r o s s the top s u r f a c e  the i n g o t , the s u r f a c e exposed b e i n g renewed every second area = area/melt r a t e ) .  of  (reduced  T h i s l e a d s him to a t t r i b u t e a reduced a r e a ,  of r e a c t i o n to the e l e c t r o d e w h i c h i s two t o t h r e e times l o w e r than the ingot.  T h i s assumption i s i n c o n t r a d i c t i o n w i t h our r e s u l t s when we  compare o x i d a t i v e l o s s e s e x p e r i e n c e d w i t h d i r e c t and r e v e r s e p o l a r i t y . The r a t e a t w h i c h the s u r f a c e a g a i n s t the s l a g i s renewed can be estimated for d i r e c t p o l a r i t y c o n d i t i o n s .  If uniform r a d i a l convection  and a p e n e t r a t i o n model a r e assumed, e q u a t i o n ( I V . 4 ) d e r i v e d i n (IV.2.2) applies:  J =  2(C. - C ) i o  /-£t i r  e  We s h a l l now c o n s i d e r the d i f f u s i o n o f ( [ T i ] ^ = 0, see I V . 2 . 2 ) .  interface  ( i n g o t s S6, M l , M2) and d i r e c t  p o l a r i t y and i s r e p o r t e d i n the f o l l o w i n g  Ingot  to a d e p l e t e d  The f l u x J i s d e r i v e d from the observed v a l u e s  of t i t a n i u m o x i d a t i o n u s i n g CaF^ s l a g s  Table  [Ti]  table:  65.  A[Ti]% Matrix  [Ti] % ° Matrix  g sec  S6  0.22  0.23  1.69  Ml  0.37  0.32  M2  ^0.40 t  e  :  "  0.29  W m  -1  10  x J -2 -1 g cm sec 4  t  P e r i o d o f drops 6  sec  sec  1.49  1.48  0.95  2.15  3.18  0.63  1.02  2.10  3.44  0.46  0.83  i s c a l c u l a t e d from J  u s i n g D - 10  -4  2 -1 cm sec  - 122  -  We see t h a t the r a t e of renewal of the s u r f a c e i s comparable  with  the frequency of the d r o p s , t h e r e f o r e the f a l l i n g of each drop w i l l create a p e r t u r b a t i o n which i s important i n b r i n g i n g f r e s h against  the i n t e r f a c e .  If  t  metal  i s found to be l a r g e r than the p e r i o d of  the d r o p s , we can assume t h a t the p e r t u r b a t i o n i s not s u f f i c i e n t renew the whole s u r f a c e .  If,  on the c o n t r a r y t  i s smaller  p e r i o d of the d r o p , r e s i d u a l c o n v e c t i o n w i l l p a r t i c i p a t e the  to  than;the  i n renewing  surface. T h i s model w i l l be v a l i d i f  the depth a t w h i c h the m e t a l  is  renewed  or  L »  (t  x D)  (see  L »  0.2  to 2 x 1 0 ~  1 / 2  2  IV.2.2)  cm.  IV.2.8 D i f f u s i o n i n the s l a g a t the s l a g - m e t a l When r e m e l t i n g 1409 A l , the c o n c e n t r a t i o n o x i d e s i n the s l a g i s n e g l i b i b l e . these components  interfaces of chromium o r i r o n  We have d i s c u s s e d p r e v i o u s l y t h a t  s h o u l d be p r e s e n t i n the s l a g when the l i m i t i n g r a t e  of d i f f u s i o n f o r the most r e a c t i v e enough t o accommodate  a l l o y i n g elements i s n o t h i g h  the r a t e of o x i d a t i o n .  T h i s i s t h e r e f o r e not the case w i t h [ A l ] when [ A 1 ] h e r e and I A I ] ^ remains >>  q  > 3.74%  as  :  0.  The r a t e c o n t r o l l i n g s t e p h e r e i s most l i k e l y to be the r a t e of s u p p l y of o x i d a n t to the i n t e r f a c e .  The s i t u a t i o n i s d e s c r i b e d on  f i g u r e 51 where the s l a g has a low c o n c e n t r a t i o n boundary and the t r a n s f e r :  occurs  i n the s l a g  of o x i d a n t a t  phase.  the  - 123 The b a s i s  f o r the c a l c u l a t i o n of f l o w c o n d i t i o n s i n the s l a g w i t h  momentum t r a n s f e r It  -  to the e l e c t r o d e  f i l m i s l a i d down i n Appendix I .  i s f e l t however t h a t too many assumptions are i n v o l v e d i n t h i s  type  of c a l c u l a t i o n and i t would be b e t t e r to assume t h a t the s l a g f l o w s over the m e t a l a t i t s coefficient -5 10  2  t e r m i n a l v e l o c i t y o r 5 to 10 cm sec ^ .  The d i f f u s i o n  of A ^ O ^ i n C a F 2 ~ A l 2 0 i s known t o be a p p r o x i m a t e l y 8.5 3  x  -1  cm sec  (15)  l i q u i d metals.  o r s l i g h t l y l o w e r than f o r a l l o y i n g elements  in  W h i l e the f l o w v e l o c i t i e s i n v o l v e d are of the same  o r d e r of magnitude as i n the e l e c t r o d e  f i l m , we s h o u l d e x p e c t  the l i m i t i n g r a t e w i l l be comparable w i t h t h a t w h i c h we have f o r the d i f f u s i o n pf [ T i ] o r [0]  i n ( I V . 2 . 5 ) and ( I V . 2 . 6 ) ,  i t w i l l d i f f e r by the c o n t r i b u t i o n of the e l e c t r i c and A . C . o p e r a t i o n  that calculated  although  f i e l d between D . C .  (IV.2.3).  T h i s mechanism w i l l produce r a t e s of r e a c t i o n d e c r e a s i n g as the o x i d a n t of the s l a g i s e x h a u s t e d , p r o v i d e d t h a t  steadily  the system i s  closed. U s i n g d i r e c t p o l a r i t y , the a n o d i c i n t e r f a c e boundary.  At t h i s b o u n d a r y , mass t r a n s f e r  same o r d e r of magnitude as at the o t h e r A l l the mass; t r a n s f e r current operation. interface  i s the  ingot-slag  i n the s l a g s h o u l d be of  the  interface.  c a l c u l a t i o n s made so f a r a p p l y to  The main r e a s o n f o r t h i s i s the f a c t t h a t  direct the  i s known to be a t an oxygen p o t e n t i a l g r e a t e r than the  c h e m i c a l d e o x i d a t i o n e q u i l i b r i u m would impose, thereby m a i n t a i n i n g a constant  driving force for d i f f u s i o n .  u s e d , on the o t h e r h a n d , the i n t e r f a c e  When a l t e r n a t i n g c u r r e n t i s a t an i n t e r m e d i a t e  p o t e n t i a l and r e s i s t a n c e to mass t r a n s f e r  is  oxygen  i n b o t h phases i n c o n t a c t  is  - 124 to be c o n s i d e r e d .  -  The f l u x per u n i t a r e a w i l l t h e n be l o w e r e d but  t h i s i s compensated f o r by the g r e a t e r i n t e r f a c i a l a r e a electrode transfer  and the i n g o t s u r f a c e s  (both  i n s t e a d o f one of them).  [M]  Although  of c u r r e n t a c r o s s the s l a g - m e t a l i n t e r f a c e must be  we are concerned w i t h the r e v e r s i b i l i t y of the r e a c t i o n M (where M can be C r , T i , F e , e t c . . . ) .  Faradaic, + ne  electrolysis  occurs  =  I t would appear t h a t t h i s  h i g h under 60 Hz c o n d i t i o n s s i n c e we observe no s i g n i f i c a n t i n the c u r r e n t wave form at c o n s t a n t  the  is  assymetry  RMS v o l t a g e and hence no net  r  (27).  IV.2.9 Comparison between oxygen and a l l o y i n g element d i f f u s i o n I n the m e t a l An e s s e n t i a l  d i f f e r e n c e between the c o n t r i b u t i o n of the d i f f u s i o n  of oxygen and of a l l o y i n g elements of the mass t r a n s f e r  process  r e s i d e s i n the p o s s i b i l i t y of  a t the o t h e r e l e c t r o d e .  argument i s a p p l i e d to the p o s i t i v e e l e c t r o d e  reversion  The f o l l o w i n g  case, but r e m a i n s . v a l i d  with direct polarity. Once oxygen has d i f f u s e d i n t o the m e t a l on the e l e c t r o d e it  i s c a r r i e d to the i n g o t p o o l w i t h the d r o p .  If  there i s a s u f f i c i e n t  amount of d e o x i d a n t i n the m e t a l , as i s the case h e r e , f o l l o w e d by p r e c i p i t a t i o n of o x i d e s t a k e s p l a c e . t i o n p r o d u c t s w i l l be removed from e i t h e r  film,  homogenization  A p a r t of the d e o x i d a -  the drop o r the m e t a l ; p o o l .  At t h i s s t a g e , the c o n c e n t r a t i o n of d i s s o l v e d oxygen i s e x t r e m e l y ; low and corresponds to the d e o x i d a t i o n e q u i l i b r i u m i n the l i q u i d m e t a l . Hence, t h e r e i s a n e g l i g i b l e f l u x of oxygen to the c a t h o d i c where i t can be e l e c t r o l y t i c a l l y removed. the m e t a l i s not e l e c t r o l y t i c a l l y  interface  T h u s , the f l u x of oxygen i n t o  reversible.  - 125  When a l l o y i n g elements  -  concentrate  i n the s l a g , on the o t h e r h a n d ,  they r a p i d l y r e a c h a c o n c e n t r a t i o n of a few t e n t h s of a p e r c e n t .  Their  r a t e of r e d u c t i o n a t the cathode cannot be c o n s i d e r e d n e g l i g i b l e because t h e i r f l u x ( i n a c o m p o s i t i o n gradient and i n an e l e c t r i c is  field)  substantial. T h i s can be v e r i f i e d d i r e c t l y from some o f the  performed.  Ingot M2 i s a good example.  experiments  A t the e a r l y s t a g e s of  the  m e l t , when the s l a g a f f o r d s l i t t l e p r o t e c t i o n t o the i n g o t , an a p p r e c i a b l e c o n c e n t r a t i o n of FeO has accummulated (0.24%, see  III.12).  Subsequently t h i s c o n c e n t r a t i o n decreased r a p i d l y and f l u c t u a t e d around 0.05%;  T h i s suggests  t h a t a steady s t a t e f o r the o x i d a t i o n and  r e d u c t i o n o f Fe was reached between the e l e c t r o d e s w h i l s t t i t a n i u m c o n t i n u e d to c o n c e n t r a t e  i n the  slag.  IV.2.10 Dependence upon time or  ^  m  ^  s  The v a r i o u s d i f f u s i o n mechanisms proposed so f a r a p p l y m a i n l y to cases where the r a t e of l o s s of r e a c t i v e elements  i s independent of  the i n g o t / s l a g r a t i o  T h i s has been  (i.e.  of the i n g o t l e n g t h ) .  g e n e r a l l y observed i n the experiments performed w i t h d i r e c t  current  under an oxygen c o n t a i n i n g atmosphere, whether t h i s i s a i r o r impure argon. When, on the c o n t r a r y , the r a t e of o x i d a t i o n i s found to d e c r e a s e w i t h t i m e , one of the two f o l l o w i n g phenomena i s l i k e l y to take a)  place:  A p r o d u c t of the r e a c t i o n i s c o n c e n t r a t i n g i n the s y s t e m , making  - 126  -  o x i d a t i o n thermodynamically l e s s f a v o u r a b l e ; d e p l e t e d i n the  b)  The o x i d a n t becomes  system.  We have observed such a d e c r e a s e i n the f o l l o w i n g - when u s i n g A . C . power as i n i n g o t s S12,  cases:  S13,  - when r e m e l t i n g under argon w i t h d i r e c t c u r r e n t  (ingot  S16).  I n b o t h cases the a c c u m u l a t i o n of r e a c t i o n p r o d u c t s was w i t h t h a t of m e l t s p r o d u c i n g i n g o t s showing no such e f f e c t . reason i s therefore  comparable The  l i k e l y to be the d e p l e t i o n of o x i d a n t i n the  system. The o x i d a n t i n the s l a g can c o n s i s t (which has been d e t e c t e d : (FeO e t c . . ) TiO^  of i m p u r i t i e s such as s i l i c a  I I I . 3 ) and the amount of m e t a l  f o r m i n g a t the s t a r t of a g i v e n r u n .  i f t h i s element i s a c o n s t i t u e n t  It  of the s l a g .  oxides  can a l s o be  All will  progress-  i v e l y become d e p l e t e d i f t h e r e i s no e x t e r n a l s u p p l y of oxygen. former w i l l d i s a p p e a r much f a s t e r t h a n the l a t t e r  owing to the  The initial  concentrations.  3(Si0 )  +  y  3 [Si]  +  K T i ^ )  2[Ti]  y  3[Fe]  +  (Ti^)  [Ti]  y  2(Ti 0 )  3 ( F e 0 ) + 2IT1]  y  (Ti^)  2  3 (FeO)  +  3(Ti0 ) 2  2  4[Ti]  +  3  P o s s i b l e ".regeneration"  or  2FeO \+  l/20  2  Ti 0 '+  l/20  2  2  Ca  3  ++•' '  +  2e  -  2  reactions  —y  2Ti0  3  +  6 (FeO)  being  2  •y Ca° (vapour) a t the cathode  (see  IV. 2.2).  - 127  -  IV.2.11 M a c r o s c o p i c assessment  bf the p r o c e s s  (oxidant i n limited supply)  An o v e r a l l a p p r a i s a l of the e f f e c t i v e n e s s  o f the o x i d a t i o n r e a c t i o n s  i n e l e c t r o s l a g can a l s o be done by t r e a t i n g the o v e r a l l mass  transfer  problem m a c r o s c o p i c a l l y . T h i s i s e s p e c i a l l y u s e f u l i n the case where the o x i d a n t i s i n l i m i t e d supply.  E. Steinmetz  (56)  g i v e s the m a t h e m a t i c a l t r e a t m e n t  the exchange of m a t e r i a l between a m o b i l e and a s t a t i c p h a s e . s l a g f u r n a c e s can be c l a s s i f i e d w i t h the p r o c e s s e s where the  for  Electrocontact  i s " t r a n s i t i o n a l " (slag s t a t i o n a r y , metal f l o w i n g ) , without h o m o g e n i z a t i o n o f the m e t a l phase subsequent segregation i n  to the r e a c t i o n  (see  IV.2.12).  W h i l e our c a l c u l a t i o n s so f a r have d e a l t m a i n l y w i t h l a m i n a r f l o w , the treatment of the problem g i v e n here r e s t r i c t s resistance  the mass  to a boundary l a y e r n e a r the i n t e r f a c e , hence i t  a p p l i c a b l e to t r a n s f e r s where the r e s i s t a n c e s l a g s i d e o f the i n t e r f a c e  is  i s c o n c e n t r a t e d on the  (note however t h a t the s l a g f l o w i s not  c l e a r l y t u r b u l e n t i n our c a s e , the v i s c o s i t y u h i g h a t ^ 0.75 p o i s e  transfer  g  being r e l a t i v e l y -  (36)).  Other assumptions a r e : - d i f f u s i o n and c o n v e c t i o n o c c u r a l o n e . i n p e r p e n d i c u l a r d i r e c t i o n s . - the mass . . t r a n s f e r c o e f f i c i e n t s  are c o n s t a n t over the a r e a o f  reaction. - the i n t e r p h a s e p a r t i t i o n c o e f f i c i e n t s  obey H e n r y ' s l a w .  The f l u x per u n i t i n t e r f a c e a r e a per u n i t time f o r a component X i s t h e r e f o r e expressed  as:  -  128  -  j = K ((X) - (X)) in the slag boundary layer 1  S o r  j  =  K  m  (W  ~  i-  n  t n e  m e t  a l boundary layer  (X) and [X] b e i n g b u l k c o n c e n t r a t i o n s of the p h a s e s , w h i l e (X)^  [ X ] ^ and  a r e l o c a l c o n c e n t r a t i o n s at the i n t e r f a c e , i n e q u i l i b r i u m w i t h  each o t h e r . K. , < a r e mass t r a n s f e r c o e f f i c i e n t s i n t h e s l a g and m e t a l s m ° limit layers. The " p r o c e s s e f f e c t i v e n e s s '  1  to, t a k e n  h e r e as the amount of  removal o f the o x i d a n t from the s l a g i s g i v e n by the r e l a t i o n (X)  nW , m  o  /'1nW11m+p s  •U  oo"  u=  r  KA  (56)  p KAt s  Os v  TTT7"w— >  expC  ( T r a n s i t i o n a l c o n t a c t c o n s i d e r i n g the t r a n s p o r t of o x i d a n t t h r o u g h the r e a c t i o n i n t e r f a c e ) where (X)  i s the i n i t i a l c o n c e n t r a t i o n o f X Henrian e q u i l i b r i u m constant: n =  [X].  a r e a o f c o n t a c t between phases  w  melt  m In  rate  o v e r a l l mass t r a n s f e r c o e f f i c i e n t . . our c a s e , the r e a c t i o n s o f o x i d a t i o n have a h i g h f r e e energy  change and  K  A i s n e g l i g i b l e when compared t o  becomes : (X) = ( X )  o  e  -p S  K  At/W S  by d i f f e r e n t i a t i o n w i t h r e s p e c t t o t i m e :  rfrl^.  The e q u a t i o n  - 129 -  -4— p KA  (X) = -(X)  The  o  W  -p e  xAt/W S  S  s  r a t e of change of  (X) or  (X) i s r e l a t e d to the r a t e  o x i d a t i o n of the a l l o y i n g element, through the ( w r i t t e n here f o r  n(X)  •  A m a t e r i a l b a l a n c e per  m(Ti)  'o  [Ti] =  [Ti]  where M r e p r e s e n t s  n  (Ti)  oxidant We and  by  n[X]  requires  W (X)  ML^  rjr—•  —W m  (Eq  IV.  18)  the atomic weight of the c o r r e s p o n d i n g element  are expressed i n weight  fraction).  (X), t h i s e q u a t i o n becomes  [Ti]  with  m  —  +  +  u n i t time  M_  Replacing  exchange r e a c t i o n  titanium):  + m[Ti]  (concentrations  of  *  O  : M  T  [Ti]  O  w  m  i  = — — — e m M.  (X)  initially  o  i n the  i s the .  stoichiometric  (Ti) equivalent  of  the  slag.  can  f u r t h e r observe t h a t i f the melt r a t e i s constant W p <A m s calling Y = — and <j> = — ; — the e q u a t i o n becomes s m  t =  W -— m W  W  r  ,  (Ti)  [ T i ] ' 0  :  =  1  " TTir * o  -<i>Y e  (Eq  -  I V , 1 9 )  - 130  -  T h i s e q u a t i o n i s l i k e l y to be the v a l i d one when the o x i d i z e d element i s at a r e l a t i v e l y h i g h c o n c e n t r a t i o n i n the m e t a l ,  the  o x i d a n t i s i n s m a l l q u a n t i t y i n the s l a g and the system does not p i c k up oxygen from the atniosphere a t an a p p r e c i a b l e  rate.  Ingot S12 i s i n f a c t the o n l y one w h i c h f u l f i l s t h e s e r e q u i r e m e n t s because our i n g o t s r e m e l t e d under argon have used a d e o x i d i z e d s t a r t and i n g o t S13 was r e m e l t e d w i t h a s l a g r i c h i n o x i d a n t (T±0^), (IV.3.2). A few c u r v e s of the f a m i l y eq ( I V . 1 9 ) a r e r e p r e s e n t e d on F i g . 52.  They show t h a t a s h o r t t a n s i e n t w i l l be o b t a i n e d when <j> i s (Ti) h i g h and the o x i d a n t (or . -•— ) l o w . o  i  f T  These c u r v e s can a l s o be a p p l i e d to the r e a c t i o n of v a r i o u s o t h e r elements w h i c h are l i k e l y to be exchanged w i t h the a.  slag:  Depending upon the b a s i c i t y of the s l a g , ( S i 0 2 ) can be reduced  by [Mn] ( 7 , 1 0 , 5 7 ) or v i c e v e r s a ( 7 , 5 8 ) .  The b e h a v i o u r of carbon i s  a l s o dependent upon the a c t i v i t y o f Si02 i n the s l a g ( 1 0 ) , to the p o s s i b l e (Si0 ) 2  according  reaction: +  2[C]  =  2(CO)  The c o m p o s i t i o n p r o f i l e of  +  IS1]  [Mn], J S i ] ,  [Cj a l o n g an i n g o t c o u l d  t h e r e f o r e obey an e x p o n e n t i a l law of the type e q .  ( I V . 1 9 ) i n the  r e g i o n where the l o s s v a r i e s w i t h the a c t i v i t y of the o x i d a n t ; a c c o r d i n g to H o l z g r u b e r ( 5 7 ) , between 0.4  t h i s r e l a t i o n s h i p i s observed f o r  and 2 f o r the r e a c t i o n of [ S i ]  between 0.3 and 1.2  f o r the r e a c t i o n of  and [Mn] and  [C].  (CaO)/(Si02) (CaO)/(Si0 )  Outside these v a l u e s ,  2  the  - 131  -  l i m i t i n g d i f f u s i v e f l u x can be reached i n one o f the phases and the system i s b e t t e r d e s c r i b e d by one of the models of s e c t i o n s to  (IV.2.3)  (IV.2.9). b.  S u l p h u r has a l s o been the s u b j e c t  of a number o f s t u d i e s and  a l t h o u g h the amount of d e s u l p h u r i z a t i o n observed i s known t o  increase  * w i t h the b a s i c i t y o f the s l a g ( i n f a c t ,  the a c t i v i t y o f CaO )  ( 1 0 , 5 8 , 5 9 , 6 0 , 6 1 ) , one of the mechanisms by w h i c h d e s u l p h u r i z a t i o n may occur i n v o l v e s the removal o f SO^ by the atmosphere  (62,63,57,58)  making the l o s s a p p r o x i m a t e l y c o n s t a n t even i n s h o r t i n g o t s o f 100 mm o r so i n l e n g t h  (21,64).  IV.2.12 M e l t r a t e and s e g r e g a t i o n 9  The symbol f o r m e l t r a t e proposed e a r l i e r .  appears i n the m e c h a n i s t i c  equations  As a l l the v a r i a b l e s are i n t e r d e p e n d e n t , a d e f i n i t e  c o n c l u s i o n as t o the i n f l u e n c e o f t h i s parameter cannot be reached by s i m p l y examining i t s i n f l u e n c e i n an a n a l y t i c a l e x p r e s s i o n . An i n c r e a s e i n m e l t r a t e can be the r e s u l t of v a r i o u s causes w h i c h a l l tend to i n c r e a s e the heat f l o w g o i n g up the  electrode:;  o v e r a l l i n c r e a s e i n power, -  c l o s i n g o f the gap between the e l e c t r o d e and i n g o t  (higher  c u r r e n t , lower v o l t r e s u l t i n g i n an i n c r e a s e i n v o l u m e t r i c heat  :  generation), it  arcing.  C h d h u r y and a l . (58) r e p o r t t h a t AI2O.3, by r e d u c i n g the a c t i v i t y of CaO has an adverse e f f e c t on d e s u l p h u r i z a t i o n a t c o n s t a n t basicity. ou  - 132 -  When o n l y a n i n c r e a s e i n power i s c o n s i d e r e d , run  at a higher  processes  the p r o c e s s i s  temperature which means t h a t the mass t r a n s f e r  a r e a c c e l e r a t e d i n the s l a g and a t the i n g o t s u r f a c e .  e l e c t r o d e , by m e l t i n g ,  provides  The  i t s own temperature r e g u l a t i o n and  mass t r a n s f e r p e r u n i t time i s f a i r l y  constant  i n the m e t a l f i l m .  f o r e , i n c r e a s i n g the melt r a t e should  reduce c o m p o s i t i o n  There-  changes when  the r a t e c o n t r o l l i n g step i s i n t h e metal f i l m o f the e l e c t r o d e . C l o s i n g t h e gap between e l e c t r o d e and i n g o t w i l l c e r t a i n l y the e x t e n t  o f the h o t zone, thereby d e c r e a s i n g  limit  exchanges w i t h the  atmosphere. V a r i a t i o n s I n the melt r a t e r e a d i l y i n f l u e n c e t h e i n g o t s t r u c t u r e . An e x c e s s i v e melt r a t e w i l l , adverse e f f e c t on  f o r example, deepen t h e p o o l and have an  dendrite o r i e n t a t i o n .  I f v a r i a t i o n s a r e pronounced,  the volume p f the p o o l and the r a t e o f s o l i d i f i c a t i o n a r e a f f e c t e d which may l e a d to s e g r e g a t i o n . We have so f a r accepted r e f l e c t s the composition  the f a c t t h a t the i n g o t  composition  o f the m e t a l m e l t e d a t a p a r t i c u l a r time  without c o n s i d e r i n g homogenization o f the p o o l .  The absence o f  s e g r e g a t i o n when the f i n a l p o o l volume s o l i d i f i e s s u p p o r t s t h i s We c o u l d n o t d e t e c t a t e r m i n a l composition probe a n a l y s i s .  t r a n s i e n t by e l e c t r o n  A zone r i c h e r i n s o l u t e i s found t o be c o n f i n e d  l a s t ^ 100 u near the s u r f a c e and may correspond t o the l i m i t f o r d i f f u s i o n during  point:  to the  layer  solidification.  I f we r e t a i n t h i s v a l u e p f the l i m i t l a y e r , measured f o r I T i ] , the e f f e c t i v e d i s t r i b u t i o n c o e f f i c i e n t a t the end o f a r u n should  be ( 6 5 ) :  - 133 -  k  = e  — 1-k jS 1 + (-jJ- -) e D o  =  0.996  (Eq IV. 20) \  f  2  i  /  - t  _2 if  f  =6.10  cm/sec (assuming t h a t a p o o l 1.8 cm deep s o l i d i f i e s i n 30 s e c )  6  = IO cm (100 y) -4 2 D = 10 cm / s e c k ( T i i n Fe) = 0.4 (35) - 2  •,  Q  k  g  i s v e r y h i g h and e x p l a i n s why we do n o t o b s e r v e l o n g i t u d i n a l  s e g r e g a t i o n a t t h e end o f a r u n .  The v a l u e chosen f o r <5 i s r e l a t i v e l y  low. Chalmers (65) g i v e s v a l u e s from 0.1 cm f o r n a t u r a l c o n v e c t i o n -3 i n m e t a l s t o 10  cm f o r v i g o r o u s  stirring.  The o v e r a l l p i c t u r e o f a s m a l l s o l i d i f y i n g e l e c t r o s l a g i n g o t i s t h e r e f o r e t h a t o f a n e a r l y c o n s t a n t c o m p o s i t i o n i n t h e i n g o t and t h e m e t a l p o o l w i t h a l i m i t l a y e r r i c h e r i n s o l u t e near t h e s o l i f i c a t i o n interface.  As .-k  f o r t i t a n i u m i s p a r t i c u l a r l y low i t i s u n l i k e l y  t h a t major a l l o y i n g elements w i l l ever s e g r e g a t e  to a high extent  when t h e p o o l i s n o t s t i r r e d . IV.2.13 Comparison between s m a l l and l a r g e u n i t s We have s a i d e a r l i e r ( I I . I ) t h a t e l e c t r o s l a g u n i t s o f a wide s i z e range a r e i n r e g u l a r p r o d u c t i o n use.  I t i s therefore of interest  t o p o n s i d e r t h e i n f l u e n c e o f u n i t s i z e on t h e c h a r a c t e r i s i t i c s o f t h e process.  - 134 I f we we  consider  -  the s i z e v a r i a t i o n of s e v e r a l r e m e l t i n g  parameters  can q u a l i t a t i v e l y draw the f o l l o w i n g o u t l i n e : a)  B i g u n i t s u s u a l l y operate at a h i g h e r v o l t a g e ,  some cases 45-50 V or more.  As  reaching  in  the c u r r e n t d e n s i t y n e c e s s a r y to  melt the metal and m a i n t a i n a s a t i s f a c t o r y p o o l geometry i s approximately i n v e r s e l y p r o p o r t i o n a l to the diameter of the e l e c t r o d e i s an i n c r e a s e i n e l e c t r o d e - i n g o t  I = j B X  bath and As  A g  w i t h s i z e i f Ohm's law  there applies:  (Eq IV.21)  s  where I i s the i n g o t - e l e c t r o d e bath, I the c u r r e n t  gap  (10),  and  gap,  V the v o l t a g e  applied across  the  B the e f f e c t i v e c r o s s - s e c t i o n o f the s l a g  i t s conductivity.  inductive  (A.C.) or r e s i s t i v e l o s s e s i n the long power  p a r t l y account f o r the h i g h v o l t a g e  a p p l i e d to b i g u n i t s , we  leads  observe  t h a t I w i l l be approximately p r o p o r t i o n a l to the l i n e a r dimension of the u n i t , or the diameter of the c r u c i b l e (2R^). constant  X  g  is relatively  because the average temperature of the s l a g does not vary  the s i z e of the u n i t p r o v i d e d  the same metal i s remelted  (27).  with  Thus,  the i n f l u e n c e pf the e l e c t r i c f i e l d on d i f f u s i o n of i o n s i n t h e , s l a g w i l l decrease w i t h  (eq.  IV.5)  and hence w i t h the i n c r e a s e i n s i z e  of the u n i t . b)  From the p r e c e d i n g  geometry w i l l be  paragraph, i t i s apparent t h a t a comparable  found i n most ESR  u n i t s . . T h i s f o l l o w s i f the m e t a l  p o o l i s to remain s h a l l o w when compared to the mold diameter (2R^) , thus m a i n t a i n i n g the mold w a l l .  a s m a l l angle between the d e n d r i t i c s t r u c t u r e  and  - 135  -  The main g e o m e t r i c a l d i f f e r e n c e may r e s i d e i n the tip profile.  electrode  The a n g l e 9 ( I V . 2 . 4 ) a t the base of the cone becomes  p r o g r e s s i v e l y s m a l l e r as the e l e c t r o d e  i n c r e a s e s i n s i z e to r e a c h  the  p o i n t where s e v e r a l drops can form a t d i f f e r e n t p o i n t s of the t i p .  It  i s not y e t c l e a r whether t h i s s i t u a t i o n can be t r e a t e d as an assembly of s m a l l s i n g l e drop c)  electrodes.  H o l z g r u b e r (66)  reports  t h a t the c l a s s i c a l law of s o l i d i f i c a -  t i o n ( a c c o r d i n g t o w h i c h the depth of the s o l i d i f i c a t i o n f r o n t , w h i c h i s h e r e i t s d i s t a n c e from the mold w a l l , i s p r o p o r t i o n a l to the r o o t of time) i s observed i n e l e c t r o s l a g .  I f a constant  p o o l geometry  i s to be r e t a i n e d , i t can be i m m e d i a t e l y deduced t h a t the melt r a t e  i s p r o p o r t i o n a l to  i n v e r s e l y p r o p o r t i o n a l to R^.  and the r i s e of the  square  possible  interface  I n s m a l l u n i t s , l i k e the one  presently  u s e d , i t seems t h a t l o w e r m e l t r a t e s than would be expected from t h i s law are a c h i e v e d .  T h i s may r e s u l t from the f a c t t h a t l a r g e u n i t s  are  u s u a l l y r u n to a c h i e v e h i g h p r o d u c t i o n r a t e s r a t h e r t h a n a s h a l l o w • pool.  In f a c t ,  o r d e r o f 1.75 d) IV.2.11,  the exponent of R^ i n the r e l a t i o n s h i p  i n most  k a  R^ i s of  cases.  R e v i e w i n g now the mechanisms proposed i n S e c t i o n s  IV.2.2  the main assumption w i l l be t h a t the t r a n s i t i o n between;  l a m i n a r and t u r b u l e n t f l o w does n o t o c c u r when s i z e i s i n c r e a s e d . T h i s i s somewhat d o u b t f u l i n the s l a g phase because i t appears  ingots. The r e a c t i o n s w i t h the atmosphere  f o l l o w a r e l a t i o n s h i p of the  type:  i  that  the s l a g i s a l r e a d y i n the t r a n s i t i o n r e g i o n when r e m e l t i n g s m a l l  e)  tp  ( I V . 2 . 2 ) were found to  ;  the  - 136  JS  -  —  oc  f o r the mass f l o w of oxygen J S . The r e s u l t i n g change i n the m e t a l c o m p o s i t i o n b e i n g  ' -1 m  JS W  cc  * - l -1/2 t ' m e  SW  The a r e a o f c o n t a c t i s the annulus exposed to the w h i c h v a r i e s w i t h the " f i l l c o n s t a n t and  •  JS w;  r a t i o " R/Rj^ = f• 1 75  to v a r y as  1  -  a  -  f>  to be  ,  P^ 0  2  Assuming t  atmosphere  2 5  f i s o f t e n c l o s e r t o u n i t y i n b i g u n i t s and the r i g h t hand s i d e v a r i e s o n l y s l o w l y w i t h R^.  Hence, the oxygen p o t e n t i a l of the  slag  may i n c r e a s e s l o w l y w i t h the s i z e of the u n i t i f p r o p e r s h i e l d i n g i s not p r o v i d e d . f)  The d i f f u s i v e f l u x i n the e l e c t r o d e  causes a c o m p o s i t i o n change  A[0]  as  t  e  o r A[M] «  * - 2 / 3 ^5/3 amw R  R t ~ e 2  W ~ m  1 / 2  , ' and W m  1  cc R_1.75  We f i n d  A10] o r A M cc R  7 / 6  W  M  _ 2 / 3  cc  f  7 / 6  film  (IV.2.3-IV.2.4)  - 137  -  H e r e , to a f i r s t a p p r o x i m a t i o n , the l o s s depends o n l y on the ratio  (W  may a l s o change w i t h f, and t  the e l e c t r o d e g) results  depends upon the geometry  of  cone).  D i f f u s i o n at the s l a g i n g o t i n t e r f a c e  l e a d s to the same  as p a r a g r a p h (e) when a p p l i e d to the whole  A[0] o r A[M] <* R  h)  fill  M  cross-section;  0 , 2 5  The m a c r o s c o p i c model used f o r A . C . ( I V . 2 . 1 1 )  AIM] <x  KA  • _ i -P W„ e M  s  K A t / W  s  2 * 1.75 I f A « R^, .W^j. = R^ " and K i s independent of ' A [M] cc A F M 1  0.25  "Ps  K A t / W  gives:  size,  s  e  w h i c h i s a f u n c t i o n of R^ the maximum of w h i c h depends upon the v a l u e of the exponent, g)  < p l a y i n g a c r i t i c a l p a r t i n the v a r i a t i o n .  I f we t u r n to the few e x p e r i m e n t a l r e s u l t s  a v a i l a b l e i n the  l i t e r a t u r e , we f i n d t h a t the o r d e r of magnitude of the t o t a l observed i n our u n i t i s comparable t o o r h i g h e r than the  loss  reported  r e s u l t s when no p r e c a u t i o n such as an argon s h i e l d i n g o r d e o x i d a t i o n of the s l a g has been t a k e n .  H o l z g r u b e r r e p o r t s l o s s e s of TMn]  t  [Si]  and  [C] i n s i l i c o n d e o x i d i z e d s t e e l s w h i c h are of the o r d e r of 5 to 50% of the i n i t i a l l e v e l  (57).  H l i n e r y and Buzek (7) [Mn] amounting to 0.20%  r e p o r t e d a combined l o s s of  i n a low chromium s t e e l  (0.9%  [C], ISi] C, 1.17%  and  Cr).  - 138 -  IV.3  Review o f the e x p e r i m e n t a l r e s u l t s w i t h r e s p e c t to c o m p o s i t i o n changes  IV.3.1 I n f l u e n c e o f the atmosphere Ingots S l and S3 have been remelted under an argon b l a n k e t which lowers the oxygen p a r t i a l p r e s s u r e without e l i m i n a t i n g the s u p p l y o f oxygen.  Ingot S2 has been remelted i n a i r , b u t i n o t h e r r e s p e c t s ,  w i t h the same c o n d i t i o n s .  A l l t h r e e i n g o t s have undergone a comparabl  l o s s of [ T i ] , h a v i n g almost  i d e n t i c a l composition p r o f i l e s  By c o n t r a s t , i n g o t s remelted under pure argon  (fig.  18).  (S16 and 17, f i g .  26 and 27) show a l o s s of t i t a n i u m which i s s u b s t a n t i a l l y  lower.  We conclude t h a t the p a r t i a l p r e s s u r e of oxygen i n the atmosphere bears l i t t l e  r e l a t i o n t o the amount o f t i t a n i u m l o s t  from 321 s t a i n l e s  s t e e l , w h i l e the a c t u a l r a t e o f supply o f oxygen c o n t r o l s the l o s s , when t h i s i s below a c e r t a i n v a l u e . The same c o n c l u s i o n a p p l i e s t o Mar. 300 when i n g o t s Ml (argon , b l a n k e t ) , M2 ( a i r ) and M7 (pure oxygen) a r e compared. The r a t e a t which the atmosphere s u p p l i e s oxygen to the system as d e r i v e d from the r a t e o f o x i d a t i o n of i r o n observed w i t h the i n g o t s F l t o F5 i s somewhat h i g h e r than would account usually  f o r the l o s s e s  observed.  IV.3.2 I n f l u e n c e o f the s l a g c o m p o s i t i o n on 321 S.S. i n g o t s When the system  i s covered by an o x i d i z i n g atmosphere,  f a c t o r has a v a r i a b l e i n f l u e n c e , depending ;  this  m a i n l y upon the m e l t i n g  - 139  -  polarity. Ingots negative.  S l to S6 have been r e m e l t e d w i t h d i r e c t c u r r e n t ,  electrode  W h i l e the s l a g used f o r i n g o t S6 was pure CaF2» S l to S5  were m e l t e d u s i n g 25% C a O A ^ O ^ .  We have seen t h a t c a l c i u m a l u m i n a t e  c o u l d be s l i g h t l y o x i d i z i n g f o r t i t a n i u m at the b e g i n n i n g of a r u n ( I V . 1 . 6 ) when l i t t l e t i t a n i u m o x i d e i s as y e t p r e s e n t i n the s l a g . T h i s s h o u l d e x p l a i n why the l o s s of T i i n S l , S2, S3 d e c r e a s e s w i t h time ( f i g . 18)  to r e a c h a f i n a l l e v e l comparable w i t h S6 ( f i g . 19)  or  about 0.31% i n the m a t r i x . Ingots  S7 and S8 ( f i g . 20) have been r e m e l t e d w i t h a more o x i d i z i n g  s l a g , c o n t a i n i n g CaTiO^. i s s l i g h t l y lower (0.20  Despite t h i s f a c t , to 0.15%).  the m a t r i x l o s s of  T h i s may be e x p l a i n a b l e on the  b a s i s of a d i f f e r e n t m e l t r a t e however (see  IV.3.6),  i n w h i c h case the  CaTiO^ a d d i t i o n would be found to have no e f f e c t on the The e q u a t i o n proposed i n s e c t i o n  (IV.2.7)  a n o d i c m e t a l at the s l a g - m e t a l i n t e r f a c e )  loss.  ( D i f f u s i o n i n the  can r e a d i l y account f o r  approximate magnitude of the l o s s and the f a c t t h a t the l o s s approximately constant  [Ti]  throughout the m e l t .  Nevertheless,  r a t e of o x i d a t i o n ( t i t a n i u m and m a t r i x elements)  the  remains  the  total  as e v i d e n c e d by the  c o n c e n t r a t i o n of these o x i d e s i n the s l a g remains dependent upon the r a t e of oxygen p i c k up from the Ingots  atmosphere.  S9-S10 and S l l have been r e m e l t e d w i t h d i r e c t  electrode p o s i t i v e ( f i g . 21).  current,  They a l l show a s u b s t a n t i a l l o s s  of  t i t a n i u m w h i c h seems to have been almost independent of the s l a g composition. for i T i ]  I t i s probable that here,  the l i m i t i n g d i f f u s i o n c u r r e n t  ( I V . 2 . 6 ) was reached i n the e l e c t r o d e  film.  V a r y i n g the  - 140  t i t a n i u m content  -  of the s l a g seems to have produced no change i n the  r a t e of r e d u c t i o n o f  (Ti)  the p r e s e n c e of about 0.5%  at the cathode w h i c h remained n e g l i g i b l e i n (Cr) i n the  slag.  The models proposed i n S e c t i o n s ( I V . 2 . 4 ) f o r the f l o w of m e t a l on the e l e c t r o d e ,  combined w i t h the e q u a t i o n o f S e c t i o n ( I V . 2 . 6 )  d e s c r i b i n g the d i f f u s i v e f l u x of a l l o y i n g elements r e a d i l y account  i n t h i s r e g i o n can  f o r the approximate magnitude and c o n s t a n c y  observed l o s s e s ,  of  the  the t o t a l r a t e of o x i d a t i o n r e m a i n i n g dependent upon  the r a t e o f p i c k up of oxygen from the atmosphere as i n the p r e c e d i n g case. I n g o t s S12-S13 have been r e m e l t e d w i t h a l t e r n a t i n g Ingot in  current.  S12, w i t h a CaF^ s l a g behaves a c c o r d i n g to the model developed  (IV.2.11).  A curve f i t t i n g the e x p e r i m e n t a l r e s u l t s  w h i c h shows t h a t  the f l u x of  [ T i ] obeys e q u a t i o n ( I V . 1 9 ) where  f o l l o w i n g c o n s t a n t s have been used Area of r e a c t i o n Mass t r a n s f e r  (fig.  (ingot + electrode  coefficient:  O x i d a n t i n the s l a g :  (Ti>  K = 3.16 e  the  22): 2 33 cm , -1  t i p + drop): -3 x 10  cm sec  = 1.90%.  The melt r a t e was almost c o n s t a n t w h i c h g i v e s the parameters  is plotted,,  ,  .  •  d u r i n g the r u n , W = 2 . 1 g sec  of the e q u a t i o n s  § ~ 0.13  and ( T i ) / [ T i ] e  Q  4.2. The i n i t i a l amount o f o x i d a n t i n the s l a g i s a r e a l i s t i c  value  f o r a s l a g , such as the one u s e d , w h i c h has not been d e o x i d i z e d . The v a l u e o f the mass t r a n s f e r c o e f f i c i e n t , when t r a n s l a t e d i n t o a : D 8 5 10~*^ e q u i v a l e n t boundary l a y e r t h i c k n e s s : - —'-—7. g i v e s 6 of K  a p p r o x i m a t e l y 270 p.  -  - 141 -  The f a c t t h a t i n g o t S12 behaves as i f i t had been s h i e l d e d from the atmosphere may r e f l e c t  an o v e r e s t i m a t i o n i n the r a t e o f  w i t h the atmosphere c a l c u l a t e d i n ( I V . 2 . 2 ) .  exchange  The v o l a t i l i z a t i o n of  c a l c i u m as a means of exchange w i t h t h e atmosphere would t h e n e x p l a i n the d i f f e r e n c e between d i r e c t c u r r e n t and A . C . o p e r a t i o n (S12 and S6 f o r instance)  (see  IV.2.2).  W. H o l z g r u b e r and a l . (11) r e p o r t an i n g o t c o m p o s i t i o n p r o f i l e where the s i l i c o n d e c r e a s e s a t t h e s t a r t o f the m e l t .  I f we t a k e t h e  p a r t o f the p r o f i l e which v a r i e s w i t h t i m e , we see t h a t , s u b s t r a c t i n g the constant  component o f the l o s s  after  (make [ S i J  Q  \  - 0.12%  instead  o f the o r i g i n a l 0.20%), the t r a n s i e n t f o l l o w s c l o s e l y e q u a t i o n (Si) ( I V . 19) w i t h t h e f o l l o w i n g c o n s t a n t s : , • — = 4.7 (equivalent o x i d a n t : o 0.56% S i ) and <(> = 0 . 1 6 . The o n l y d i m e n s i o n r e p o r t e d : i s the d i a m e t e r e  l b l J  of t h e i n g o t , 2  2200 cm  43 cm and we can assume the a r e a of c o n t a c t , '  and t h e m e l t r a t e (57) W = 0 . 4 3 m  coefficient  ton/haur.  A -  The mass  transfer  comes t h e n to be remarkably c l o s e t o the v a l u e we have -3  calculated: <  - 3 . 3 6 x 10  -1 cm sec  I n g o t S13 was r e m e l t e d w i t h a s l a g c o n t a i n i n g 35.3% CaTiO^ and the d e p l e t i o n of the o x i d a n t i s not o b v i o u s on f i g . 2 3 . expected as ( T i ) / [ T i ] e  Q  i s at least  7.2, counting T i  4 +  T h i s i s to be as t h e o n l y  o x i d a n t , and t h i s would g i v e a n e g a t i v e o r i g i n t o e q u a t i o n ( I V . 1 9 ) w h i c h i s t h e r e f o r e meaning l e s s f o r the b e g i n n i n g o f t h e m e l t ,  ( f i g . 47).  the r a t e of l o s s i s the r e s u l t of the sum of t r a n s p o r t r a t e s a t t h e various slag metal interfaces  and t h e r e f o r e  s h o u l d be n e a r l y  constant  w i t h time. There i s l i t t l e d i f f e r e n c e between i n g o t s S16 and S17, w h i c h were r e m e l t e d under a r g o n .  The l o s s i s s m a l l but remains  Here  - 142  measurable.  -  The main d i f f e r e n c e i s i n the i n f l u e n c e of the s l a g on the  starting conditions.  The d e o x i d i z e d s l a g w h i c h c o n t a i n s no CaTiO^  s t i l l causes a h i g h e r l o s s of T i a t the s t a r t w h i l s t the aluminium a d d i t i o n has reduced some ( T i ) from the T i O ^ r i c h s l a g (compare f i g . 26 and 2 7 ) .  T h i s i s i n agreement w i t h e q u i l i b r i u m c o n s i d e r a t i o n s .  IV.3.3 I n f l u e n c e of the s l a g c o m p o s i t i o n on M a r a g i n g 300  steel  The r e s u l t s o b t a i n e d here agree q u a l i t a t i v e l y w i t h those found f o r 321 s t a i n l e s s  steel.  I n g o t s M l , M3, M4 ( f i g . 28) have been r e m e l t e d w i t h d i r e c t  current,  e l e c t r o d e n e g a t i v e , they e x h i b i t comparable l o s s e s o f m a t r i x t i t a n i u m w h i l s t the molybdenum c o n t e n t has v a r i e d l i t t l e .  The d i f f e r e n c e i n  c o m p o s i t i o n r e s u l t i n g from the use of d i f f e r e n t s l a g s i s not s i g n i f i c a n t . I f we compare t h e s e r e s u l t s w i t h the e q u i v a l e n t ones found f o r stainless steel  (IV.3.2),  we f i n d t h a t the magnitude o f the T i l o s s  i s i n c r e a s e d w i t h the c o n c e n t r a t i o n o f  [ T i ] i n the s t e e l .  This could  i n d i c a t e t h a t the r a t e l i m i t i n g s t e p i s a l s o d i f f u s i o n i n the m e t a l at the s l a g - i n g o t i n t e r f a c e  (IV.2.7).  The i n c r e a s e i n A [ T i ] i s  greater  than t h a t of [ T i ] because of v a r y i n g p r o p e r t i e s o f the s t e e l s ( d i f f u s i o n c o e f f i c i e n t , v i s c o s i t y , melting temperature). I n g o t s M5 and M6 r e m e l t e d w i t h d i r e c t c u r r e n t , e l e c t r o d e p o s i t i v e are a l s o i n d i s t i n g u i s h a b l e from each o t h e r on the b a s i s o f s l a g composition.  The l o s s of T i i s l o w e r than i n the e l e c t r o d e  case and lower even than t h a t o b t a i n e d w i t h S . S . conditions.  negative  321 i n comparable  I n a d d i t i o n to the p h y s i c a l p r o p e r t i e s of the s t e e l ,  s l a g - m e t a l i n t e r f a c i a l t e n s i o n may v a r y w i t h the m e t a l c o m p o s i t i o n  the  - 143  -  and w i l l r e s u l t i n d i f f e r e n t l o s s e s  i n the two c a s e s .  In these cases,  a r i m of s o l i d i f i e d s l a g was observed over p a r t of the e l e c t r o d e around the drop a r e a . f o r 321 S . S .  cone  T h i s i m p l i e s t h a t a l t h o u g h the m e l t i n g c o n d i t i o n s  and M a r . 300 were h e l d c o n s t a n t ,  the l o c a l heat  flux  around the e l e c t r o d e was d i f f e r e n t i n the two c a s e s . T h i s would l e a d t o a d i f f e r e n c e i n c h e m i c a l l y a c t i v e a r e a between the two c a s e s , M a r . 300 b e i n g exposed to the s l a g f o r a much s h o r t e r time and over a s m a l l e r a r e a .  Thus the t i t a n i u m r e a c t i o n c o u l d f o l l o w  the model o f S e c t i o n ( I V . 2 . 6 ) as does S . S . lower m e l t l o s s o f t i t a n i u m .  321, w h i l s t l e a d i n g t o a  The a d d i t i o n a l oxygen brought i n by the  atmosphere would then cause the o x i d a t i o n of some i r o n over the titanium depleted surface,  l e a d i n g t o the observed h i g h content  of  (FeO) i n the s l a g .  IV.3.4 I n f l u e n c e of the s l a g c o m p o s i t i o n on 1409 A l s t e e l A l t h o u g h the m e l t c o n d i t i o n s were not as w e l l c o n t r o l l e d as w i t h the o t h e r s t e e l s ,  i t i s g e n e r a l l y apparent t h a t the s l a g c o m p o s i -  t i o n had l i t t l e e f f e c t on the r a t e of aluminium l o s s . expected because  T h i s was  the s l a g c o n t a i n s i n i t i a l l y no o x i d a n t and the  of l o s s i s c o n t r o l l e d by the r a t e of t r a n s f e r t h r o u g h the d e o x i d i z e d s l a g . c o n s t a n t r a t e of l o s s  rate  (or s u p p l y ) of oxygen  I n t h e s e c o n d i t i o n s , we would observe a  (model I V . 2 . 2 ) .  The f i n a l l o s s i s s l i g h t l y h i g h e r than t h a t observed f o r T i w i t h the o t h e r s t e e l s  (see r e s u l t s i n I I I . 1 3 ) .  - 144  -  IV.3.5 I n f l u e n c e of the s l a g c o m p o s i t i o n on the o x i d a t i o n of Fe I n g o t s F l to F5 have been r e m e l t e d under v a r i o u s s l a g s direct p o l a r i t y .  With i r o n ,  the s l a g s h o u l d be r a t e The presence  the r a t e of t r a n s f e r of oxygen t h r o u g h  limiting.  i n the s l a g of an i n c r e a s i n g amount of CaTiC^ has  been found to cause a p r o g r e s s i v e i n c r e a s e  on the t o t a l amount of  o x i d a t i o n as e v i d e n c e d by i n g o t s F2 to F 5 , i n t h a t o r d e r The o x i d a t i o n was h i g h e s t  with  (III.14).  was about 20% h i g h e r when the c o n c e n t r a t i o n of CaTiO^  (52%)  than was the case f o r pure CaF2«  I t i s l o g i c a l t h a t the p r e s e n c e of a redox c o u p l e i n the should increase A constant  the r a t e of t r a n s f e r w i t h the atmosphere  slag  (IV.2.2).  component of the o x i d a t i o n would be the d i r e c t o x i d a t i o n  of the e l e c t r o d e  (IV.2.1).  Ingot F l ( F e r r o v a c E) shows a h i g h e r r a t e than F4 (1018 s t e e l ) w i t h v i r t u a l l y t h e same s l a g .  The o x i d a t i o n of  [MnJ may have p l a y e d a r o l e i n the f i n a l m a t e r i a l  mild  [C], [Si]  and  balance.  IV.3.6 I n f l u e n c e of the m e l t r a t e The e f f e c t of melt r a t e can o n l y be observed w i t h the series a)  of  following  experiments: Ingots  S l , S2, S 3 , S6, S7, S8, a l l r e m e l t e d i n  electrode  n e g a t i v e c o n d i t i o n s showed o n l y a s m a l l i n f l u e n c e of the s l a g c o m p o s i t i o n on A l T i ] . against  F i g . 53 shows t h i s same parameter  A[Ti] plotted  the m e l t r a t e at v a r i o u s l e v e l s o f these i n g o t s .  The c u r v e  p l o t t e d i s A [ T i ] x W = k w h i c h i s e q u i v a l e n t to d i f f u s i o n c o n t r o l a t  - 145 the s l a g - i n g o t i n t e r f a c e (IV.2.2).  -  ( I V . 2 . 7 ) o r at the s l a g - a t m o s p h e r e  k has been found from the l e a s t  the s t r a i g h t  interface  square f i t r e l a t i o n s h i p on  line  A[Ti] = k x i  W  -  m  The f a c t t h a t A [ T i ] seems to be s l i g h t l y h i g h f o r the h i g h v a l u e s of W may r e s u l t from the a c c e l e r a t i o n of mass t r a n s f e r a t the h i g h temperatures a s s o c i a t e d w i t h h i g h m e l t  rates.  The bottom s e c t i o n s of i n g o t s S l , S2, S3 have been e x c l u d e d because the e q u i l i b r i u m r a t i o o f T i / A l was not a c h i e v e d i n the s l a g b)  The same r e l a t i o n s h i p drawn f o r Maraging 300 i n g o t s M 1 , M 3 ,  M4 l e a d s t o i n c o n c l u s i v e r e s u l t s because of a h i g h c)  (IV.3.2).  I n g o t s S9, S10,  scatter.  S H show no i n f l u e n c e o f the m e l t r a t e on the  l o s s o f i T i ] ( e l e c t r o d e p o s i t i v e ) . The r e l a t i o n s h i p s proposed i n . ( I V . 2 . 5 ) to IV.2.5).  ( I V . 2 . 7 ) suggest t h a t A [ T i ] s h o u l d v a r y as W  '  (see  m As the l o s s was v e r y h i g h , i t i s p o s s i b l e t h a t p a r t of  e l e c t r o d e f i l m was d e p l e t e d  the  w h i l e the r e g i o n near the t i p r e t a i n e d ; '  some I T i ] .  >  -2/3  I n t h i s case the l o s s would t h e n be A [ T i ] = k + k '  k b e i n g l a r g e and k ' b e i n g c o n c e a l e d by the s c a t t e r on A l T i ] . IV.3.7 I n f l u e n c e of the oxygen p o t e n t i a l of the s l a g The e f f e c t o f d e o x i d i z i n g the s l a g i s most o b v i o u s when redox c o u p l e s are  present.  The c o n t i n u o u s d e o x i d a t i o n of the s l a g i n i n g o t s S14 and S15  ,  - 146  has caused p r o g r e s s i v e i n c r e a s e the i n i t i a l v a l u e .  -  i n the  [ T i ] c o n t e n t of the i n g o t s  The f i n a l c o m p o s i t i o n of i n g o t S15 shows t h a t  r e d u c t i o n has been almost s t o i c h i o m e t r i c , i n d i c a t i n g t h a t the  above the  capacity  of the s l a g f o r t r a n s p o r t i n g oxygen has been g r e a t l y r e d u c e d . When r e m e l t i n g 1409 A l , i t i s g e n e r a l l y apparent t h a t [ A l ] tends to d e c r e a s e w i t h t i m e .  This i s consistent with  p r o g r e s s i v e d e p l e t i o n of o x i d a n t s i n a s l a g h a v i n g l i t t l e c a p a b i l i t i e s f o r oxygen.  the l o s s  of  the transport  E v a p o r a t i o n of p a r t of the a l u m i n i u m makes  any d i s c u s s i o n d i f f i c u l t however, as i t has f i n a l l y the same i n f l u e n c e as the e v a p o r a t i o n of c a l c i u m mentioned e a r l i e r .  IV.3.8 I n f l u e n c e of r e m e l t i n g p o l a r i t y T h i s i n f l u e n c e has a l r e a d y been d i s c u s s e d on s e v e r a l ( S e c t i o n s I V . 3 . 1 to  occasions  IV.3.7).  A marked d i f f e r e n c e was found between r e m e l t i n g w i t h d i r e c t o r reverse p o l a r i t y (D.C.)  (see  IV.3.2-IV.3.3).  l o s s of t i t a n t i u m and the oxygen c o n t e n t , p a s s i n g from 321 S . S .  The i n f l u e n c e on the  however, was r e v e r s e d when  to Maraging 300.  When u s i n g a n e g a t i v e e l e c t r o d e ,  the l o s s e s  are comparable f o r  two m e t a l s i f we take i n t o account the d i f f e r e n c e i n i n i t i a l tion  the  concentra-  (IV.3.3). When u s i n g a p o s i t i v e e l e c t r o d e ,  lower w i t h Maraging 300 s t e e l hand i s l a r g e r .  the observed l o s s i s s i g n i f i c a n t l y  (M5, M6).  The oxygen c o n t e n t on the  other  Delayed p r e c i p i t a t i o n f o r o x i d e s may have a l l o w e d the  p i c k up of oxygen i n the e l e c t r o d e  film (IV.2.6),  most of the d e o x i d a t i o n  147  -  -  t a k i n g p l a c e i n the p o o l where the m e t a l i s c a t h o d i c a l l y When u s i n g A . C . power, i t i s found ( i n g o t s observed o x i d a t i o n i s more c o n s i s t e n t  p o s i t i v e and n e g a t i v e  the d i f f e r e n c e  electrode  S12-S13) t h a t t h e  w i t h an o x i d a t i o n mechanism w h i c h  does not i n v o l v e p o l a r i z a t i o n o f the e l e c t r o d e s W i t h 1409 A l i n g o t s ,  protected.  (IV.2.11-IV.3.2).  between d i r e c t c u r r e n t .  was n o t found to be  significant,  presumably because v o l a t i l i z a t i o n o f aluminium i s t h e predominant t r a n s f e r mechanism, w h i c h would be independent o f p o l a r i t y .  IV.3.9 I n c l u s i o n and oxygen c o n t e n t s I t has been suggested  (67,68)  t h a t i n c l u s i o n s a r e removed from  the m e t a l by f l o t a t i o n i n the m e t a l p o o l .  This conclusion i s arrived  at by comparing t h e v e l o c i t y of r i s e o f the i n c l u s i o n s , c a l c u l a t e d S t o k e ' s l a w , w i t h the v e l o c i t y of t h e s o l i d i f i c a t i o n i n t e r f a c e .  from  This  g i v e s an upper s i z e l i m i t f o r the i n c l u s i o n s r e t a i n e d i n a g i v e n electroslag  ingot  (67,69).  Other w o r k e r s suggest however t h a t i n c l u s i o n s a r e most l i k e l y to be removed when they come i n c o n t a c t w i t h t h e s l a g , f i l m on t h e e l e c t r o d e  o r i n the drop  (15,52,69).  For a s t e e l h a v i n g v e r y h i g h counts of c a r b i d e o r inclusions  i n the l i q u i d  carbo-nitride  (1409 A l ) , we f i n d t h a t o n l y l i t t l e removal i s  achieved.  The i n i t i a l s i z e of t h e i n c l u s i o n s i s n e a r , b u t n o t c l e a r l y a typical cut-off  size for flotation.  f o r a normal E . S . m e l t  above  T h i s would be i n the range 15-20 u  (69).  T i ( C , N ) i n c l u s i o n s are s o l u b l e i n the s t e e l at h i g h  temperature.  - 148 -  The time of d i s s o l u t i o n i n the l i q u i d m e t a l c a n be a p p r o x i m a t e l y c a l c u 2 l a t e d by the c l a s s i c a l r e l a t i o n s h i p : t ^ — f o r an i n c l u s i o n o f diameter u . -4 of 10  Diffusion coefficients  i n l i q u i d m e t a l s a r e o f the o r d e r  2 - 1 cm sec  and t h e l i f e of a 10 u i n c l u s i o n s h o u l d n o t exceed  a f r a c t i o n o f a second.  In short,  a T i ( C , N ) i n c l u s i o n would p r o b a b l y  d i s s o l v e i n the metal before reaching the p o o l . The oxygen c o n t e n t  of 1409 i n g o t s was n e g l i g i b l e and no  o x i d e i n c l u s i o n c o u l d be d e t e c t e d , calculations  i n agreement w i t h t h e d e o x i d a t i o n  (IV.1.6).  The r a t e a t w h i c h oxygen d i f f u s e s i n t o t h e s t e e l of o x i d e p r e c i p i t a t i o n a r e t h e p r i n c i p a l f a c t o r s count o f o x i d e p a r t i c l e s found i n the m e t a l .  and t h e k i n e t i c s  c o n t r o l l i n g the f i n a l  I n b o t h 321 S . S . and  Mar. 300, t h e amount of oxygen found i n t h e s t e e l (y 100 ppm) when compared w i t h t i t a n i u m c o n t e n t  remains l o w  (2000 ppm) and does n o t  s t o i c h i o m e t r i c a l l y account f o r t h e d i f f e r e n c e observed between m a t r i x and t o t a l c o n c e n t r a t i o n s carbide content.  w h i c h i s thus l a r g e l y r e p r e s e n t e d by the  The range o f oxygen c o n c e n t r a t i o n s  observed i n t h e  i n g o t s i s compared w i t h t h e d e o x i d a t i o n e q u i l i b r i a on f i g u r e s 42 and 43. I n a l l c a s e s , the c o n c e n t r a t i o n i s l o w e r than t h a t imposed by t h e m a t r i x (see chromium l i n e on f i g . 4 2 ) . Having argued t h a t t h e m e t a l s u r f a c e was not c o m p l e t e l y  depleted  i n d e o x i d a n t when a l t e r n a t i n g c u r r e n t i s u s e d , we s h o u l d expect a lower oxygen content w i t h t h i s mode, i f the i n c l u s i o n c o n t e n t ingot r e s u l t s  from a d i f f u s i v e f l o w o f oxygen.  of the  T h i s i s observed  on f i g . 42 (321 S . S . ) b u t n o t on f i g . 43 (Mar. 3 0 0 ) . The i n c l u s i o n counts g i v e n i n S e c t i o n s  I I I . 1 1 and I I I . 1 2  indicate  - 149  -  t h a t t h e r e e x i s t s a q u a l i t a t i v e c o r r e l a t i o n between a h i g h oxygen content  (80 ppm o r more) and the presence of l a r g e o x i d e i n c l u s i o n s  (5 to 10 y and > 10 y ) . I n most c a s e s , t h e r e was a s i g n i f i c a n t d e c r e a s e i n the T i ( C , N ) c l a s s f o r s i z e s > 5 y , the e x t e n t o f w h i c h v a r i e d w i t h the s l a g and the p o l a r i t y u s e d , t h i s may p o s s i b l y be due to v a r i a t i o n s i n the t h e r m a l h i s t o r y of t h e . i n g o t . A number of w o r k e r s have suggested a c o r r e l a t i o n between s l a g c o m p o s i t i o n ( c o n c e n t r a t i o n and type o f o x i d e components) and the n a t u r e and count o f i n c l u s i o n s ( 6 7 , 4 ) .  I n the p r e s e n t s y s t e m , where the  d e o x i d a t i o n r e a c t i o n i s w e l l d e f i n e d , we f i n d t h a t the o x i d e i n c l u s i o n s c o n t a i n almost e x c l u s i v e l y t i t a n i u m . Choudhury (58)  r e p o r t s a marked decrease i n the oxygen c o n t e n t  o f the m e t a l w i t h an i n c r e a s e i n A ^ O ^ content i n C a F ^ - C a O - A l ^ O ^ w h i l e the i n c l u s i o n count remains u n a f f e c t e d .  slags  The b a s i c i t y o f the s l a g ,  on the o t h e r h a n d , has no v i s i b l e e f f e c t . I f we c o l l e c t our r e s u l t s o b t a i n e d w i t h 321 S . S .  ingots remelted  i n e l e c t r o d e n e g a t i v e c o n d i t i o n s , we f i n d a comparable r e l a t i o n s h i p . Fig.  54 r e p r e s e n t s  the i n f l u e n c e o f the A l ^ O ^ c o n t e n t o f s l a g s i n the  system CaF2~CaAl20^ and the i n f l u e n c e of the T i C ^ c o n t e n t i n the system C a F 2 ~ C a T i 0 ; b o t h o x i d e s cause a s i m i l a r r e d u c t i o n o f the oxygen 3  l e v e l i n the i n g o t , w h i c h cannot be accounted f o r by a change i n ,the melt  rate.  - 150  -  I V . 3.10 Importance  o f the a n a l y s i s methods  (see Appendix IV)  When a n a l y s i n g the m e t a l , we have t u r n e d our a t t e n t i o n b o t h the m a t r i x c o n t e n t of r e a c t i v e  elements (microprobe  and the t o t a l c o n t e n t ( s p e c t r o g r a p h s  toward  analysis)  or wet c h e m i c a l a n a l y s i s ) .  d i s t i n c t i o n i s i m p o r t a n t because of the need to d i f f e r e n t i a t e  This  between  the a l l o y i n g elements w h i c h are c o n t a i n e d i n i n c l u s i o n s and those w h i c h are a v a i l a b l e to f u l f i l a s t r u c t u r a l f u n c t i o n i n the m a t r i x ( p r e c i p i t a t i o n hardening  etc...).  T h i s d i s t i n c t i o n i s somewhat a r b i t r a r y i n the case of p r e c i p i t a t i o n since  t h e r e would be c a r b i d e p r e c i p i t a t e s  which would r e g i s t e r  as a m a t r i x c o n t e n t i n m i c r o p r o b e  carbide  i n a size  range  analysis.  However, our d i s s o l u t i o n c a l c u l a t i o n i n d i c a t e s that s i z e ranges observed i n b o t h the e l e c t r o d e tion reaction,  and the i n g o t r e p r e s e n t  and t h e r e f o r e the m a t r i x t i t a n i u m c o n t e n t and the;  c a r b i d e i n c l u s i o n c o n t e n t bear a c o n s t a n t It  appears  an e q u i l i b r i u m p r e c i p i t a -  relationship.  t h a t i n most i n g o t s the amount of element  (titanium)  t i e d up i n o x i d e i n c l u s i o n s i s always s m a l l (100 ppm of oxygen r e a c t s w i t h 200 ppm T i ) .  C a r b i d e s are u s u a l l y i n h i g h e r  concentrations  w i t h a r e l a t i v e l y c o n s t a n t s i z e d i s t r i b u t i o n from sample to The same m e c h a n i s t i c element a n a l y s i s ,  conclusions  can be drawn from the  as from m a t r i x t i t a n i u m a n a l y s i s .  an answer to one of the o r i g i n a l q u e s t i o n s ,  This  sample. total constitutes  the t i t a n i u m l o s s does not  r e p r e s e n t s i m p l y a removal of o x i d e i n c l u s i o n s .  Any change i n s i z e  d i s t r i b u t i o n of c a r b i d e i n c l u s i o n s between the e l e c t r o d e  and the  ingot  i s not l i k e l y to r e p r e s e n t a s o l u t i o n of c a r b i d e i n c l u s i o n s but m e r e l y different precipitation  conditions.  CHAPTER V CONCLUSIONS The p r e s e n t o b s e r v a t i o n s c o n f i r m t h a t a l o s s o f r e a c t i v e f o r an a l l o y s t e e l can o c c u r d u r i n g e l e c t r o s l a g r e m e l t i n g .  elements  I n the  experiments p e r f o r m e d , t h i s l o a s was between <v> 5 and 80% o f the content w h i c h was 0.5  to 4 wt %.  original  I n many c a s e s , the v a r i a t i o n o f  c o m p o s i t i o n f e l l o u t s i d e the range w h i c h would be r e q u i r e d by commercial specifications. The o x i d a t i v e l o s s i s comparable i n magnitude w i t h the r a t e of o x i d a t i o n of pure i r o n i n s i m i l a r c o n d i t i o n s but may be l o w e r i n some cases where the m a t r i x i s a l s o o x i d i z e d .  When the l i m i t i n g r a t e o f  o x i d a t i o n i n a i r was a c h i e v e d , 0 . 3 to 0.6 wt % o f the t o t a l m e t a l was l o s t d u r i n g the r e m e l t i n g o p e r a t i o n u s i n g D . C .  I n the cases where  s l a g and m e t a l c o n s t i t u t e a c l o s e d s y s t e m , a m a t e r i a l b a l a n c e can be w r i t t e n w i t h a s a t i s f a c t o r y p r e c i s i o n ; when p a r t i a l v o l a t i l i z a t i o n  of  a component (aluminium) o r e x c e s s i v e s e g r e g a t i o n o f the i n g o t o c c u r s , d i s c r e p a n c i e s i n the m a t e r i a l b a l a n c e s are o b s e r v e d . Q u a n t i t a t i v e or s e m i - q u a n t i t a t i v e models have been developed w h i c h can account f o r the observed r a t e of t r a n s f e r a c r o s s interfaces.  These a r e :  - Atmospheric o x i d a t i o n o f the  the  following ;  electrode.  - 152  -  - D i f f u s i o n of oxygen and a l l o y i n g elements of the e l e c t r o d e  (electrode  i n the l i q u i d  film  positive).  - D i f f u s i o n of oxygen and o x i d i z a b l e s p e c i e s at the  slag-atmosphere  interface. Models i n v o l v i n g unknown parameters  a r e proposed f o r  the  following: - D i f f u s i o n t h r o u g h the s l a g - i n g o t  interface.  - Rate of r e a c t i o n w i t h an o x i d a n t i n l i m i t e d s u p p l y Two d i s t i n c t f a c t o r s  i n the s l a g c o m p o s i t i o n have been found to  i n f l u e n c e the l o s s of r e a c t i v e of an o x i d a n t , FeO, S±0^  (A.C).  elements.  The f i r s t one i s the p r e s e n c e  or H O 2 w h i c h , b e i n g i n l i m i t e d s u p p l y  w i l l cause a d e c r e a s i n g l o s s w i t h time i f the system i s c l o s e d .  The  second f a c t o r i s the presence of an element w i t h m u l t i p l e v a l e n c y s t a t e s , such as T i  3+  atmospheric oxygen.  /Ti  4+  or Fe  2+  3+  /Fe  w h i c h a c t s as a c a r r i e r  for  A c o l d s l a g s t a r t i n a i r w i l l n o r m a l l y produce  enough i r o n o x i d e to i n i t i a t e such a  process.  The c o n v e c t i v e mass f l o w of oxygen b r o u g h t t o the m e l t r e g i o n by the atmosphere i s the most i m p o r t a n t f a c t o r S i n c e b o t h o x i d a t i o n of the e l e c t r o d e  i n the o x i d a t i o n  and of the s l a g o c c u r a t  l i m i t i n g r a t e s f o r v e r y low p a r t i a l p r e s s u r e s interface, reactions  rate.  at the  their  atmospheric  the e q u i l i b r i u m oxygen p a r t i a l p r e s s u r e o f the o x i d a t i o n i s n o r m a l l y exceeded i n open atmosphere m e l t i n g .  E l e c t r o c h e m i c a l p r o c e s s e s p l a y an i m p o r t a n t r o l e i n d i r e c t  current  o p e r a t i o n , i n t h a t a n o d i c p o l a r i z a t i o n i s a c h i e v e d by p r o g r e s s i v e s a t u r a t i o n i n o x i d e components a t the s l a g s i d e of the anode, o n l y a s m a l l c o n c e n t r a t i o n of o x i d e i o n s i s p r e s e n t h i g h r a t e of o x i d a t i o n i s t h e r e f o r e  even.if  i n the s l a g .  A  a c h i e v e d at the anode, w h i c h i n the  -  153  -  case of low c o n c e n t r a t i o n r e a c t i v e elements processes  i n the m e t a l .  i s c o n t r o l l e d o n l y by d i f f u s i o n  A t the c a t h o d e , r e d u c t i o n of the l e a s t  stable  o x i d e s of the s l a g o c c u r s at a r a t e w h i c h i s d i m i n i s h e d by a t m o s p h e r i c o x i d a t i o n of the s l a g .  Both t h e s e e l e c t r o c h e m i c a l p r o c e s s e s  a n e t l o s s of r e a c t i v e e l e m e n t s . of l e s s r e a c t i v e elements transfer  toward the  I n most c a s e s , the s l a g  result i n  concentration  s t a b i l i z e s r a p i d l y a c c o r d i n g to t h e i r r a t e of  cathode.  When a l t e r n a t i n g c u r r e n t slag metal interfaces  (60 Hz) i s u s e d , t r a n s f e r  through the  o c c u r s at a l o w e r r a t e w h i c h seems to be  independent of p r o c e s s c u r r e n t d e n s i t y .  As mass t r a n s f e r  same d i r e c t i o n over the whole a r e a of s l a g - m e t a l c o n t a c t ,  ;  o c c u r s i n the the  net  r e s u l t i s comparable i n magnitude w i t h d i r e c t c u r r e n t o p e r a t i o n .  The  r a t e of a b s o r p t i o n of oxygen by the s l a g from the atmosphere i s r e d u c e d , r e s u l t i n g i n a l o s s w h i c h d e c r e a s e s w i t h the d e p l e t i o n of oxidant.  initial  The r e l a t i o n s h i p proposed to e x p l a i n t h i s c o m p o s i t i o n p r o f i l e  seems to be d i r e c t l y t r a n s f e r a b l e a b l e mass t r a n s f e r The a d d i t i o n of  to d i f f e r e n t s i z e i n g o t s w i t h compar-  coefficients. ( T i C ^ ) to the s l a g , as has been suggested  does not seem, per s e ,  (9),  to reduce the r a t e of o x i d a t i o n of t i t a n i u m .  Continuous d e o x i d a t i o n of t h e s l a g w i t h aluminium can cause the t r a n s f e r of t i t a n i u m from a ( T i C ^ ) r i c h s l a g to the m e t a l and a l s o reduce the r a t e a t w h i c h a t m o s p h e r i c oxygen i s a c c e p t e d by redox reactions  i n the s l a g .  - 154  Appendix I . 1.  -  Flow o f m e t a l i n the e l e c t r o d e  film  C a l c u l a t i o n o f the exposure t i m e i n the f r e e f l o w case  t  =  e  Equation (IV.7)  g i v e  f°  d  _ R cos9  s  (IV.2.4)  x  v . xi  v . = P , g  XI  9  s l n  44ry  S  2  m  o r , r e p l a c i n g 6* by i t s e x p r e s s i o n ( I V . 11)  =  xi  Pg  s  l  •n  6  o  ,  3/ \/  2u  3p W „ m m .2 2irpg s i n e cos 0 -)  (  V  m  1  —J U  K S  Therefor, t . - 2  2 2„ x cos 6 . 2  .  y  R  x  V — i  1  '  '  3  m  "  ,  x ' <l 2  3  cos 9  1^  I f we r e p l a c e -  = A , t h e e q u a t i o n becomes '  cos 0  t  M  =2 ...) / (...) / 2  (  3  1  ° x/a -  3  2  f  3  A  or nr-  9 f( . . . ) -- -2  tr  .  2  /  3  (...)  / '  .  1  /  3  A /3 A 4  /, o A  4r  2 / 3  dx  A  , .2 - x 2), - 2 / 3 dx , x 2 / 3(A  The i n t e g r a l reduces to the E u l e r i a n i n t e g r a l B e t a :  /  o  ...dx = 1/2A  .1/3 1  r  ,  l  k  B(  1 / 3  r  2 / 3  2  +  1  ,  - 2/3 + 1) =  (5/6) r d / 3 ) (7/6) r  i s a t a b u l a t e d f u n c t i o n (70).  l/2A  1 / 3  B(5/6,l/3)  ;  0  - 155 -  Therefore:  e  2.  6,2/3  ,2TTCOS 3  ^  f  m  m ) / ( 1 pg s i n B cos Q l  y  }  3  R  }  l  Flow p a t t e r n c o n s i d e r i n g momentum t r a n s f e r See f i g u r e  (Al.l)  5  /  3  1-129 x 2.678 0.9027  w i t h the s l a g .  f o r a d e s c r i p t i o n o f the geometry o f the  system. L e t us c o n s i d e r a s m a l l annulus o f l e n g t h dx around t h e cone a t distance  x from t h e t i p , t h e t h i c k n e s s  of the f i l m being 6 ^ , the  o r i g i n o f y can be t a k e n as t h e m e t a l - s l a g a)  The m e t a l f i l m obeys the c l a s s i c a l  p r o f i l e of v e l o c i t i e s , IV.2.4  interface.  equivalent  p a r a b o l i c l a w f o r the  t o t h e r e l a t i o n used f o r  b u t r e w r i t t e n f o r the f o l l o w i n g boundary  / =— 7  v . .6  V  f  X  /  x  d y  6  m  A  o  = W (1 -  dy  2  "  r,2  R  2  in  conditions:  Q  ° ) (The t o t a l f l o w i s t h e , . m e l t r a t e a t x)  T h i s l e a d s t o v = f u n c t i o n o f v . , y , W and the v a r i o u s x x,i m geometrical  and p h y s i c a l c o n s t a n t s  6:  FT  1  (v  ., x , i '  6,  '  x, W) = 0 ' m  ( 6 , p , u ^ . . . ) and an e x p r e s s i o n f o r  -  b)  The  156  -  s l a g d e v e l o p s a l a m i n a r boundary l a y e r , o v e r the  i n the case r e p r e s e n t e d , v  i s d i r e c t e d upwards and  electrode,  the boundary l a y e r  oo  thickness  d increases  toward the base of the  cone.  To t r e a t t h i s k i n d of problem, the v e l o c i t y p r o f i l e i s u s u a l l y assumed e m p i r i c a l l y ( r e f . 25,  p. 144);  v -v . = v (3/2n - l/2n x X,l » .'  3  )  we may  s e l e c t f o r example:  where n =  y/d  d i s a f u n c t i o n of the p h y s i c a l c o n s t a n t s of the s l a g ( u , 6, v  ,..)  g  and x, l e a d i n g to the  F (v 2  c) (v  x  x j i  relation  , d, x) - Q  At the s l a g - m e t a l boundary, we have a l r e a d y w r i t t e n  ^)g^ g>  a d d i t i o n , dynamic e q u i l i b r i u m r e q u i r e s  i n  a  momentum between the phases.  I f we  call T  x  (v  .)  conservation  t h i s shear f o r c e per  -= of  unit  area  (T  x  j L  ) s l a g = (x  x  i  )metal  Sv °  r  y  s  X ly"  ... .3v I  !  =  - »m W  X  y—K)  | I.  <  y—>• 0  T h i s l e a d s to a t h i r d e q u a t i o n a l s o c o n t a i n i n g constants:  F (v 3  i )  . , d, 6,  x, W ) m  - 0  the v a r i o u s  physical  - 157 d)  -  The t h r e e e q u a t i o n s F ^ , F2, F^ can be s o l v e d f o r a g i v e n  v a l u e of the m e l t r a t e W , l e a d i n g , i f d and <5 are e l i m i n a t e d , to a m p r o f i l e f o r the v e l o c i t y at the i n t e r f a c e : 0  v  . = x,i  f(x)  I n t e g r a t i o n of t h i s p r o f i l e over x g i v e s the exposure time  ;  R cos 8  ^ — x,i  = t  V  Equation F^,  and F^ are p o l y n o m i a l s of the t h i r d power (maximum)  i n the v a r i a b l e s v  x,  . , d , 6 , and x .  1  The unknown c o n s t a n t s  They can be s o l v e d n u m e r i c a l l y ,  i n the equations are p r i n c i p a l l y v ^ , w h i c h  would r e q u i r e a good knowledge of c o n v e c t i o n p a t t e r n s e l e c t r o d e and u  g  w h i c h depends upon temperature  The f a c t t h a t the i n t e r f a c e  against  the  gradients.  i s not everywhere convex w i l l  certainly  i n t e r f e r e w i t h d , m a i n l y near the d r o p . I n the absence of f u r t h e r d a t a on v , n u m e r i c a l c a l c u l a t i o n s CO  n o t attempted  here.  are  - 158 -  Appendix I I .  S e g r e g a t i o n i n ESR  We a r e concerned here w i t h g r o s s l o n g i t u d i n a l s e g r e g a t i o n as o u r discussion  i s based on t h e assumption t h a t no m i x i n g o c c u r s between  the b u l k o f t h e l i q u i d p o o l o f m e t a l and t h e s o l i d i f y i n g  liquid.  L e t us c o n s i d e r a s o l u t e X h a v i n g p a r t i t i o n c o e f f i c i e n t k , b e i n g Q  the r a t i o o f c o n c e n t r a t i o n s  i n t h e s o l i d and l i q u i d phase i n e q u i l i b r i u m  at t h e s o l i d i f i c a t i o n i n t e r f a c e :  k  o  =  [X]  s  For t i t a n i u m i n d i l u t e s o l u t i o n i n i r o n : k = 0 . 4 0 o For n i c k e l i n d i l u t e s o l u t i o n i n i r o n : k =0.83 o In p r a c t i c e ,  (72) (72)  the e f f e c t i v e p a r t i t i o n c o e f f i c i e n t k , i s h i g h e r , £  owing t o t h e h i g h e r c o n c e n t r a t i o n i n t h e l i q u i d near t h e s o l i d i f i c a tion interface If  (see I V . 2 . 1 2 ) .  the l i q u i d i s homogeneous, t h e " w o r s t ' c a s e " o f l o n g i t u d i n a l  segregation i s encountered.  During a r u n , the composition of the  l i q u i d m e t a l would i n c r e a s e from t h e average c o m p o s i t i o n o f t h e o and tend toward — — a t i n f i n i t y . e I X ]  electrode:  [Xj  Q  The e l e c t r o s l a g  p r o c e s s would t h e n produce a c o m p o s i t i o n p r o f i l e i d e n t i c a l t o t h e first  pass o f a zone r e f i n i n g o p e r a t i o n ( 7 3 ) : 1X3. 1X1  =  1 - (1 - k ) e ^  e  k. W \ e  s  (A2.1)  where W and W. a r e the w e i g h t o f t h e s o l i d i f i e d m e t a l and of t h e s £  - 159 l i q u i d zone  -  respectively.  W  g  I f we assume t h a t a steady s t a t e i s a c h i e v e d (— = °°) £  [ X ]  ™  =  s  ^o>  o  -n^  w  t  L e t us now s i m u l a t e a sharp v a r i a t i o n i n the m e l t r a t e  comparable  to the o p e r a t i o n of s w i t c h i n g the power from D . C . t o A . C . w i t h our unit. I n the f i r s t s t a g e , the power i s shut o f f and we assume t h a t electrode  s t o p s m e l t i n g , w h i l e a f r a c t i o n g of the l i q u i d  (0 < g < 1 ) .  solidifies  The c o m p o s i t i o n p r o f i l e f o l l o w s the r e l a t i o n s h i p  IX]  = [X] s  (1 - g )  the  (73):  (A2.2)  k _ 1  o  At the end of t h i s s t a g e , g = G. I n the second s t a g e , the power i s r e s t o r e d and we assume the i n t e r f a c e  remains s t a t i o n a r y u n t i l the i n i t i a l volume of  i s reestablished. c o m p o s i t i o n IX]  IX]  that liquid  The l i q u i d i s s i m p l y d i l u t e d by f r e s h m e t a l of .  =  A t the end of t h i s I X j  o e  (1 - G)  k-1  the  stage  + GjX]  (A2.3)  °  I n the t h i r d s t a g e , s o l i d i f i c a t i o n o c c u r s a g a i n and we are i n the i n i t i a l transient resolidification.  s i t u a t i o n , the o r i g i n f o r W b e i n g the b e g i n n i n g of g  Multiplying  (A2.3) by k g i v e s the i n i t i a l  of the s o l i d a t the b e g i n n i n g o f t h i s  stage:  composition  - 160  [X]  a  = [ X ] (1 - G)  k  -1  +  Q  When the p r o c e s s c o n t i n u e s ,  [X]  s  G k [X] e  (A2.4)  Q  v a r i e s a c c o r d i n g to  (A2.1)  r e w r i t t e n f o r the new o r i g i n :  [x]q S  =  1  +  - -  [(1 - G ) e + K  W G  ek  - 1] e  6  W  £  The t o t a l c o m p o s i t i o n p r o f i l e i s r e p r e s e n t e d g r a p h i c a l l y on figure  (AII.l).  The maximum c o n c e n t r a t i o n s t e p o c c u r s between p o i n t s A and B , we can c a l c u l a t e v a l u e s a c c o r d i n g t o the v a l u e of G:  Matrix  k  Ti  Fe  Ti Ni  Solute  G  [X]  0.40  0.2  1.14[X]  Fe  0.40  0.5  1.51[X]  Fe  0.83  0.5  1.12[X]  o  r x ] „ B  A  A o o o  .995[X] .958[X] .978[X]  o o o  I n p r a c t i c e , G = 0.2 would c o n s t i t u t e an i m p o r t a n t d i s c o n t i n u i t y . No c o n c e n t r a t i o n s t e p was observed f o r the f o l l o w i n g elements i n i n g o t s where power was changed from D . C . to A . C : T i and Cr i n 321  S.S.  T i and N i i n M a r . 300  steel.  - 161 Appendix I I I .  A3.1  -  A c t i v i t y of TiC> i n CaF ~CaO 2  D e s c r i p t i o n of the  2  experiment  CaF ~CaO s l a g s p r e p a r e d from the pure c h e m i c a l s C a F 2  (British  2  Drug House C o . ) and c a l c i n e d CaCO^ have been e q u i l i b r a t e d w i t h t i t a n i u m carbide  (Cerac C o r p . ) i n a g r a p h i t e c r u c i b l e at temperatures  1700 and 1900°K.  The atmosphere was carbon monoxide a t  between  atmospheric  pressure. The c r u c i b l e ( 6 . 3 mm of i n s i d e d i a m e t e r and c o n t a i n i n g two to t h r e e grams of s l a g and 0.5 g of c a r b i d e was heated i n the i s o t h e r m a l zone of a g r a p h i t e tube w h i c h a c t e d as a s u s c e p t o r (fig.  f o r a 12 KW R . F .  set  A3.1). The t i t a n i u m c a r b i d e was i n the form of a hot p r e s s e d  w h i c h remained s o l i d t h r o u g h the experiment and c o u l d be  pellet  easily  s e p a r a t e d from the s l a g a t the end of the r u n . The time o f e q u i l i b r a t i o n was v a r i e d from 10 minutes to 3 hours a t the l o w e s t temperature a n d , as no c o m p o s i t i o n d i f f e r e n c e was o b s e r v e d , subsequent m e l t s were k e p t a t h i g h temperature f o r 20 m i n u t e s . The temperature was r e g u l a t e d w i t h i n ±5°C u s i n g a P t - P t 10% Rh thermocouple.  The time needed to s o l i d i f y the sample a f t e r the power was  shut o f f was always l e s s than 30  seconds.  The s l a g was t h e n a n a l y s e d f o r t i t a n i u m .  The f l u o r i n e  content  was a l s o v e r i f i e d and found to have v a r i e d by no more than the e q u i v a l e n t of +1.2% analytical  C a F , w h i c h i s s l i g h t l y o u t s i d e the range of 2  error.  I i  the  - 162  A3.2  -  R e l a t i o n t o ESR The experiment was d e s i g n e d t o measure the a c t i v i t y of  o x i d e s i n c o n d i t i o n s comparable  to e l e c t r o s l a g  titanium  r e m e l t i n g as  this  r e l a t e s to the d e o x i d a t i o n e q u i l i b r i a between m e t a l and s l a g  (see  IV.1.5). To c i r c u m v e n t the d i f f i c u l t y caused by the v a r i o u s o x i d a t i o n s t a t e s o f t i t a n i u m , the system had t o be k e p t at an oxygen  potential  comparable  [Ti].  t o t h a t p r e v a i l i n g when i r o n i s d e o x i d i z e d w i t h  C/CO e q u i l i b r i u m p r o v i d e s t h i s oxygen p o t e n t i a l t h r o u g h the  2C(gr) + 0 ( g ) 2  = 2CO(g)(A3.1)  The  reaction.  AF° at 1700°K = - 1 2 5 , 0 0 0 c a l AF° a t 1900°K = - 1 3 3 , 0 0 0 c a l  hence the oxygen p a r t i a l p r e s s u r e  P  0  = 10~  2  1 6  = 10~  *°  1 5 - 3  under one atmosphere  atm at  1700°K  atm a t  1900°K  By c o m p a r i s o n , an i r o n a l l o y c o n t a i n i n g 0.5% [ T i ] with Ti0  2  0 (g) 2  at an a c t i v i t y of 0.01 w i l l  + [Ti](s)  =  o f CO i s ;  i n equilibrium  impose:  ( T i 0 ) ( s ) AF° = - 1 5 0 , 0 0 0 c a l / m a t  1800°K  2  _ AF°  K  or  a  p ~  pO.  Y  o T i  TiQ?  lTi]  - 10  P  0  0.03 2  atmosphere.  0.01 x 0.005 x pO  RT  _  18.2 U  - 163 A3.3  R e a c t i o n s ot T i C S i n c e oxygen (CO) can be t r a n s f e r r e d t h r o u g h t h e s l a g , t h e f o l l o w -  ing  r e a c t i o n s w i l l l e a d t o t h e d i s s o l u t i o n o f t i t a n i u m i n t o the s l a g .  TiC  +  2C0  y  2TiC +  3C0  y  TiC  CO  >• TiO +  +  Ti0 T i  +  2  2°3  +  3C  (A3.2) (A3.3)  5 C  2C  (A3.4)  In t h e e x p e r i m e n t s , the a c t i v i t i e s of T i C and C were u n i t y , as t h e s l a g was i n c o n t a c t w i t h the pure s o l i d s , a n d the p a r t i a l p r e s s u r e of CO was a l s o u n i t y . reactions w i l l  The f r e e energy change o f t h e above  t h e r e f o r e l e a d t o t h e a c t i v i t y of t h e t i t a n i u m o x i d e s  through the r e l a t i o n  AF° = - RT l n a  :  T i 0  x  T a b l e ( A . l ) g i v e s t h e v a l u e o f f r e e e n e r g i e s of f o r m a t i o n o f the compounds of i n t e r e s t a t v a r i o u s t e m p e r a t u r e s : T a b l e A . l Free e n t h a l p i e s of f o r m a t i o n T°K  CaO  CaF  2  Ti0  (74),cal/mole  2°3  TiO(31)  TiC  (solid)  (solid)  (solid)  (solid)  (gas)  2  T i  CO  1600  -110,320  -226,900  -157,250  -260,600  -88,450  -39,850  -60,600  1700  -110,000  -222,100-153,100  -254,750  -86,400  -39,550  -62,650  1800  -106,200  -219,800  -149,350  -249,450  -84,400  -39,400  -64,450  1900  -101,575  -214,600  -145,500  -244,000  -82,450  -39,200  -66,500  - 164  -  C a l c u l a t i o n of the f r e e e n e r g i e s  of r e a c t i o n s  A 3 . 2 to A 3 . 4 i s  s t r a i g h t f o r w a r d and l e a d s to the diagram o f f i g u r e ( A I I I . 2 ) the a c t i v i t i e s of the v a r i o u s t i t a n i u m o x i d e s w i t h r e s p e c t  representing to  temperature. In  a s l a g where the a c t i v i t y c o e f f i c i e n t s  of the v a r i o u s t i t a n i u m  i o n s would be e q u a l , the r a t i o of t h e i r r e s p e c t i v e c o n c e n t r a t i o n would be g i v e n by  (Ti  4 +  Z(Ti  ) n +  ^ ° 2 )  a  Ti0  2  (Ti  + 2a . T  Q  ^ + a  T i Q  3 +  E(Ti  )  n +  2 a T i  )  a  2°3  Ti0 +2...  etc.  2  T h i s i s r e p r e s e n t e d by f i g u r e A I I I . 3 . T h i s r a t i o i s not n e c e s s a r i l y f o l l o w e d as the a c t i v i t y  coefficients 2-  can be d i f f e r e n t .  I n p a r t i c u l a r , when the s l a g c o n t a i n s CaO(0 ) , the 4+ 3+ a c t i v i t y c o e f f i c i e n t of T i w i l l be d e c r e a s e d . T i a l s o forms complex 2ipns with 0 a l t h o u g h t h e i r f r e e energy of f o r m a t i o n i s not known. 2+ 2Ti s h o u l d have l i t t l e a f f i n i t y f o r 0 because of the l a r g e r s i z e and lower e l e c t r i c  A3.4  charge of t h i s i o n .  Range of the  experiments  S e v e r a l d e t e r m i n a t i o n s of the m e l t i n g p o i n t of the c a l c i u m f l u o r i d e used were done and we found 1410 ± 5°C. CaF  2  The m e l t i n g p o i n t of pure  has r e c e n t l y been g i v e n as 1423°C and our m a t e r i a l  shows a d e p r e s s i o n i n the f r e e z i n g p o i n t e q u i v a l e n t to CaO i s the o n l y i m p u r i t y ( 1 4 ) .  We s h a l l n e v e r t h e l e s s  5+2%  therefore CaO i f  c o n s i d e r i t t o be  pure when we w r i t e the f o l l o w i n g t a b l e g i v i n g the s l a g c o m p o s i t i o n s used i n the  experiments:  - 165  -  Table A.2  Slag  % CaO  1  mole f r a c t i o n CaO  range o f temperatures i n v e s t i g a t e d °K 1727 -  1901  0.125  1693 -  1935  0  0  2  9.35  3  19.31  0.25  1726 -  1901  4  31.45  0.39  1728 -  1875  5  37.45  0.47  1723 -  1901  The p a r t i a l p r e s s u r e of CO was h e l d c o n s t a n t  A3.5  Activity coefficient  at one  atmosphere.  of TIO^  A f t e r the e x p e r i m e n t s ,  the s l a g s were a n a l y s e d f o r t i t a n i u m . 3+  The method used i s d e s c r i b e d i n Appendix IV ( t i t r a t i o n of T i The r e s u l t s were then c o n v e r t e d i n t o mole f r a c t i o n of T i 0 [or N ^  T i Q  ^] and the R a o u l t i a n a c t i v i t y c o e f f i c i e n t  by Ce  i n the  2  of T i 0  4+  2  slag  calculated  a c c o r d i n g t o the f o r m u l a o ^TiO,  Ti0 j — 2 a  9  (Ti0 ) 2  a^,^Q  i s known from f i g u r e A I I I . 2 .  The c o m p o s i t i o n s measured are r e p o r t e d i n T a b l e A . 3 , w h i l e the r e s u l t s of the c a l c u l a t i o n s are g i v e n i n f i g .  AIII.4.  I t can be r e a d i l y seen t h a t the i n t r o d u c t i o n of CaO i n the  )  -166  -  Table A . 3  Slag  Temperature  Ti  content  N  Ti0  2  X  X  °  2  Y  o Ti0  2  (Table A . 2 )  °K  1 1 1 1 1 1  1727 1733 1773 1851 1863 1901  .162 .072 .570 .366 .336 .126  .264 .120 .930 .596 .550 .210  2 2 2 2 2 2 2  1693 1701 1718 1798 1828 1893 1935  0.318 0.498 1.89 1.35 1.05 .618 .747  .500 .780 2.99 2.12 1.65 .970 (11.7)  3 3 3 3 3 3  1726 1743 1813 1828 1893 1901  2.04 3.01 1.90 1.49 1.62 1.70  3.10 4.58 2.87 2.24 2.44 2.57  .580 .284 .125 .125 .042 .036  4 4 4 4 4 4  1728 1728 1738 1778 1803 1875  1.03 1.81 2.62 3.17 1.58 3.00  1.49 2.62 3.80 4.60 2.29 4.35  1.15 .656 .376 .144 .186 .031  5 5 5  1723 1782 1901  2.00 2.24 1.13  2.84 3.18 1.60  .674 .192 .058  %  as o f % T i 7.2 13.1 .785 .324 .291 .440 7.2 3.87 .703 .217 .170 .105 (.005)  - 167  -  s l a g has s u b s t a n t i a l l y decreased y , as would be expected w i t h any i o n f o r m i n g a complex w i t h 0  2-  .  When a l a r g e excess of CaO i s used (0.125  o r more) y° v a r i e s l i t t l e w i t h N _ . below 1800°K w h i l e the e f f e c t CaO  is  s l i g h t l y more pronounced between 1800 and 1900°K. The f a c t t h a t the c u r v e s pass from y° > 1 t o y° < 1 when the temperature i n c r e a s e s  c o u l d i n d i c a t e t h a t the v a l e n c e s t a t e of T i  passes from a form w h i c h has a p o s i t i v e d e v i a t i o n (at low temperature) 2to another form w h i c h tends t o be complexed, presumably by 0 high temperature).  If  (at  t h e r e was no change i n the s t a t e of o x i d a t i o n o f  T i , y° would be e x p e c t e d to t e n d toward u n i t y as the  temperature  increases. T h i s o b s e r v a t i o n i s r a t h e r s u r p r i s i n g as the i o n w h i c h i s most s t a b l e a t low temperature ( T i 2form complexes w i t h 0  4+  , see f i g u r e A I I I . 2 )  i s a l s o known to  - 168 A3.6  -  Discussion The r e s u l t s o b t a i n e d are of l i m i t e d v a l u e m a i n l y because the  state  of o x i d a t i o n of T i i s not known q u a n t i t a t i v e l y . X - r a y a n a l y s i s of the quenched s l a g was i n c o n c l u s i v e as a l a r g e number of weak l i n e s on the Debye S h e r r e r p a t t e r n c o u l d n o t be identified. 0.39  and 0.47  CaF^ i s always the dominant p a t t e r n .  Specimens c o n t a i n i n g  CaO would g e n e r a l l y r e v e a l the p a t t e r n of C a ( 0 H )  2  ( h y d r a t e d CaO) w h i l e a compound of TiO,, c o u l d sometimes be d e t e c t e d the few samples where t h e r e was 3% o r more T i . either 4Ca0-3Ti0  2  2  from the presence of c a r b o n .  +  3C(gr)  • CaC (s)  +  2  C0(g)  r e l a t i v e l y s m a l l and p o s i t i v e  AF° = 22,300 at 1700°K =  or,  The f r e e  reaction  CaO(s)  is  T h i s compound was  or C a O ' T i 0 .  Another problem a r i s e s energy of the  in  8,100  CaC„2 a.  f o r pCO = 1 atm.  The f o r m a t i o n of C a C  at 1900°K  2  at h i g h t e m p e r a t u r e . a v a r i a b l e amount (0.1  K  p  K  P  = 1.35 x =  at  3  0.117  , o r x -1 = 1.35 1/1— 0 "3 " . a .t = 0.117  10~  i-7rt«o 1700°K T  1900°K  i n the s l a g w i l l o c c u r to some e x t e n t ,  especially  Carbon a n a l y s i s of the specimen i n d i c a t e s to 3%) w i l l be p i c k e d up i n the s l a g .  It  that is  - 169 however i m p o s s i b l e t o s e p a r a t e t h a t p a r t o f the c a r b o n w h i c h i s a c t u a l l y r e a c t e d w i t h the components o f t h e . s l a g from the p a r t s w h i c h r e s u l t from the r e a c t i o n s o f T i C w i t h CO o r from s t r a i g h t m e c h a n i c a l erosion  of the c r u c i b l e .  As the c a l c u l a t i o n i n S e c t i o n A 3 . 2 shows, the p a r t i a l p r e s s u r e o f oxygen e x i s t i n g i n the e l e c t r o s l a g p r o c e s s i n g of the a l l o y s u s e d , i s comparable w i t h t h a t e x i s t i n g i n the p r e s e n t e x p e r i m e n t s .  The r a t i o s  of v a l e n c e s t a t e s o f t i t a n i u m s h o u l d t h e r e f o r e be q u i t e s i m i l a r .  Hence,  a l t h o u g h we have c a l c u l a t e d an a c t i v i t y c o e f f i c i e n t w h i c h i s a r t i f i c i a l l y a t t r i b u t e d t o T i 0 2 i t w i l l be l e g i t i m a t e t o use t h i s n u m e r i c a l v a l u e i n c a l c u l a t i o n s r e l a t i n g m e t a l - s l a g r e a c t i o n s i n the p r e s e n t c a s e . .  - 170  -  Appendix I V . A4.1  A n a l y s i s of  A n a l y t i c a l methods  slags  A l l slags were f i r s t c r u s h e d to a f i n e powder b e f o r e b e i n g f u s e d . I t has been found t h a t an e x t r e m e l y f i n e powder was needed i f the a c i d f u s i o n ( d i s s o l u t i o n of T i and Fe) was to be completed i n a  reasonable  time. Samples were c o l l e c t e d as o u t l i n e d i n S e c t i o n  (III.10).  j  A4.1.1 T i t a n i u m and i r o n T i t a n i u m and i r o n were d i s s o l v e d by p e r f o r m i n g an a c i d f u s i o n Two methods were used w i t h comparable a)  (75).  results:  To a sample w e i g h i n g Q . l to 0.5 g (the sample s h o u l d be  larger  i f b o t h i r o n and t i t a n i u m a r e to be determined from the same s o l u t i o n ) add 10 ml c o n c e n t r a t e d H^SO^.  Fuse f o r a t l e a s t 30 minutes w i t h  the  e v o l u t i o n of w h i t e fumes i n a p l a t i n u m o r a s i l i c a c r u c i b l e . b)  To the same sample, add 3 g of anhydrous p y r o s u l p h a t e (K^S^O^)  and f u s e w i t h the e v o l u t i o n of w h i t e fumes i n p l a t i n u m o r s i l i c a u n t i l a deep r e d c o l o r i s o b t a i n e d .  Add 7 g of p y r o s u l p h a t e and f u s e a g a i n  f o r 10 minutes a v o i d i n g any o v e r h e a t i n g (< 3 5 0 ° C ) . be  The l i q u i d s h o u l d  clear. A f t e r c o o l i n g , the sample i s d i l u t e d i n about 20 ml of 1 N . H^SO^.  The s o l u t i o n i s a l l o w e d to s t a n d f o r 48 h r d u r i n g w h i c h time p r e c i p i t a t i o n of CaSO^ o c c u r s .  I t i s t h e n f i l t e r e d , the f i l t r a t e i s  recovered  q u a n t i t a t i v e l y and a n a l y s e d i m m e d i a t e l y by one of the f o l l o w i n g methods:  - 171 a)  -  T i t r i m e t r i c a n a l y s i s of t i t a n i u m and i r o n :  A p p r o x i m a t e l y 200 ml  of s o l u t i o n c o n t a i n i n g 5 to 100 ppm T i , an e q u i v a l e n t  q u a n t i t y of  iron  f o u r drops of methylene b l u e and 1 gram H^BO^ i s passed t h r o u g h the Jones r e d u c t o r and c o l l e c t e d  i n a f l a s k w h i c h has been f l u s h e d w i t h CO^ to 3+  p r e v e n t the o x i d a t i o n of T i  .  T i t r a t i o n i s c a r r i e d out a t -3  u s i n g a s t a n d a r d s o l u t i o n of C e r i c s u l f a t e  5 x 10  end p o i n t f o r t i t a n i u m i s i n d i c a t e d by a l i g h t b l u e  60°C  N) u n t i l  the  color.  The t i t r a t e d s o l u t i o n i s then c o o l e d to room t e m p e r a t u r e . drops of o - p h e n a n t h r o l i n e  (76)  f e r r o u s complex a r e added and the  Eight  titration  i s c o n t i n u e d u n t i l a b l u i s h - g r a y c o l o r i s o b t a i n e d , i n d i c a t i n g the end p o i n t f o r  iron.  A b l a n k v a l u e i s t a k e n f o r b o t h p a r t s of the The c e r i c s u l f a t e of a r s e n i o u s b)  oxide  analysis.  s o l u t i o n i s standardized against  a weighed amount  (77).  Spectrophotometric  d e t e r m i n a t i o n of t i t a n i u m :  i s added to the s o l u t i o n c o n t a i n i n g 0.5  to 2 mg T i .  1.5 m l ^2®2  1 ml H^PO^ s h o u l d  a l s o be added i f the b l a n k ( s o l u t i o n w i t h o u t H2O2) i s s l i g h t l y c o l o r e d by the presence of i r o n . 2 N . H^SO^. is stable c)  The s o l u t i o n i s then d i l u t e d to 50 m l w i t h  Spectrophotometry  i s performed a t 410 my.  (75). Spectrophotometric  d e t e r m i n a t i o n of F e :  been c a r r i e d out at 500 my u s i n g 1-10 f o r Fe  The c o l o r a t i o n  i n an a c e t a t e b u f f e r  This determination  p h e n a n t h r o l i n e as an i n d i c a t o r  (2 M sodium a c e t a t e )  (75).  •  Both T i and Fe can be determined from known amounts of the same fusion solution.  has  - 172 A4.1.2 Chromium An a l k a l i n e f u s i o n i s performed i n an i r o n c r u c i b l e , u s i n g 8 g Na20 f o r a specimen up t o 1 g .  C o l o r i m e t r y i s c a r r i e d out on a c i d i f i e d  s o l u t i o n c o n t a i n i n g 0 . 1 to 1 ppm C r ( I V ) i n t h e presence o f d i p h e n y l carbazide; wavelength:  540 my ( 6 4 , 6 5 ) .  A4.1.3 Fluorine A sample c o n t a i n i n g about 0 . 2 g CaF^ i s f u s e d i n a p l a t i n u m c r u c i b l e w i t h the e u t e c t i c :  3 g K  2  C  °3  a n d  2  ,  5  8 Na C0 . 2  The p r o d u c t of f u s i o n i s e x t r a c t e d  3  i n an a c i d s o l u t i o n o f 1:2 H C l .  I t i s d i l u t e d w i t h 350 m l H 0 and 50 m l of 0 . 1 M EDTA i s added. 2  The  volume i s brought t o 500 m l and the s o l u t i o n i s a l l o w e d to s t a n d f o r one h o u r .  Immediately a f t e r the pH has been brought t o 9 w i t h  NaOH, t h e a c t i v i t y o f F  i s read u s i n g a saturated calomel  and a s p e c i f i c i o n f l u o r i n e e l e c t r o d e Measuring i n s t r u m e n t :  saturated  electrode  ( O r i o n Research I n c . , model 9 4 - 0 9 ) .  potentiometric electrometer,  K e i t h l e y 630.  The p o t e n t i a l i s c a l i b r a t e d w i t h weighed amounts of pure d r y CaF » 2  (fig. AIV.l).  A t l e a s t one r e f e r e n c e specimen i s a n a l y s e d each day as  the p o t e n t i a l c a l i b r a t i o n may s h i f t s l i g h t l y .  This a l l o w s the  .{  e x p e r i m e n t a l e r r o r to be reduced from c i r c a 10% t o 1%. Reaction constants  (75):  O a * * ..+  2F~  =  CaF  Ca" "*"  +  Y "  =  C a Y ~ (EDTA): p K = 10.7  +  4Y  =  3HY :  p K = 10.3  +  HY " =  HY ":  pK =  1  + H H  +  ;  4  3  :  2  2  pK = 1 0 . 3 ( s o l u b i l i t y p r o d u c t ) . Q  2  x  6.2  - 173  -  Hence, the apparent pK f o r the complex CaY (75),  a s s u r i n g the s o l u b i l i t y o f CaF .  A4.2  A n a l y s i s of the  2-  i s about 8.6  a t pH 9  steels  A4.2.1 Total metallic  elements  T h i s was c a r r i e d out by i n d u s t r i a l l a b o r a t o r i e s  according to  the  o  following table;  some samples were d u p l i c a t e d between  laboratories.  Table A.4 Steel 321  Mar.  Element  Method  Done by  S.S.  Ti Mn Si Ti  Spectrographic Spectrographic Spectrographic Wet c h e m i c a l  ( U n i v e r s a l Cyclops JSpecialty Steels Div. (. ( B r i d g e v i l l e , P a . Esco ( P o r t l a n d , O r e . )  300  Ti Ti Mo  Spectrographic Spectrographic Spectrographic  Vasco ( L a t r o b e , P a . ) j l n t . N i c k e l Co. 1 (New Y o r k )  Al Ti Si Mn  Spectrographic  1409 A l  U n i v e r s a l Cyclops Specialty Steels Div.  A4.2.2 Matrix metallic Microprobe:.  elements  JEOL, model J X A - 3 A .  M i c r o p r o b e a n a l y s i s was used f o r t i t a n i u m i n 321 s t a i n l e s s and Maraging 300 s t e e l . each s t e e l  I n a d d i t i o n , the main a l l o y i n g element  steel of  (Cr and N i r e s p e c t i v e l y ) was a n a l y s e d on the second c h a n n e l  - 174 of the p r o b e .  K  r  a  -  e m i s s i o n was d e t e c t e d  f o r each  element.  The e l e c t r o n beam had an average i n t e n s i t y o f 8 x 10 accelerated  under a 25 KV p o t e n t i a l .  _g  Amp and was  The c o u n t i n g time was 10  seconds.  Aluminium and chromium were a n a l y s e d i n s i m i l a r c o n d i t i o n s i n 1409 A l s t e e l u s i n g an a c c e l e r a t i n g  v o l t a g e o f 20 KV.  Specimens were m e c h a n i c a l l y p o l i s h e d , the f i n a l s t e p b e i n g 1 u d i a m o n d p a s t e f o l l o w e d by a l i g h t pass on 0.05 Titanium:  u alumina.  The probe c o r r e c t i o n f o r t i t a n i u m i s e s t i m a t e d by the  f o l l o w i n g ^method: A r e l a t i o n s h i p of the type N - B [Ti] = k - E g s i s assumed to be v a l i d a t the peak of i n t e n s i t y of the K ^ l i n e of titanium: N  p  B N  g  =  counts measured on specimen,  =  counts measured on T i f r e e m a t r i x .  =  counts measured on pure T i .  If  the r e l a t i o n s h i p between the t o t a l c o n t e n t [ T i ] measured by N - B s p e c t r o g r a p h i c methods i s p l o t t e d v e r s u s —^g f o r a g r e a t number s of specimens,  i t i s found t h a t  the c l e a n e s t s t e e l s  (absence of b i g  i n c l u s i o n s ) c o n s i s t a n t l y l e a d to a s m a l l e r v a l u e of k . The b e s t specimens  show k = 0.9  f o r the c o r r e c t i o n to be a p p l i e d (see  and t h i s v a l u e has been  retained  f i g . A4.2).  The c u r v e d e r i v e d from computer c a l c u l a t i o n s u s i n g the program MAGIC (78)  i s p l o t t e d f o r comparison ( f i g .  T h i s program computes the c o r r e c t i o n s characteristic  fluorescence,  A4.2). f o r background, a b s o r p t i o n ,  b a c k s c a t t e r and i o n i z a t i o n p e n e t r a t i o n .  - 175  -  As t h i s type of c a l c u l a t i o n l e a d s o n l y to approximate v a l u e s f o r low concentrations,  the e x p e r i m e n t a l c u r v e was p r e f e r r e d .  l i t t l e consequence  T h i s choice b e a r s  to the g e n e r a l c o n c l u s i o n s o f the w o r k , however.  I n o r d e r to d i f f e r e n t i a t e between m a t r i x and p r e c i p i t a t e s ,  at  50 p o i n t counts were t a k e n at random from each specimen and those w h i c h exceeded t w i c e the average v a l u e were r e j e c t e d .  The  least counts  percentage  of r e j e c t i o n was u s u a l l y below 10%. A l t h o u g h t h i s p r o c e d u r e gave coherent r e s u l t s f o r most i n g o t s ,  it  seems to have f a i l e d w i t h i n g o t s S14 and S15 where p a r t of the t i t a n i u m c o n t e n t was reduced from the s l a g .  Excessive segregation i s suspected  have d e f e a t e d the method i n t h i s c a s e , as an e x c e s s i v e  difference  to  is  found between m a t r i x and t o t a l T i . Major Elements The e l e c t r o d e s ,  the c o m p o s i t i o n of w h i c h i s known p r o v i d e d one  s t a n d a r d f o r Cr i n S . S .  321, N i i n Maraging 300 and Cr i n 1409 A l . :  The v a r i a t i o n of c o n c e n t r a t i o n was assumed to f o l l o w a l i n e a r r e l a t i o n s h i p w i t h probe r e s p o n s e ,  the s l o p e of w h i c h was c a l c u l a t e d w i t h the computer  program. F i f t y p o i n t counts were a l s o t a k e n on each specimen.  A4.2.3 Oxygen Each r e p o r t e d v a l u e r e p r e s e n t s a d j a c e n t specimens w e i g h i n g 0.5  the average c o m p o s i t i o n of  to 3 g .  three  The Leco Oxygen A n a l y s e r  w h i c h was used performs the f o l l o w i n g f u n c t i o n s :  The m e t a l i s m e l t e d  by i n d u c t i o n h e a t i n g i n a g r a p h i t e c r u c i b l e at a p p r o x i m a t e l y 2000°C; CO  !  - 176  -  i s p r o d u c e d , swept away by a c a r r i e r gas and t r a p p e d i n a m o l e c u l a r s i e v e . by a chromatographic method.  ( h e l i u m ) , c o n v e r t e d to CC^  CO^ i s then r e l e a s e d and a n a l y s e d  - 177 -  BIBLIOGRAPHY 1.  Duckworth W . E . and Wooding P . J . , T r a n s . e n c e , Am. V a c . S o c , N . Y . ( 1 9 6 8 ) .  Vacuum M e t a l l u r g y  2.  Roberts R . J . , Trans. ( V I , 1969).  3.  Duckworth W . E . and H o y l e G . , E l e c t r o - s l a g H a l l (}969).  4.  H o l z g r u b e r W . , P r o c . F i r s t I n t . Symp. on ESR t e c h n o l o g y Mellon I n s t i t u t e , P i t t s b u r g h ( V I I I , 1967).  5.  H o l z g r u b e r W . , P e t e r s e n K . and S c h n e i d e r P . E . , T r a n s . M e t a l l u r g y C o n f . , Am. V a c . S o c , N . Y . ( 1 9 6 8 ) .  6.  Thomas R . D . and Parsons R . C , P r o c . F i r s t I n t . Symp. on ESR Technology V o l I , M e l l o n I n s t i t u t e , P i t t s b u r g h ( V I I I , 1 9 6 7 ) .  7.  H l i n e r y J . and Buzek Z . , S b o r n i k V e d . P r a c i V . S . B . O . 1 1 , 483 ( I I I , 1 9 6 5 ) .  8.  Yuassa G . , P r o c S e c o n 4 Symp. on ESR t e c h n o l o g y I n s t i t u t e , ( I X , 1969).  9.  Medovar B . I . , L a t a s h Y u . V . , Maksimovich B . I . and Stupark L . M . , E l e c t r o s l a g R e m e l t i n g , S t a t e S c i e n t i f i c and Techn. P u b l . House o f L i t e r a t u r e on F e r r o u s and N o n - F e r r o u s M e t a l l u r g y , Moscow (1963).  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Standart n° E45-63 (1969) book of s t a n d a r d s p a r t 3 1 .  17.  J e r n k o n t o r e t Research O r g . , E x a m i n a t i o n of s t e e l f o r s l a g i n c l u s i o n s , A l m g v i s t and W i k s e l l , Stockholm (1966).  E . , Trans V a c . M e t .  of M e t a l l u r g y , U . B . C  - 178 -  18.  M i t c h e l l A. and Etienne M . , Trans A . I . M . E . ,  242, 1462  (1968).  19.  S i n g h a l L . K . and M a r t i n J . W . , J . I . S . I . ,  20.  B o n i s z e w s k i T. and B o n i s z e w s k i E . , J . I . S . I . ,  21.  Whittaker D . A . , Ph.D. t h e s i s ,  22.  Sun R . C . and P r i d g e o n J . W . , P r o c . Second I n t . Symp. on ESR Techn. V o l I I I , M e l l o n I n s t i t u t e ( I X , 1 9 6 9 ) .  23.  Bodsworth C . , P h y s i c a l Chem. of I r o n and S t e e l M a n u f a c t u r e , Longmans, ( 1 9 6 3 ) .  24.  Towers H. and Chipman J . , Trans A . I . M . E . ,  25.  D e l i m a r s k i i and P a v l i n o v , c i t e d by D i f f u s i o n D a t a , V o l 3 , n ° 4 , 507 (1969).  26.  Kubaskewski 0 . and Hopkins B . E . , O x i d a t i o n of M e t a l s and A l l o y s . B u t t e r w o r t h s , (1962).  27.  M i t c h e l l A . , J o u r n a l of Vacuum S c i e n c e and T e c h n o l o g y , (to be p u b l i s h e d ) .  28.  M i t c h e l l A . , T r a n s . Faraday S o c , 63^, 1408 ( V i , 1 9 6 7 ) .  29.  Evceef P . P . ; a n d a l . , I z v . V y s s h i k h Uchebnykh Z a v e d i n c e , 1 2 , ! 47, ( I X , 1 9 6 9 ) .  30.  H i l l e r t L . , A c t a Chemica S c a n d . , 1 9 , 1516 ( V I , 1 9 6 5 ) .  31.  E l l i o t t J . and G l e i s e r M . , Thermochemistry A d d i s o n - W e s l e y , 1960.  32.  Segawa K . and Tsunetomi E . , Trans I . S . I . J . ,  33.  H o l f e r t C , Lambrecht J . and P r a s k e W . , F r e i b u r g e r F o r s c h u n g . , 122, 155 (1966).  34.  R o s s o k i n B . G . , Smirnov M . V . and L o g i n o v N . A . , i n E l e c t r o c h e m i s t r y of m o l t e n and s o l i d e l e c t r o l y t e s , V o l I V , B a r a b o s h k i n N . A . and P a l ' g u e v ( e d i t o r s ) , C o n s u l t a n t B u r e a u , N . Y . (1967).  35.  Campbell J . , J o u r n a l of M e t a l s , 2 2 , 23 ( V I I , 1 9 7 0 ) .  36.  J o s h i S . , unpublished research,  37.  Cameron J . , E t i e n n e M . and M i t c h e l l A . , M e t a l l u r g i c a l 1 , 1839 ( V I , 1 9 7 0 ) .  205, 947 ( I X , 1 9 6 7 ) . 360 ( I V , 1 9 6 6 ) .  McMaster U n i v e r s i t y ( V I I I ,  209, 769  1967).  (1957).  Dec 1970  f o r S t e e l making, 9_, 89 (1969).  Department of M e t a l l u r g y , U . B . C . Transactions  - 179  -  38.  P a n i n V.V., B o r o v s k i y O.B. 52 ( I , 1964).  and a l . , R u s s i a n M e t a l l u r g y and M i n i n g ,  39.  Schuhmann R., (1952).  40.  I n t . N i c k e l Comp., Data Sheet on the Foundry C h a r a c t e r i s t i c of 17% N i Cast Maraging S t e e l (W.A.K. 7-1-1966).  41.  M e t a l s Handbook V o l I , 8th e d i t i o n , Am.  42.  M i t c h e l l A. and J o s h i S., O b s e r v a t i o n s on the e l e c t r i c a l and thermal p r o p e r t i e s of the s l a g - s k i n r e g i o n i n ESR., Accepted by M e t a l l u r g i c a l T r a n s a c t i o n s , no. 69-358-C.  43.  G r j o t h e i m and Zuca, c i t e d by D i f f u s i o n Data, V o l 2, no. 3-4, (1968).  44.  Whitman W.G.,  45.  Danckwerts P.V., Ind. Eng. Chem.,43, 1460 J o u r n a l J L . 456 (1955).  46.  Toor H.L.  47.  B r e b i g M.A. i n Molten S a l t Chemistry, B l a n d e r M. I n t e r s c i e n c e (1964).  48.  S p o n s e l l e r D.L.  49.  H u l t g r e n R.R., S e l e c t e d Values f o r the Thermodynamic P r o p e r t i e s of Metals and A l l o y s , C a l . Min. Res. Lab., U. of C., B e r k e l e y , (1956).  50.  J o s t W.,  51.  B i r d R.B., Stewart W.E. John W i l e y , (1965).  52.  Klyuev M.M.  53.  M i t c h e l l A., P r o c . Second I n t . Symp. on ESR Technolgoy V o l I , M e l l o n I N s t i t u t e , (IX, 1969).  54.  Darken L.S. and Gurry R.W., H i l l , (1953).  55.  Wanibe Y. and Sano K., 60 (1967).  56.  Steinmetz E., A r c h i v . f u r Eisenhuttenwesen, 36, 421  57.  Holzgruber W.  M e t a l l u r g i c a l Engineering Vol I,  Chem. & Met.  Eng.,  and M a r c h e l l o J.M.  Addison-Wesley  Soc. Met.,  29^, 147  (1967).  (1923). (1951) and A.I.Ch.E.  A.I.Ch.E. J o u r n a l 4^, 97  and F l i n n R.A.,  (1958).  (editor),  T r a n s . A.I.M.E. 230, 876  D i f f u s i o n , Academic P r e s s , N.Y.  358,  (VI, 1964).  (1960).  and L i g h t f o o t E.N.,  T r a n s p o r t Phenomena  and Mironov Yu.M. , S t a l i n E n g l i s h , 480  (VI, 1967)..  P h y s i c a l Chemistry o f M e t a l s , McGraw  c i t e d by D i f f u s i o n Data, V o l I , no.  3,  (VI, 1968).  and P l o e c k i n g e r E., S t a h l and E i s e n , 88,  638:(1968).  - 180 58.  Choudhury A . , K l i n g e l h o f e r H . J . and W a h l s t e r M . , Second I n t . Symp. on ESR Technology V o l I I , M e l l o n I n s t i t u t e ( I X , 1 9 6 9 ) .  59.  H l i n e r y J . and Buzek Z . , H u t n i c k e L i s t y , 524 ( V I I I ,  60.  L a t a s h Y u . V . , Avtom S v a r k a , n o . 9 , 30 (1965).  61.  H l i n e r y J . , C h i n e l a r I . and Buzek Z . , S b o r n i k V e d . P r a c i V . S . B . O . (Ostrava) ' 1 1 , 477 ( I I I , 1 9 6 5 ) . ,  62.  H l i n e r y J . and Buzek Z . , i b i d . ,  63.  K a s i n V . I . and a l . , F r e i b e r g e r F o r s c h u n g . , 126, 97 ( 1 9 6 7 ) .  64.  H l i n e r y J . , Chmelar I . and Buzek Z . , S b o r n i k V e d . P r a c i V . S . B . O . (Ostrava) 1 1 , 471 ( I I I , 1 9 6 5 ) .  65.  Chalmers B . , P r i n c i p l e s o f S o l i d i f i c a t i o n , John W i l e y (1964).  66.  H o l z g r u b e r W . , P r o c . Second I n t . Symp. on ESR Technology V o l I , Mellon I n s t i t u t e (IX, 1969).  67.  L a t a s h Y u . V . , M a k s i m o v i c h B . I . and Medovar B . I . , Avtom. n o . 9 , 17 (1960).  68.  Madono 0 . , P r o c . Second I n t . Symp. on ESR T e c h n o l o g y , V o l I , Mellon I n s t i t u t e (IX, 1969).  69.  K l y u e v M . M . and S h p i t s b e r g V . M . , S t a l i n E n g l i s h , 168 ( I I ,  70.  Handbook o f M a t h e m a t i c a l F u n c t i o n s , U . S . N a t . Bureau of A p p l i e d Mathematics S e r i e s N o . 5 5 , Wash. ( V I , 1 9 6 4 ) .  71.  Shewmon P . G . , D i f f u s i o n i n S o l i d s , McGraw H i l l  72.  Chipman J . and E l l i o t t J . F . , i n E l e c t r i c Furnace S t e e l making V o l I I , Sims C . E . ( e d i t o r ) , I n t e r s c i e n c e P u b l (1962).  73.  P f a n n W . G . , Zone M e l t i n g , J . W i l e y ( 1 9 6 6 ) .  74.  G l a s s n e r A . , The Thermochemical P r o p e r t i e s o f the Oxides F l u o r i d e s and C h l o r i d e s to 2 5 0 0 ° K . , Argonne N a t . L a b . , U . S . Atomic Energy Commission (1957).  75.  C h a r i o t G . , L e s methodes de l a c h i m i e a n a l y t i q u e , A n a l y s e q u a n t i t a t i v e m i n e r a l e , Masson (1961),  76.  Shippy B . A . , A n a l y t i c a l Chem., 2 1 , 698 ( V I , 1 9 4 9 ) .  1 1 , 505 ( I I I ,  1966).  1965).  Svarka,  1969).  Standards,  (1963).  - 181 -  77.  Vogel A . I . , Q u a l i t a t i v e Inorganic  Analysis, J . Wiley,  78.  Microprobe A n a l y s i s General I n t e n s i t y C o r r e c t i o n , Fortran Program adapted by O ' B r i e n T . E . , Department of M e t a l l u r g y , U . B . C .  i  (1960).  - 182  -  er  F i g u r e 1.  P r i n c i p l e of e l e c t r o s l a g  F i g u r e 2.  Cold s t a r t c o n f i g u r a t i o n .  remelting.  U.B.C.  unit.  -  tATUMM  IS  3  M»CTO»\  RAM »0»ITIOM V rtlOIMK U N I T /  CMttHtftO^.  ELtCTMM %U>*<*1\ CVLINMM X  mom  CONTACT  eucTRooc>  RCTUtM  BU»j>  F i g u r e 3.  Commercial u n i t (Consarc C o r p o r a t i o n  -  cu rH CO  •H JH  cd  >  C •H  4-1  rH ai  s  0)  QJ  185  -  rH O  H  4-1  ft a o W  o  u OJ o  CU  Pi  o o o CM 4j  > E  .C  O  00 LO  Figure  5.  Schematic  d r i v e motor  circuit.  -  Figure  6.  Electrode  holder.  186  -  Copper b a s e p l a t e  F i g u r e 7.  Baseplate  arrangement.  (1:2.5 approximately)  -  Figure  9.  Fume h o o d  188  (1:2.5).  -  Water c o o l e d (copper)  Rubber b e l l o w s  F i g u r e 10.  Atmospheric s h i e l d ( 1 : 2 . 5  approx.).  stub  F i g u r e 11. Powder f e e d e r . ' &  m - , ^ -i), tr,, , . f i g u r e 14. View o f the l a b o r a t o r y .  Electrode holder  Water c o o l e d copper e x t e n s i o n  VO Mold  Electrode Slag  F i g u r e 12.  Non consumable  electrode  F i g u r e 13.  Sampling of the  slag.  TI  HOQ  Bus bars  C f-i  CO  Electrode leads  n  n> 3 tu rt Hn  o fD  o  c  Exchangeable connections DC - AC  switch  DC  bias  - 193 -  D.C. Volts  0  200  400  600  800 D . C . Amperes  F i g u r e 16.  Hobart W e l d e r s .  Output  Characteristics.  1000  - 194  2S  -  7— / 2M 2C  /  /  /  /  I  F i g u r e 17.  IS  /  IM  IC  Sampling of i n g o t and s l a g .  - 195 -  - 196 -  - 197  F i g u r e 20.  I n g o t s S7,  S8.  -  E l e c t r o d e n e g a t i v e , CaF„ + CaTiO  198  (0  ro  d  d 0^  Figure  I n g o t s S9,  CM  d  d  r-, 21,  -  S10,  Sll.  Electrode  positive.  0.6 tot.  [Ti]o  Wt  %  rnrrrnrr  [Ti]  Inclusions removed ' ' I I I I I I I  i  111711 mtx.  [Ti]o  SI2  0.4  total  03  ^  ^  matrix (calculated)  0.2  0.1  1_  DC | A C I  ±  _L 6  « m  /  V  V  Y  8  0.6  [Ti]o °z  Wt  tot.  %  [Til  - T T - T T - r - r T T - T  m SI3  0.4  n  y  Inclusions removed.  [TiJo mtx.  total 0.3  0.2  0.1  DC } A C I J  L  w /w m  8 s  - 201 -  - 202 -  aluminium  added.  F i g u r e 26.  Ingot S16. CaF .  Electrode  Argon.  negative/A.C.  - 204  d  p  •-•  F i g u r e 27.  iq  ro  <fr  d  d  Ingot S17. CaF„  -  +  d  Electrode  CaTiO .  negative/A.C.  Argon.  CM  d  - 205  Figure  28.  I n g o t s M l , M3, M4.  -  Electrode  negative.  - 206 -  16 M2 (Ti) in slag  A [Ti] from slag composition. •  [Ti] as analysed  o  (total).  /-  wt % cm  /  10  /  t m  0.8  12  8  0.6  0.4  0.2  u // i  '  —  A  0.0 0 F i g u r e 29.  - ~ x w r  2 Ingot  M2.  1  l  4  6  Electrode negative.  8 CaF,  10  12  w /w "nrr  w  s  -  207  -  CTi]o total  %[Ti]  (•)  [ T i ] o matrix total  •  0.6  •  A 0 0.5 A  fi  A  A  A  matrix 0.4  AO  O  o  0.3  —  0.2 • 0.1  O  M5  A A  M6 —  0  1  1  1  1  1  2  3  4  .  1  1 5  6  w Figure  30.  I n g o t s M5, M6.  Electrode  positive.  m  % [Ti]  [Ti]o total •  o • Inclusions removed  [Ti]o matrix  O  o  *  O 0.6  matrix  O  0  o  o •  05  o  —  M7  0.4  • O  total  —  DCJAC  matrix  _  1  0.3  1  1  1  i  i  !  i  i  1  8  r-  Figure J2.  macrograph, ingot  S17.  2Q9 -  Figure 33.  Macrograph, ingot  Slh.  - 210  F i g u r e 3^. Macrograph, i n g o t M5.  -  F i g u r e 3 6 . Macrograph, i n g o t A l  - 211 -  F i g u r e 3 5 . Macrograph, i n g o t  M7  D.C.  A.C.  electrode positive. F i g u r e 37.  P a t t e r n o f g r a p h i t e p a r t i c l e s on the ( t h e e l e c t r o d e i s shaded).  D.C. slag  electrode negative.  F i g u r e 39.  Thermocouple l o c a t i o n s on the  electrode.  - 214 -  0 F i g u r e 40.  20  40  60  80 Distance  100 mm.  T  Figure 4 i .  - 215 -  1  1  1  Distance  r  mm.  - 217 -  -  218  -  f  log p 0  %Ti+0  2  2  (g)  - 12  Ti0  {  2  2°3 CaTiO, T i  -13  SS-TiO,  -14  Fe-Ti0  2  SS-Ti 0  -15  2  3  Fe- T i 0 2  3  -16  SS-CaTiO, -17 Fe-CaTiO.  J  L  O.OI Figure  J  L  O.I 44,  Deoxidation as  a  1.0  e q u i l i b r i a of  function  of  pO .  Fe  and  321  S.S,  Wt %  [Ti]  -  Figure  46.  F l o w on t h e  219  electrode  -  tip.  - 220 -  -  221 -  concentrations Oxygen ( O ' l  [ML  electode.  slag, reaction plane.  Figure 49. Oxidation of an a l l o y i n g element i n small concentration (electrode p o s i t i v e ) .  0 F i g u r e 50.  10  20  t as a f u n c t i o n o f e  30  40  Q  o  50  - 222 -  gure  51.  Oxidation film  of  a major  (electrode  alloying  positive).  element  i n the  electrode  - 223 -  ure 52.  Equation  (IV.19).  - 224 -  321 S.S.  - 225  F i g u r e 54.  Influence  -  of the s l a g c o m p o s i t i o n on the oxygen c o n t e n t .  - 226 -  - 227  -  CX]  k-1  [X],  D Q = C X ] C X 3 C ( I - G U Gk - I] " S  0 +  0  k ( W  e  W. Figure A2.1.  C o m p o s i t i o n p r o f i l e due to melt r a t e  discontinuity.  s  / V  V  I atm. CO  o o o o o o o  O o o o o o  4  Pt-Rh tc 1.  C r u c i b l e c o n t a i n i n g s l a g and T i C compact,  2.  Graphite  3.  Graphite f e l t  4.  Quartz  susceptor. insulation.  tube.  Figure A3.1.  E q u i l i b r a t i o n apparatus  (1:1  approximately)  - 229 -  -  230  R a t i o of activities  C/CO  1700  Figure A 3 . 3 .  of titanium  (I atm.)  1900  1800  R a t i o of a c t i v i t i e s  ions  of t i t a n i u m  ions.  - 231 -  - 232  ppm  -  CaR  L.600  .400  200 •  I  20 Figure A 4 . 1 .  .  30  I  40  •  mV  P o t e n t i a l of f l u o r i n e i o n e l e c t r o d e v e r s u s s a t u r a t e d c a l o m e l electrode.  Range of c a l i b r a t i o n s .  - 233  Total  Ti  -  %  .4  0  .2  Figure A4.2.  C a l i b r a t i o n curves f o r m i c r o p r o b e a n a l y s i s of titanium.  .6  .8  1.0  matrix  

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