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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 Ingenieur 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 accept t h i s t h e s i s as conforming to the r e q u i r e d s tandards THE UNIVERSITY OF BRITISH COLUMBIA October , 1970 In present ing th i s thes is in pa r t i a l f u l f i lmen t o f the requirements fo r an advanced degree at the Un ive rs i t y of B r i t i sh 'Co lumb ia , I agree that the L ib ra ry sha l l make it f r ee l y ava i l ab le for reference and study. I fu r ther agree that permission for extensive copying of th i s thes is fo r s cho la r l y purposes may be granted by the Head of my Department or by h is representat ives . It is understood that copying or pub l i c a t i on o f th i s thes is fo r f i nanc i a l gain sha l l not be allowed without my wr i t ten permiss ion. Department of The Un ivers i ty of B r i t i s h Columbia Vancouver 8, Canada Date - 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 'Elements d ' a l l i a g e 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 ra i te*s sont l e s per tes de t i t a n e dans l ' a c i e r i n o x y d a b l e AISI 321 et l ' a c i e r maraging type 300 et 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 1 ' i n f l u e n c e s o n t : l a p r e s s i o n p a r t i e l l e d'oxygene dans 1 'atmosphere , l a c o m p o s i t i o n 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 et l a p o l a r i t e " du conrant d l e c t r i q u e , l a v i t e s s e de f u s i o n . On a f a i t l a d i s t i n c t i o n ent re 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 ' i n c l u s i o n s . 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 t r a n s f e r t par d i f f u s i o n de l 'oxyg&ne 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 et 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 d u r i n g E l e c t r o s l a g Remelt ing have been i n v e s t i g a t e d i n the a l l o y s AISI 321 s t a i n l e s s s t e e l and Maraging 300 s t e e l which 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 , c o n t a i n i n g a luminium. 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 mel t p a r a -meters , i . e . , the p a r t i a l pressure 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 t r a n s p o r t p r o p e r t i e s , 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 the mel t r a t e . 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 compos i t ion v a r i a t i o n s which may r e s u l t from the removal of i n c l u s i o n s d u r i n g the r e m e l t i n g 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 of oxygen • and a l l o y i n g elements between the v a r i o u s phases p r e s e n t , and are compared w i t h the exper imenta l data ob ta ined i n t h i s work or c o l l e c t e d from the l i t e r a t u r e . - i v -TABLE OF CONTENTS VOLUME I P a ? e TITLE PAGE . . , 1 ABSTRACT 1 1 TABLE OF CONTENTS i v LIST OF FIGURES i x LIST OF TABLES x i i LIST OF SYMBOLS x i i i ACKNOWLEDGEMENTS x v i CHAPTER I . INTRODUCTION 1 1.1 The ESR process 1 1.2 Statement of the problem 3 1.3 Choice of the m a t e r i a l s ^ o 1.4 P r e l i m i n a r y d i c u s s i o n 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 . THE U . B . C . ELECTROSLAG UNIT I1 11.1 G e n e r a l i t i e s , ^ 1 o 11.2 Framework and e l e c t r o d e t r a v e l mechanism 11.3 E l e c t r o d e h o l d e r 1 3 I I . 4 Mold b a s e p l a t e 1 3 - v -Page I I . 5 Mold and water j a c k e t 14 I I . 6 Atmosphere c o n t r o l 15 I I . 7 Continuous s l a g or meta 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 supply 16 11.10 A . C . power supply 17 11.11 D . C . b i a s 18 11.12 D . C . c o n t r o l s 19 11.13 A . C . c o n t r o l s 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 the 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 Procedure and p r e c a u t i o n s 24 111.2 Composi t ion of the a l l o y s s t u d i e d 25 111 .3 S lags 26 111.4 Remelt ing c o n d i t i o n s f o r 321 s t a i n l e s s s t e e l 28 111.5 Remelt ing c o n d i t i o n s f o r Maraging 300 s t e e l 39 111.6 Remelt ing c o n d i t i o n s f o r 1409 A l s t e e l 43 I I I . 7 Remelt ing of Ferrovac E and m i l d s t e e l 46 111.8 A n a l y s i s and sampling 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 I I I . 10 A n a l y s i s and sampling of the s l a g 51 111.11 Composi t ion and m a t e r i a l ba lance of 321 S .S . i n g o t s . . 53 111.12 Composi t ion and m a t e r i a l balance of Maraging 300 s t e e l i n g o t s ^ y •A - v i -Page I I I . 1 3 Composi t ion 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 of 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 . DISCUSSION 8 5 I V . 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 86 I V . 1 . 1 Atmospheric oxygen 86 I V . 1 . 2 O x i d a t i o n of the e l e c t r o d e 8 7 I V . 1 . 3 E l e c t r o l y s i s 90 I V . 1 . 4 Thermochemical data 91 I V . 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 I V . 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 I V . 1 . 7 S ta te 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 I V . 2 P h y s i c a l d e s c r i p t i o n of the process 100 I V . 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 1 0 0 I V . 2 . 2 D i f f u s i o n at the s l a g atmosphere i n t e r f a c e . . 1 0 2 I V . 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 I V . 2 . 4 Flow of m e t a l on the e l e c t r o d e 1 1 0 I V . 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 ) I V . 2 . 6 D i f f u s i o n of a l l o y i n g elements i n the l i f t e l e c t r o d e f i l m ( reverse p o l a r i t y ) ± x o I V . 2 , 7 D i f f u s i o n i n the meta l at the s l a g - i n g o t i n t e r f a c e ( d i r e c t p o l a r i t y ) l 2 - ^ - v i i -Page I V . 2 . 8 D i f f u s i o n i n the s l a g at the s l a g - m e t a l i n t e r f a c e s , 122 I V . 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 124 I V . 2 . 1 0 Dependence upon time or W^/W 125 I V . 2 . 1 1 Macroscopic assessment of the process (Oxidant i n l i m i t e d supply - 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 I V . 2 . 1 3 Comparison between s m a l l and l a r g e u n i t s . . 133 I V . 3 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 compos i t ion changes 138 I V . 3 . 1 I n f l u e n c e of the atmosphere 138 I V . 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 O Q i n g o t s x J O I V . 3 . 3 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 Mar. 300 i n g o t s l ^ 2 I V . 3 . 4 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 1409 A l i n g o t s i 4 3 I V . 3 . 5 I n f l u e n c e of s l a g compos i t ion on the o x i d a t i o n of Fe I 4 4 I V . 3 . 6 I n f l u e n c e of the melt r a t e i 4 4 I V . 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 1 4 5 I V . 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 4 6 I V . 3 . 9 I n c l u s i o n and oxygen contents I 4 ' ' I V . 3 . 1 0 Importance of the a n a l y s i s methods 1 ^ CHAPTER V . CONCLUSIONS VOLUME I I Appendix I . Flow of m e t a l on the e l e c t r o d e 151 154 - v i i i -Page A l . l C a l c u l a t i o n of t e , f r e e f l o w i n g f i l m 154 A1.2 Momentum t r a n s f e r w i t h the s l a g . , 155 Appendix I I . Segregat ion i n ESR 158 Appendix I I I . A c t i v i t y of T10 2 i n CaF^CaO 161 A 3 . 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 React ions of T i C 1 6 3 A3.4 Range of the experiments 164 A3.5 A c t i v i t y c o e f f i c i e n t of TiC>2 165 A3 . 6 D i s c u s s i o n 168 Appendix I V . A n a l y t i c a l methods 1^0 A4 .1 A n a l y s i s of s l a g s 1^0 A 4 . 1 . 1 T i t a n i u m and I r o n I 7 0 A 4 . 1 . 2 Chromium i 7 2 A 4 . 1 . 3 F l u o r i n e i 7 2 A4.2 A n a l y s i s of the s t e e l s , A 4 . 2 . 1 T o t a l m e t a l l i c elements i 7 3 1 TO A 4 . 2 . 2 M a t r i x m e t a l l i c elements X / J A 4 . 2 . 3 Oxygen 1 7 5 BIBLIOGRAPHY 1 7 7 FIGURES 182 y - i x -LIST OF FIGURES F i g u r e 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 Remel t ing 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 d r i v e motor c i r c u i t 185 6 E l e c t r o d e h o l d e r 186 7 Basepla te 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 lag and meta l a d d i t i o n device 190 12 Non consumable e l e c t r o d e c o n f i g u r a t i o n 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 power c i r c u i t s 192 16 Welders output c h a r a c t e r i s t i c s 193 CHAPTER I I I 17 Sampling of i n g o t and s l a g 19^ 18 Composi t ion p r o f i l e - Ingot S l , S2, S3 i 9 5 19 Composi t ion p r o f i l e - Ingot S6 1^6 20 Composi t ion p r o f i l e - Ingot S7, S8 1 9 7 21 Composi t ion p r o f i l e - Ingot S9, S10, S l l i 9 8 - X -F i g u r e Page 22 Composi t ion p r o f i l e . Ingot S12 199 23 Composi t ion p r o f i l e . Ingot S13 200 24 Composi t ion p r o f i l e . Ingot S14 201 25 Composi t ion p r o f i l e . Ingot S15 202 26 Composi t ion p r o f i l e . ' Ingot S16 203 27 Composi t ion p r o f i l e . - Ingot S17 204 28 Composi t ion p r o f i l e . Ingot M l , M3, M4 205 29 Composi t ion p r o f i l e . Ingot M2 206 30 Composi t ion p r o f i l e . Ingot M5, M6 207 31 Composi t ion 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 Macrograph, 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 p a r t i c l e s on the s u r f a c e of the s l a g 212 38 Suggested c o n v e c t i o n c u r r e n t s 213 39 Thermocouple arrangement ( e l e c t r o d e ) 213 40 E l e c t r o d e temperature g r a d i e n t s ( s t a i n l e s s s t e e l ) . . 214 41 E l e c t r o d e temperature g r a d i e n t s (mi ld s t e e l ) 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 2 218 45 D i f f u s i o n i n the s l a g 219 - x i -F i g u r e Page 46 Flow on the e l e c t r o d e 219 47 F i l m of meta 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 . 220 49 Oxygen d i f f u s i o n i n f:he e l e c t r o d e f i l m 221 50 V a r i a t i o n of t (on the e l e c t r o d e ) w i t h 0 221 e 51 O x i d a t i o n of major a l l o y i n g elements i n the e l e c t r o d e f i l m 222 52 E q u a t i o n ( IV.19) 223 53 I n f l u e n c e of melt r a t e on A [ T i ] e l - v e 321 S . S . . . . . . 224 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 225 Appendix I A l . 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 s l a g 226 Appendix I I A2 .1 Composi t ion p r o f i l e due to mel t r a t e d i s c o n t i n u i t y . 227 Appendix I I I ' 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 ox ides under 1 atm C0/C(gr) . . . 229 A 3 . 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 ions 230 A3.4 y° = f u n c t i o n of T 231 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 [ T i ] 2^3 - x i i -LIST OF TABLES Table Page CHAPTER I I I 1 to 19 Remelt ing c o n d i t i o n s f o r i n g o t s S l to S19 (321 S . S . ) 30-38 20 to 26 Remelt ing c o n d i t i o n s f o r i n g o t s M l to M7 (Mar. 300) 39-42 27 to 30 Remelt ing c o n d i t i o n s f o r i n g o t s A l to A4 (1409 A l ) 44-45 31 to 35 Remelt ing c o n d i t i o n s f o r i n g o t s F l to F5 (Ferrovac -E 46-47 and 1018 M . S . ) 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 Composi t ion of i n g o t s S l to S3 54-55 40 to 51 Composi t ion of i n g o t s S6 to S17 56-68 52 to 58 Composi t ion of i n g o t s M l to M7 69-74 59 to 62 Composi t ion of i n g o t s A l to 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 at 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 ) l ^ 3 A. 2 Range of experiments ( A I I I ) 1 ^ A. 3 E x p e r i m e n t a l r e s u l t s , Y ^ i 0 (AI I I ) 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 ( A I V ) . 173 - x i i i -LIST OF SYMBOLS a . a c t i v i t y of i 1 2 A t o t a l area of s l a g - m e t a l c o n t a c t , cm -3 C c o n c e n t r a t i o n of d i f f u s i n g spec ies i n the s l a g , g cm -3 C 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 same, g cm -3 C b u l k c o n c e n t r a t i o n of same, g cm o d t h i c k n e s s of s l a g boundary l a y e r , cm 2 -1 D d i f f u s i o n c o e f f i c i e n t , cm sec 2 D. . d i f f u s i o n c o e f f i c i e n t of i i n d i l u t e s o l u t i o n i n i , cm sec e r f x e r r o r f u n c t i o n of x e~! 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 i f_^  Henryan a c t i v i t y c o e f f i c i e n t of i F F a r a d a y ' s 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 e l e c t r i c c u r r e n t , A -2 - 1 J d i f f u s i v e mass f l u x per u n i t area , g cm sec k constant k 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 ( s o l i d i f i c a t i o n ) e k e q u i l i b r i u m d i s t r i b u t i o n c o e f f i c i e n t ( s o l i d i f i c a t i o n ) 2 -1 k ' p a r a b o l i c constant of o x i d a t i o n cm sec P . K r e a c t i o n constant K r e a c t i o n constant ( p a r t i a l pressures ) P 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 s u b s c r i p t f o r meta l - x i v -_3 [M] 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 (M) 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 " 3 _3 (M) ox idant i n the s l a g e q u i v a l e n t to [M], g cm N . mole f r a c t i o n of i x p^ p a r t i a l pressure qf i , atm. i n t e g r a t e d mass f l u x of i , g sec 1 r r a d i u s c o o r d i n a t e R r a d i u s of the e l e c t r o d e , cm R^ r a d i u s of the mol4, cm s s u b s c r i p t f o r s l a g S i n t e r f a c i a l area t t i m e , sec t exposure t i m e , sec e T a b s o l u t e temperature , °K u . m o b i l i t y of i o n i x v , v v e l o c i t i e s i n p o l a r c o o r d i n a t e s ( s l a g ) , cm sec r 6 v , v , v v e l o c i t i e s i n C a r t e s i a n c o o r d i n a t e s , cm sec 1 x y z v t e r m i n a l v e l o c i t y , cm sec 1 oo ' * V process v o l t a g e , V W mass of remelted m e t a l , g , m a w ' W = - — melt r a t e , g sec m 3 t • » 6 • - 1 .V w I- ' - 3 - 1 W - m melt r a t e , cm sec m — Pm W mass of the s l a g b a t h , g - X V -x , y , z C a r t e s i a n c o o r d i n a t e s , cm v w Y = _m W 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 of i B , r E u l e r i a n f u n c t i o n s 6 Thickness 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 , cm 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 x AH Entha lpy of r e a c t i o n , c a l mole X AS Entropy of r e a c t i o n c a l mole X ° K X G B a s a l angle of the e l e c t r o d e t i p cone K mass t r a n s f e r c o e f f i c i e n t , cm sec X 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 x u ( w i t h s u b s c r i p t ) v i s c o s i t y , p o i s e -4 p (without s u b s c r i p t ) 10 cm II product p=pm-ps g c m - 3 -3 P m >P s 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 t summation u , - 1 - 2 x shear s t r e s s , g cm sec $ e l e c t r i c f i e l d , v o l t cm x p KA s (j) =— d i m e n s i o n l e s s W m % weight percent p r o p o r t i o n a l to - x v i -ACKNOWLEDGEMENT The author i s indebted 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 work. Thanks are a l s o due to D r . K . Brimacombe and f e l l o w graduate s tudents 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 a p p r e c i a t e d . 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 grants i s g r a t e f u l l y acknowledged. CHAPTER I INTRODUCTION 1 . 1 The ESR process The electroslag process (ESR) is 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) is possible 5 . Solidification pattern which is 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 limited to some pressure sensitive reactions and the floating out of inclusions. The schematic diagram of figure 1 shows immediately that the situation is very different in a typical ESR unit. There are several interfaces of contact between the metal and i t s environment, resulting in many possible sites for chemical reactions. The most important sites are the electrode-atmosphere, the slag-atmosphere, the electrode-slag and the ingot pool-slag interfaces. While i t is 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 industrial operation (1 ) . The lower capital cost and slightly higher production rate of electroslag -melting equipment are offset by the higher specific 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 aircraft industry and the difference in the level pf development attained in various countries is more the result of tradition or of the available technology at a given time than of a deliberate choice. ESR has a clear advantage in the production of shaped ingots (hollows, slabs) which cannot be made by VAR, mainly because of the pressure differential to which the copper mold is subjected between the vacuum and the cooling water ( 2 ) . The world production of ESR steel is very unevenly distributed among developed countries. Duckworth and Hoyle (3) report that,the production of ESR steel in the USSR is the largest in the world: because i t provides a substitute for VAR material in that country, whereas in the-rest of the world, ESR is regarded as competitive or - 3 -complementary, depending on the c i r c u m s t a n c e s . 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 producer r e p o r t s h a v i n g produced i n g o t s of 23 tons (4 ) . A f u r t h e r i n c r e a s e to about 70 tons ( 1.8 m i n diameter) i s foreseen 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 changes. The optimum r a t e of p r o d u c t i o n i s l i m i t e d by the type of s o l i d i f i -c a t i o n p a t t e r n which i s d e s i r e d , the a v a i l a b l e power and , i n VAR, the maximum r a t e at which heat can be e x t r a c t e d through 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 ton i n g o t of 1 m i n diameter (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 i n c r e a s e 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 S e c t i o n I V . 2 . 1 3 ) , the l e n g t h of the i n g o t be ing then the main f a c t o r l i m i t i n g the p o s s i b l e p r o d u c t i o n . VAR i n g o t s are s o l i d i f i e d at a s l i g h t l y lower r a t e (25 to 50% l e s s ) . T y p i c a l power consumption i s 1200 to 2000 KWh per ton of m e t a l f o r ESR, and somewhat l e s s f o r VAR( l ) ( 5 ) . Both processes are r e c o g n i z e d to add approx imate ly the same cost per ton of m e t a l , depend-i n g upon the s i z e of the o p e r a t i o n (5 ) . 1.2 Statement of the problem I n m o s t i n s t a n c e s , the E l e c t r o s l a g process 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 process where the c o m p o s i t i o n of the f i n a l 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 e l e c t r o d e minus i t s most conspicuous 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 " process (5) where cont inuous 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 achieve 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 patents 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 lements . - 4 -N e v e r t h e l e s s , t h i s g o a l i s not always a c h i e v e d , e s p e c i a l l y where 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 . 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 important consequences i n caus ing the f i n a l product to l o s e i t s r e q u i r e d p r o p e r t i e s and hence f a i l to meet commercial s p e c i f i c a t i o n . 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 methods of p r e v e n t i n g o x i d a t i o n . Buzek and a l . (7) mention the use of an argon atmosphere or 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 r e s i s t a n t p a i n t s . Most o f t e n i t i s c la imed tha t a d j u s t i n g the s l a g compos i t ion 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) suggests tha 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 t i t a n i u m . E o l z g r u b e r (10) recommends that r e d u c i b l e ox ides should be avoided 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 e lements . A more recent p u b l i c a t i o n (11) r e p o r t s tha t " b a l a n c e d " s l a g s , c o n t a i n i n g TiO^ and A ^ O ^ a ^ e 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 both t i t a n i u m and aluminum. There remains however the q u e s t i o n of whether or not an i n c r e a s e i n the t i t a n i u m ox ide content of the s l a g prevents o x i d a t i o n by s h i f t i n g the chemical e q u i l i b r i u m . The apparent r e t e n t i o n of t i t a n i u m or aluminium may r e f l e c t an i n c r e a s e d content of ox ide i n c l u s i o n s . I t has a l s o been suggested (11) t h a t e l e c t r o l y t i c removal of i o n s 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 prevent the t r a n s p o r t of oxygen from the atmosphere to the s l a g . 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 , s i n c e - 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 p r e s e n t , 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 -t i o n s . In Maraging S t e e l type 300, f o r i n s t a n c e , which c o n t a i n s about 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 content w i l l cause the 7 2 y i e l d s t r e n g t h s to drop by 6 x 10 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 l i m i t s (.38 to .43% i n the case of AIS I 4340 which p r e c i p i t a t e s 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 users of the process seem to i n d i c a t e that l o s s e s of r e a c t i v e elements cannot be t o t a l l y a v o i d e d , i t i s of paramount importance tha t these l o s s e s should at l e a s t be known i n advance and reproduced from mel t to m e l t . The sampling and a n a l y s i s of the remel ted m e t a l d u r i n g the process i s not a p r a c t i c a l p r o p o s i t i o n . 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 at by a d j u s t i n g the i n i t i a l ( e l e c t r o d e ) compos i t ion to compensate f o r 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 . In i n d u s t r i a l p r a c t i c e , t h i s c o r r e c t i o n i s a r r i v e d at 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 tha t the a p p l i c a t i o n of mathemat ica l models to mass t r a n s f e r c o n d i t i o n s i n the E l e c t r o s l a g process cou ld r e p l a c e the " t r i a l and e r r o r method" i n de termining c o m p o s i t i o n changes i n the 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 these changes. In t h i s c o n t e x t , i t should be noted tha 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 compos i t ion 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 ( " segregat ion" ) i n a, g i v e n - 6 -i n g o t . I n summary, the purpose of t h i s study was to answer the f o l l o w i n g p o i n t s : - 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 occur 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 , which cannot be accounted f o r by the s imple removal of ox ide i n c l u s i o n s ? - can a m a t e r i a l balance be w r i t t e n f o r the elements i n c o n s i d e r a t i o n ? - 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 composi t ion? - are the l o s s e s e l e c t r o c h e m i c a l i n nature? - where are the r e a c t i o n s i t e s ? - to what extent 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 Choice of m a t e r i a l s 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, experiments have been c a r r i e d out w i t h the U . B . C . E l e c t r o s l a g u n i t . The d e s i g n of t h i s u n i t i s s p e c i f i c a l l y adapted to the requirements of a range of r e s e a r c h p r o j e c t s . I t w i l l be d e s c r i b e d i n Chapter I I . 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 or 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 designed equipment; they are r e f e r r e d to i n the t e x t (see a l s o appendix 3 ) , and w i l l be the b a s i s f o r the choice of composi t ions l i k e l y to be compat ib le w i t h the chemica l requirements of the remel ted m e t a l s . This w i l l be done by adding to the b a s i c CaF^ s l a g v a r i o u s amounts of the ox ide 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 the m e t a l l i c phase ( (T iC^) f o r [ T i ] or ( A l ^ ) f o r [ A l ] . . . ) An important problem has been the s e l e c t i o n of s u i t a b l e a l l o y s f o r the purpose pf the s t u d y . The f o l l o w i n g ones have been s e l e c t e d (exact composi t ions are g i v e n i n Chapter I I I , 2 ) . 321 S t a i n l e s s S t e e l T h i s a l l o y has a b a s i c compos i t ion 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 content should be at l e a s t f i v e t imes carbon and i s u s u a l l y kept around 0.5%. The presence i n the e l e c t r o d 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 creates i n t e r e s t i n g problems as to the a c t u a l d e o x i d a t i o n r e a c t i o n s and the c o m p o s i t i o n of 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%) are the p r i n c i p a l elements sub jec ted to o x i d a t i o n w h i l e the m a t r i x i s b a s i c a l l y i r o n -18% N i . The d e o x i d a t i o n products should here be e s s e n t i a l l y t i t a n i u m o x i d e s . The c o m p o s i t i o n changes are to be compared w i t h those of the p r e v i o u s m a t e r i a l , both 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 A l A h i g h c o n c e n t r a t i o n (3.74%) of aluminium i s present i n t h i s a l l o y along 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 along w i t h the r e a c t i o n between aluminium and t i t a n i u m ox ides d u r i n g r e m e l t i n g i s of i n t e r e s t . - 8 -Ferrovac E (Fe , 0.01% C, 0.001% Mn, 0.006% S i . . . ) 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 c o n d i t i o n s can be compared w i t h the behaviour of the o ther a l l o y s . 1.4 P r e l i m i n a r y d i s c u s s i o n 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 ox ides 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 has been completed. I t i s reasonable to assume t h a t the mechanism by which they are removed from the meta l i s , e i t h e r t h e i r d i s s o l u t i o n 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 , or t h e i r o x i d a t i o n at a s l a g - m e t a l i n t e r f a c e . D i r e c t d i s s o l u t i o n of m e t a l l i c 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 at the s lag-atmosphere i n t e r f a c e w i l l be r u l e d out when i t can be argued that 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 metals i n i t i s l o w . 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 l a t e r . 1.4.1 Atmospheric oxygen i s one important f a c t o r of o x i d a t i o n . I t can enter the system i n two ways: 1 . 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 i o n s . A few p o s s i b l e r e a c t i o n s a r e : 2 ( F e 2 + ) + l / 2 0 ? " - • 2 C F e 3 + ) + ( 0 ~ ) 2 ( T i 3 + ) + l / 2 0 2 • 2 ( T i 4 + ) + ( 0 ~ ) (Ca). + 1/20 • ( C a 2 + ) + (o"~) _ 9 -2. The e l e c t r o d e i t s e l f i s o x i d i z e d be fore i t p h y s i c a l l y enters the molten s l a g . The extent of t h i s r e a c t i o n can be d i r e c t l y observed or c a l c u l a t e d w i t h the h e l p of temperature p r o f i l e s measured on the e l e c t r o d e s (Chapter XV, 1 , 2 ) . 0 o + Fe »- FeO ( s o l i d ) ^ { FeO (s lag) 2 . v ' L [0] 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 meta l and s l a g i s to take p l a c e . In most of the a l l o y s we c o n s i d e r h e r e , the normal d e o x i d a t i o n product i s the pure o x i d e of the r e a c t i v e element ( T i and A l ) . The removal of the prpduct of the o x i d a t i o n r e a c t i o n or the supply of oxygen through a s l a g boundary l a y e r i n contac t w i t h the meta l may both 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 heterogeneous . , . 0 ( s lag) — ; •> 0 (metal) r e a c t i o n D i f f u s i o n of bo th oxygen and r e a c t i v e elements i n the meta l i t s e l f 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 s l a g — ^ m e t a l * 3 ^ + 2 ^ — A 1 2 ° 3 In t h i s case , d i f f u s i o n r a t e s of both 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 r e a c t i o n . / i - 10 -1 .4 .3 In the p a r t i c u l a r case of 1409 s t e e l , a s u b s t a n t i a l vapour pressure of aluminium develops i n the l i q u i d m e t a l . D i r e c t e v a p o r a t i o n of aluminium i s t h e r e f o r e to be c o n s i d e r e d . 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 whether they are ox ides or c a r b i d e s , s u l p h i d e s , e t c . . . i s t r a n s l a t e d i n a composi-t i o n change. In numerous cases , the r e s u l t i n g e f f e c t w i l l be f a v o u r a b l e to the p r o p e r t i e s of the f i n i s h e d p r o d u c t . I t i s important t h e r e f o r e to d i s s o c i a t e compos i t ion changes due to o x i d a t i o n of m a t r i x elements and compos i t ion changes due to the removal of i n c l u s i o n s . 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 w i t h l i m i t i n g r a t e s of; r e a c t i o n r e q u i r e s a knowledge of p h y s i c a l and chemical parameters i n the system. These are a c t i v i t i e s of the c o n s t i t u e n t s i n the s l a g and 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 , v i s c o s i t i e s , c o n v e c t i o n p a t t e r n s i n the l i q u i d phases e t c . . . T h e i r measurement or computat ion w i l l . a l s o be d i s c u s s e d . CHAPTER I I THE U . B . C . ELECTROSLAG UNIT I I . I G e n e r a l i t i e s 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 processes 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 c h a r a c t e r i s t i c s . Smal l l a b o r a t o r y u n i t s can produce i n g o t s which present the s i x main q u a l i t i e s l i s t e d i n (1.1) to an extent comparable w i t h the l a r g e s c a l e i n g o t s . Res i s tance 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 . As s m a l l u n i t s r e q u i r e h i g h e r c u r r e n t d e n s i t i e s , i n t e r m i t t e n t 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 lower l i m i t on workable i n g o t diameters (around 5 cm, see a l s o I V . 2 . 1 3 ) . Commercial o p e r a t i o n i s done on i n g o t s (or s l a b s e t c . . ) which range i n s i z e from 0.15 to 1 m i n diameter (1970) a l t h o u g h the upper l i m i t seems to be a v e r y temporary one. A t y p i c a l f u r n a c e , produced 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 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 , overhead, f l o o r loads - 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 l a b o r a t o r y . • In the present case , 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 imple framework must accept 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 a long w i t h a u x i l i a r i e s f o r the c o n t r o l of the atmosphere, the cont inuous a d d i t i o n of s l a g or d e o x i d a n t , the p r o b i n g and sampling of the b a t h . - 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 e i t h e r , p o l a r i t y or A . C . or a m i x t u r e of b o t h ; both 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 . - I n s t r u m e n t a t i o n and access p o i n t s have to be p r o v i d e d f o r the measurement of a l a r g e number of parameters which can a f f e c t the p r o c e s s . I I . 2 Framework and e l e c t r o d e t r a v e l mechanism The framework i s designed ( f i g u r e 4) to accommodate v a r i o u s molds and a c c e s s o r i e s . I t i s r i g i d enough to support e l e c t r o d e s weighing 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 bending of the 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 e l e c t r o d e ) . The e l e c t r o d e h o l d e r i s secured 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 automatic w e l d e r . (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 C a r ) . The normal rack and p i n i o n d r i v e of the r a i l i s not used . In i t s p l a c e the moving t r o l l e y i s d r i v e n by a cont inuous 3 /4" p i t c h c h a i n 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 behind the u n i t (Boston Gear R a t i o t r o l BG29005). The 1750 RPM motor i s geared down i n 2 stages by a f a c t o r of 2000, thereby p r o v i d i n g a good c o n t r o l over the e l e c t r o d e t r a v e l v e l o c i t y between 0.25 and 12 cm min \ The c a r r i a g e t r a v e l i s l i m i t e d at both ends of the r a i l by m i c r o -- 1 3 -switches which 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 ) . S i m i l a r l y , 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 e l e c t r o d e to approx imate ly 100 k g f . T h i s prevents damage to the t r a n s m i s s i o n and c h a i n attachment 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. P r o v i s i o n f o r 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 p r o v i d e d . I I . 3 E l e c t r o d e h o l d e r ( f i g u r e 6) The e l e c t r o d e i s h e l d from an arm overhanging 0.21 m from the beam, permanently a l i g n e d and secured on the moving t r o l l e y . The e l e c t r o d e i s clamped by a s p l i t copper p l a t e , water cooled and cored to an i n s i d e diameter of 1 . 5 " . B igger diameters of e l e c t r o d e s are accommodated by w e l d i n g a stub at the h o l d i n g end; s m a l l e r diameters by i n s e r t i n g s p l i t bushings of the a p p r o p r i a t e s i z e . Power i s fed through f o u r i n s u l a t e d c a b l e s , each r a t e d at 500 Ampere and supported by a cross member secured to the main v e r t i c a l beam. I I . 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 . S i x threaded b l i n d ho les on a 159 mm c i r c l e r e c e i v e studs used to clamp down the mold on top of the p l a t e . Power i s c a r r i e d to the b a s e p l a t e by a 1" diameter copper rod threaded at b o t h ends and secured i n a b l i n d h o l e a t the bottom center of the p l a t e . Four 500 A m p e r e power leads are connected to the lower end of the r o d . - 14 -A water j a c k e t covers the bottom face of the p l a t e . To prevent f o r m a t i o n of any a i r or steam pocket on the w a t e r c o o l e d s i d e of the b a s e p l a t e , t h i s has been turned 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 the 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 i n i e t s p r o v i d e a s w i r l motion 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 b a s e p l a t e . P r o t e c t i o n to the upper face of 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 , approx imate ly 5 mm t h i c k , which 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 are i n s u l a t e d from the mold by a l a y e r of asbestos and prevent overhea t ing of the copper base . The faces of the dummy p l a t e s are 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 . In the event tha 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 to the upper dummy p l a t e , a r c i n g w i l l o n l y occur between the two dummy p l a t e s , thereby s a v i n g the s u r f a c e of the copper . I I . 5 Mold and w a t e r j a c k e t ( f i g u r e 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 chosen. 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 l a r g e diameter near the t o p . Warm water (30°C) i s used to a v o i d condensat ion 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 -f l a n g e . The mold i s e l e c t r i c a l l y i n s u l a t e d ( 10 Kfi) from the base -p l a t e by an asbestos gasket . M i l d s t e e l molds have a l s o been t r i e d ; i n t h i s case , the t h i c k n e s s of the w a l l i s reduced to a maximum of 3 mm to avo id 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 . T h i s l a t t e r c o n d i t i o n 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 to process i n s t a b i l i t y and a r c i n g . Mold s i z e s range from 30 to 80 cm i n u s e f u l h e i g h t (40 to 90 cm o v e r a l l ) and 5 to 10 cm i n i n s i d e d i a m e t e r . I I . 6 Atmosphere c o n t r o l Two types of hoods are i n use : one, which p r o v i d e s an argon b l a n k e t and e x t r a c t i o n 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 sea led chamber i n which the e l e c t r o d e i s h e l d onto a watercoo led s t u b . A rubber b e l l o w s clamped to the stub at 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 pressure i n case of e x p l o s i o n . T h i s l a s t 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 which depends almost e x c l u s i v e l y upon the p u r i t y of the argon used . A f t e r the i n i t i a l purge , a v e r y s m a l l f l o w (200 ml hr x ) i s s u f f i c i e n t to . prevent back d i f f u s i o n of a i r through the s m a l l exhaust and the asbestos gasket at the base of the m o l d . By c o n t r a s t , chromatographic a n a l y s i s of the argon b l a n k e t i n d i c a t e s that i t may c o n t a i n up to 1% 0^ w i t h a f l o w rate of 100 1 / h r . - 16 -I I . 7 Continuous s l a g or 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 designed 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 d u r i n g the m e l t . A v e r t i c a l c a n n i s t e r whose base i s c l o s e d by the 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 or s m a l l granules ) through a c a l i b r a t e d g a t e . The stream of m a t e r i a l i s then wiped over the edge of the p l a t e i n t o the mold . The 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 Corp. Motomatic E5503) geared down by a f a c t o r of 3000:1 thereby 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 of 0 to 10 g/min of m a t e r i a l . (Maximum RPM of . r o t a t i n g t a b l e : 1.7) I I . 8 Other r e m e l t i n g equipment A water cooled e l e c t r o d e h o l d e r , made of copper , and a l s o used w i t h the t i g h t atmospheric s h i e l d ( I I . 6 ) i s a v a i l a b l e f o r non-consumable e l e c t r o d e experiments ( f i g u r e 12) . A movable arm which can be brought near the e l e c t r o d e p r o v i d e s a guide f o r a s l a g temperature probe , p a s s i v e e l e c t r o d e or s l a g sampler ( f i g u r e 13) . The v e r t i c a l mot ion of the arm i s c o n t r o l l e d manual ly or w i t h an e l e c t r i c a l motor . The whole e l e c t r o s l a g r i g i s enc losed on three s i d e s to p r o t e c t the opera tor and the l a b o r a t o r y i n the event of an e x p l o s i o n ( P i c t u r e f i g u r e 14) . I I . 9 D . C . Power supply ( f i g u r e 15) D . C . power i s s u p p l i e d by two commercial Eobart Welders (Hobart RC750) connected i n p a r a l l e l . Maximum c u r r e n t i s l i m i t e d to 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 approx imate ly 10 to 50 V (open c i r c u i t v o l t a g e 55 V ) . The welders 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 which 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 r e a c t o r s (the pr imary power i s t h e r e f o r e 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 s l o p e of the c u r r e n t v o l t a g e curve on the secondary a l l o w s these u n i t s to operate s a t i f a c t o r i l y 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 process 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) (11 .16) . R e c t i f i c a t i o n i s done by a set of s i l i c o n r e c t i f i e r s which are p a r t of the u n i t . 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 approx imate ly 10% (RMS) of the t o t a l D . C . c u r r e n t . T h i s i s cons idered to have a n e g l i b i b l e i n f l u e n c e on the e l e c t r o c h e m i c a l processes 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 remains 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 decomposi t ion p o t e n t i a l of the s l a g components. Furthermore , i t has been observed (13) that p o l a r i z a t i o n processes on the e l e c t r o d e s are s low 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 welders i s grounded. Each u n i t has i t s own c i r c u i t breaker w h i l e the s t a r t i n g r e l a y s are operated 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 supply ( f i g u r e 15) The A . C . stepdown transformer operates from one phase of the mains . The t ransformer e x h i b i t s n e a r l y constant v o l t a g e c h a r a c t e r i s t i c s 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 of the v o l t a g e i s a v a i l a b l e through a set of tappings on the pr imary c o i l . T h i s - 18 -p r o v i d e s 4 v o l t a g e s of 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 narrower l i m i t s i s done by v a r y i n g the pr imary v o l t a g e of the s tep down t r a n s f o r m e r . A booster t rans former whose pr imary c o i l i s f ed by a v a r i a c (0 to 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 the secondary of the s tep down t rans former f o r the o p e r a t i o n of the ESR r i g : 28.3 to 35 V 24.3 to 30 V 20 to 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 pr imary c u r r e n t which can be drawn from the mains (230 amps). T h i s corresponds to a p p r o x i -mately 50 KW ( t y p i c a l l y 1600 amps, 24 v o l t s ) on the secondary. 11.11 D . C . b i a s A recent 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 of r e c t i f i e r s i s f ed by the secondary c i r c u i t of the t rans former w i t h a v a r i a b l e r e s i s t o r (0 to 10 mO) i n s e r i e s ( f i g u r e 15) . 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 va lue of the v a r i a b l e r e s i s t o r . - 19 -11.12 C o n t r o l s ( 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 both welders - s i n g l e power r h e o s t a t ( r e s i s t o r ) v a r y i n g the degree of s a t u r a -t i o n of the r e a c t o r s on both welders - D . C . ammeter connected across 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 and the e l e c t r o d e (0-50 V) - coulometer : the s i g n a l from the shunt i s a m p l i f i e d through a K e i t h l e y 153 microvol t -ammeter and i n t e g r a t e d by a L e c t r o c o u n t I I coulometer (Royson Engineer ing) w i t h d i g i t a l r e a d i n g - An automatic 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 constant c u r r e n t d u r i n g the process i s a l s o a v a i l a b l e . The s i g n a l from the shunt i s opposed by a mV source corresponding to the d e s i r e d s i g n a l . The r e s u l t i n g ) s i g n a l (shunt + source) i s f ed i n t o a cont inuous balance or n u l l d e t e c t o r c i r c u i t (Brown Instruments I n c . , M i n n e a p o l i s - H o n e y w e l l ) . , The output of the cont inuous balance u n i t i s f ed to a r e v e r s i b l e D . C . motor which operates a m i c r o s w i t c h through a cam arrangement. T h i s cuts on and o f f the e l e c t r o d e d r i v e to compensate f o r c u r r e n t v a r i a t i o n s ( f i g u r e 5 ) . - Mains A . C . current (0 to 250 amps). 11.13 C o n t r o l s ( 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 : - 20 -- o n - o f f 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 15) - RMS process v o l t a g e (EICO 250 v o l t m e t e r ) - RMS process c u r r e n t : 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 b e i n g fed to the D . C . Ammeter and the coulometer - Mains pr imary c u r r e n t (0 to 250 amps). 11.14 C o n t r o l s (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 : - d i g i t a l counter 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 - cont inuous r e c o r d i n g of the process c u r r e n t ( i n p a r a l l e l w i t h the ammeter) - measurement of the mold p o t e n t i a l r e l a t i v e to the e l e c t r o d e or b a s e p l a t e p o t e n t i a l - measurement of both the i n l e t and o u t l e t temperature of the mold c o o l i n g water (Thermistor probe and Y . S . I . 42SC telethermometer) - t w i n channel o s c i l l o s c o p e which d e t e c t s the ampl i tude and the phase angle of b o t h process c u r r e n t (shunt) and process 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 e l e c t r o d e 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 . , type 564 s torage o s c i l l o s c o p e ) . 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 S e v e r a l 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 e s e a r c h 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 most obvious one i 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 . - 21 -Most i n d u s t r i a l u n i t s operate on A . C . power as t h i s mode has been found to be economical both i n investment and power consumption. I t 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 : The s l a g i s an i o n i c conductor used w e l l above i t s decomposi t ion ( 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 anodic and c a t h o d i c r e a c t i o n c o u l d have important consequences on the f i n a l l e v e l of a l l o y i n g or i m p u r i t y elements ( s u l p h u r , o x y g e n . . . ) . T h i s i s examined i n more d e t a i l i n a l a t e r s e c t i o n . 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 or the use of A . C . c u r r e n t on the behaviour of the remel ted m a t e r i a l . S i m i l a r l y , the i m p o s i t i o n of a s m a l l D . C . component (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 c u r r e n t 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 i n s t a n c e i n enhancing d e o x i d a t i o n . The c h o i c e of a cont inuous c h a i n d r i v e f o r the e l e c t r o d e i s i n keeping 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 weight of the e l e c t r o d e 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 s o l i d b r i d g e s which i n t e r f e r e w i p h the downward m o t i o n . A p o s i t i v e p r e s s u r e on the e l e c t r o d e i s r e q u i r e d . T h i s system a l s o ensures a smooth and cont inuous mot ion 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 bes 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 that 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 se t -up and . i n i t i a l run p r e p a r a t i o n . - 22 -For 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 weights and not 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 , the cont inuous c h a i n d r i v e has been found s a t i s f a c t o r y . 11.16 S t a r t i n g 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. Switchover to A . C , when r e q u i r e d , i s 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 the use of an A . C . t rans former as the source of power i m p r a c t i c a l because of i t s constant v o l t a g e c h a r a c t e r -i s t i c s (see power s u p p l i e s 1 1 . 9 , 1 0 ) : 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 shor 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 supply to have a s i g n i f i c a n t r e a c t a n c e . 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 l i q u i d s l a g poured i n s i d e the -mold would immediate ly 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 p a t h . S t a r t i n g c o n f i g u r a t i o n i s represented on f i g u r e 2 . The s t a r t i n g compact c o n s i s t s of a m i x t u r e of m e t a l t u r n i n g s having a m e l t i n g p o i n t near tha t of the e l e c t r o d e , 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, a c c o r d i n g to the 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 . The e l e c t r i c a l r e s i s t a n c e of the c o l d compacts i s ad jus ted to be comparable to that of the running u n i t , i . e . 8 to 50 -m^. The compacts are u s u a l l y 37 mm i n diameter and the t o t a l h e i g h t v a r i e s between 25 mm Cpositive e l e c t r o d e of 25 mm i n diameter) and 60 mm 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 compos i t ion - 23 -o f t h e t u r n i n g s i s a d j u s t e d to t h a t o f t h e r e m e l t e d m a t e r i a l i f i t i s i m p o r t a n t t o do s o . The s o l i d s l a g s h o u l d be c r u s h e d i n t o g r a n u l e s l e s s t h a n 6 mm i n s i z e and 15 to 30% powder as t h i s has been f o u n d to f a v o u r r a p i d m e l t i n g w h i l e p r e v e n t i n g e l e c t r o d e hangups (a p r o b l e m w i t h s m a l l u n i t s ) . CHAPTER I I I REMELTING OF INGOTS I I I . l Procedure and p r e c a u t i o n s The s t a r t i n g procedure has been d e s c r i b e d (11 .16) . The c o o l i n g water i s turned on at the same time as the power to a v o i d condensat ion i n the melt zone and on the e l e c t r o d e h o l d e r . T h i s i s a u s e f u l p r e c a u t i o n as the r e a c t i o n between molten c a l c i u m f l u o r i d e and water ( h y d r o l y s i s ) i s u s u a l l y e x p l o s i v e . Warm water 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 t u r n i n g s of the remelted meta l compos i t ion or of pure i r o n (Ferrovac E) to avo id chemica l i n t e r f e r e n c e w i t h the m e l t . . They are kept 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 kept i n a dry atmosphere. The e l e c t r o d e s are degreased and desca led be fore r e m e l t i n g . M e t a l 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 melt and act as a source of oxygen i n the system. I t i s important to remove the i n i t i a l 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 which the e l e c t r o d e o x i d i z e s as i t enters the p r o c e s s . - 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 the experiments performed. Both l i v e and i n s u l a t e d molds are used i n d u s t r i a l l y , the l a t t e r be ing more common. From an e l e c t r i c a l p o i n t of v i e w , the mode chosen w i l l determine the c u r r e n t pa th through the s l a g to a great extent as a great p a r t (80% a c c o r d i n g to (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 . In 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 are r e a c t i o n i n t e r f a c e s f o r e l e c t r o c h e m i c a l r e a c t i o n s and are known to 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 , the mold w a l l s t i l l p l a y s a p a r t i n the 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 . I t c o u l d have an i n f l u e n c e on the content of t r a c e elements i n the meta l (oxygen, i f the system i s s h i e l d e d ) but would probably not be an important 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 processes which we c o n s i d e r l a t e r . The e l e c t r i c a l r o l e of the mold has been the s u b j e c t of a separate i n v e s t i g a t i o n (67) . I I I . 2 Composi t ion of the a l l o y s s t u d i e d ( i n wt %) A u s t e n i t i c s t a i n l e s s s t e e l : 321 grade . A i r m e l t e d . ( S u p p l i e d 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 N i T i S i Mn C P S O . B a l 17.78 10.60 .58 .56 1.86 .05 .031 .018 .0009 - 26 -Maraging S t e e l : 300 grade . Vacuum arc r e m e l t e d . ( S u p p l i e d by R e p u b l i c S t e e l C o r p . , C l e v e l a n d ) . Fe N i Co Mo T i Mn S i A l 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 grade . A i r mel ted ( S u p p l i e d by U n i v e r s a l C y c l o p s , B r i d g e v i l l e , P a . ) Fe Cr A l T i Mn S i Mo Cu N i 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 Ferrovac 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 S i N i Cr Mo N 0 H B a l 0.010 0.001 0.002 0.004 0.006 0.01 <.01 .001 .0002 .00092 .000018 Co Cu V 1 W .006 .006 <.002 .02 I I I . 3 S lags Calc ium 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 . P a r t of t h i s m a t e r i a l ( f l o t a t i o n concentrate ) i s used as a dry powder to make up f o r the r e q u i r e d p r o p o r t i o n of powder and granules (11 .16 ) . Most of 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 c r u c i b l e . I n d u c t i o n h e a t i n g and an argon b l a n k e t are used. The - 27 -m a t e r i a l i s kept at h i g h temperature o n l y f o r the t ime r e q u i r e d f o r f u s i o n . T h i s p r e c a u t i o n minimizes carbon p i c k u p from the c r u c i b l e (3 ) . 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 , the amount v a r y i n g s l i g h t l y from run to r u n , depending m a i n l y upon the mois ture content of the i n i t i a l m a t e r i a l . Traces of s i l i c a and i r o n ox ide have a l s o been d e t e c t e d . 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 presence 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 i n f l u e n c e on the D . C . process 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 composi t ions I I I . 1 1 to I I I . 1 4 ) . Both CaO and s i l i c a t e s depress the m e l t i n g p o i n t of CaF^. Ko j ima and Masson (14) have r e c e n t l y p u b l i s h e d t h e i r experiments on the s u b j e c t and reviewed the a v a i l a b l e l i t e r a t u r e . T h e i r d e t e r m i n a t i o n of the f r e e z i n g temperature of CaF^ i s 1423°C, depressed by °^2.3°C per 0.01 mole f r a c t i o n of CaO and depressed by ^3°C per 0 .01 mole 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 are p r e s e n t . We have measured a f r e e z i n g p o i n t of 1405°C and t h e r e f o r e have up to 5 wt % of such i m p u r i t i e s . Alumina i s used e i t h e r as powder ( A l c a n , 99.9% p u r i t y ) or granules of e l e c t r o f u s e d alumina (Norton Co . ) of e q u i v a l e n t p u r i t y . Ca lc ium monoalumihate ( C a O ' A ^ O ^ ) i s pre fused 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 technique as f o r c a l c i u m f l u o r i d e . S t a r t i n g m a t e r i a l s are l i m e prepared from c a l c i u m carbonate ( t e c h n i c a l grade) f i r e d 12 hrs at 1100°C and alumina powder (see above) i n s t o i c h i o -m e t r i c q u a n t i t i e s . - 28 -Calc ium monot i tanate (CaO'TiO^) i s commerc ia l ly 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 spray c o a t i n g . I t i s c o l d pressed and s i n t e r e d i n a i r be fore use . T h i s procedure has the advantage of producing a dry m a t e r i a l i n lump form. 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 ox ide cou ld be detected i n the c a l c i u m t i t a n a t e . X-Ray examinat ion of the powder r e v e a l s the c h a r a c t e r i s t i c p a t t e r n of CaTiO^ along w i t h the three s t ronges t l i n e s of T i O ^ . Chemical a n a l y s i s however i n d i c a t e s tha t the t i t a n i u m content 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 the method (appendix IV) - i . e . 35.10 wt % versus a t h e o r e t i c a l v a l u e of 35.25 %. I I I . 4 Remelt ing c o n d i t i o n s f o r 321 S t a i n l e s s S t e e l The c o n d i t i o n s f o r r e m e l t i n g are kept w i t h i n the s t a b l e range of ESR - i . e . power i n p u t and melt r a t e are set 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 s t r u c t u r e . Tables 1 to 19 rev iew the power c o n d i t i o n s i n which the v a r i o u s i n g o t s have been remelted and the mel 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 which s t a b l e c o n d i t i o n s have been achieved are r e p o r t e d . Values are averaged over 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 producing an average i n g o t diameter of 5.60 cm when the s l a g s k i n has been removed. The e l e c t r o d e diameter i s 2.54 cm which corresponds to a cross s e c t i o n r a t i o of 4.87 between the i n g o t i and the e l e c t r o d e . The amount ,'vof 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 equal \ 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 : £ IP i I - 29 -L ( - — ) = 1.26 L i ± _ 4.87 L i s measured d i r e c t l y from the d i g i t a l counter (11.14). -3 The d e n s i t y of the s t e e l was measured to be 7.916 g cm at room temperature . The i n i t i a l q u a n t i t y of s l a g was 380 g of which 80 g was i n the s t a r t i n g compacts. The v a r i a b l e s l i s t e d below are the ins tantaneous weight of the dW * m remelted meta l ( i n g o t ) W , the s l a g weight W , the melt r a t e W = - — , rn. s m d t 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 are l i s t e d f o r p o i n t s i n the process which correspond to the m e l t i n g of the p o r t i o n of the i n g o t from which samples were taken f o r chemica l a n a l y s i s . These samples extend f o r a h e i g h t of about 6 mm or 120 g of i n g o t . The W v a r i a t i o n of _m over a sample i s approx imate ly 0.3 w h i l e the time needed W s to remelt 100 g ; i s o40 to 80 seconds. The margin: .of random e r r o r over the v a r i o u s measurements corresponds t o : : i 30 g on ( repor ted to the neares 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 . ± 0.1V 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 cut i n t o samples. Ingots and have been used to measure e l e c t r o d e temperature g r a d i e n t s ( I I I .17) and S Q l o and S^g to study convec t ion p a t t e r n s i n the s l a g ( I I I .16). - 30 -Table 1 . Ingot S l E l e c t r o d e n e g a t i v e Argon B lanket S lag ( i n wt %): 75% C a F 2 , 25% C a A l ^ Sample Ingot I n g o t - s l a g M e l t Oper. Oper. number weight weight r a t i o r a t e c u r r e n t v o l t a g e W W m W W~ s g sec 1 Amp V o l t IS 550 1.6 1.62 615 23.7 1C 750 2.2 1.84 625 23.8 2S 1260 3.7 1.57 635 23.7 2C 1470 4 .3 1.62 630 23.8 3S 1770 5.2 1.56 630 23.8 3C 1980 5 .8 1.66 630 23.8 4S 2600 7.6 1.68 630 23.6 4C 2800 8.2 1.70 620 23.5 Table 2. Ingot S2 E l e c t r o d e n e g a t i v e In a i r S l a g : 75% C a F 2 > 25% C a A l 2 0 Sample w , g m' 6 W m W s - 1 W , g sec m Amp V o l t IS 650 1.9 0.85 660 23.7 1C 850 2.5 1.27 650 23.6 2S 1650 4 .8 1.52 650 23.4 2C 1850 5.4 1.45 650 23.7 3S 2600 7.6 1.52 640 23.7 3C 2800 8.2 1.50 650 23.7 - 31 -Table 3. Ingot S3 E l e c t r o d e n e g a t i v e Argon B lanket S l a g : 75% C a F 2 > 25% C a A l ^ W " - 1 Sample W , g m W , g sec Amp V o l t m — m' ° w s IS 610 1.8 1.78 720 24.5 IC 820 2.4 1.93 720 24.4 2S 1460 4 .3 1.97 720 24.4 2C 1670 4 .9 1.98 715 24.3 3S 2020 6.1 2.03 710 24.4 3C 2210 6.5 2.08 Table 4. Ingot S4 E l e c t r o d e n e g a t i v e Argon B lanket S l a g : 75% C a F 2 , 25% C a A l ^ W * - 1 Sample W , g m W , g sec Amp V o l t m 7 7 - m W s 400 1.1 1.61 670 23.7 900 2.6 1.49 670 23.5 1350 3.9 1.37 660 23.8 Table 5 . 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 S l a g : 75% CaF„, 25% CaAl^O . . . . . . . 2 . . . . . z W " - 1 Sample W , g m W , g sec Amp V o l t W s 600 1.8 1.52 560 22.8 1200 3.6 1.81 575 22.8 1760 5 .3 1.68 570 22.8 - 32 -Table 6. Ingot S6 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 S l a g : 100% C a F 2 Sample W , g __m W , g sec Amp V o l t IS 700 2.2 1.37 680 22.8 1C 920 2.6 1.37 670 23.1 2S 1740 5.4 1.48 700 22.7 2C 1950 6.0 1.69 700 22.5 3S 3000 " 9.4 1.56 710 22.5 3C 3200 10.0 1.51 Table 7. Ingot S7 E l e c t r o d e n e g a t i v e Argon B lanket S l a g : 84.4% C a F 2 , 15.6% C a T i 0 3 W •* - 1 Sample W , g m W , g sec Amp V o l t r m r— m W s IS 560 1.6 2.00 680 23.2 1C 780 2.2 2.00 680 23.3 2S 1060 3.0 1.97 670 23.6 2C 1270 3.6 1.97 675 23.6 3S 2130 6.0 1.99 670 23.7 3C 2330 6.6 1.95 670 23.7 - 33 -Table 8. Ingot S8 E l e c t r o d e n e g a t i v e Argon B lanket S l a g : 68.4% C a F 2 > 31.6% C a T i 0 3 Sample W W,.g _m W W , g sec m -1 Amp V o l t IS 1C 2S 2C 3S 3C 590 810 1080 1290 2000 2200 1.7 2.3 3.1 3.7 5.7 6.3 2.05 2.33 2.35 2.35 2.52 2.54 680 670 660 660 640 640 23.4 23.6 23.3 23.4 23.7 23.7 Table 9. Ingot S9 E l e c t r o d e p o s i t i v e Argon B lanket S l a g : 100% CaF 2 Sample W W , g m m — W s W , g sec m -1 Amp V o l t IS 1C 2S 2C 3S 3C 810 .1010 1630 1830 2050 2250 2.3 2.9 4 .6 5 .2 5.8 6.4 1.52 1.59 1.32 1.35 1.34 1.36 630 600 630 650 650 650 22.3 22. 22. 22. 22. 22. - 34 -Table 10. Ingot S10 E l e c t r o d e p o s i t i v e Argon B lanket S l a g : 92.1% C a F 2 , 7.9% C a T i 0 3 W • - 1 Sample W , g m W , g sec Amp V o l t r m ° 7 7 - m 0 r W s IS 620 2.0 2.60 630 22.6 IC 820 2.6 2.18 630 22.4 2S 1450 4 .7 1.57 640 21.7 2C 1650 5 .3 1.67 640 21.7 3S 2040 6.8 1.72 660 21,6 3C 2240 7.2 1.62 650 21.7 Table 11. Ingot S l l 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 S l a g : 68.4% C a F 2 , 31.6% C a T i 0 3 W * - 1 Sample W , g m W , e sec Amp V o l t v m' ; 0 — m' & r w s . . IS 720 2.1 1.72 620 22.4 IC 920 2.6 1.92 650 22.2 2S 1450 4 .1 1.82 650 22.2 2C 1650 4 .7 1.82 650 22.2 3S 2010 5 .7 1.77 630 22.3 3C 2200 6.3 1.82 640 22.1 - 35 -Table 12. Ingot S l 2 A l t e r n a t i n g c u r r e n t Argon B lanket S l a g : 100% CaF 0 W * - 1 Sample W , g m W , e sec Amp V o l t m ° — m ° W s IS 540 1.8 1.99 590 27.2 1C 740 2.4 2.03 600 27.2 2S 1180 3.9 2.08 580 27.3 2C 1380 4.5 2.20 570 27.2 3S 2410 8.0 2.25 570 27.3 3C 2550 8.5 2.25 580 27.3 Table 13. Ingot S13 A l t e r n a t i n g c u r r e n t Argon B lanket S l a g : 64.7% C a F 2 , 35.3% C a T i 0 3 W ' -1 Sample W , g m W , g sec Amp V o l t r m ~— m • W s IS 1000 3.2 2.08 630 26.7 1C 1200 3.8 2.13 600 26.7 2S 1760 5.7 2.09 580 26.8 2C 1960 6.3 2.10 580 26.8 3S 2670 8.3 2.07 580 26.8 3C 2730 8.8 2.07 580 26.8 - 36 -Table 14. Ingot S l 4 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 S l a g : 68.4% CaF~, 31.6% CaTiO W Aluminium a d d i t i o n s t a r t s at - ~ = 1 .6 : r a t e W 1 g mm W " - 1 Sample W , g m W , g sec Amp V o l t s m ° — m W s . . . - 360 1.1 1.45 670 23.8 IS 520 1.6 1.90 690 23.7 IC 730 2.2 2.08 690 23.6 2S 1260 3.8 2.18 680 23.0 2C 1440 4.4 , 1.94 670 23.2 3S 1740 5 .3 2.45 670 23.2 3C 1940 5.9 2.41 680 23.1 4S 2170 6.6 2.35 670 23.2 4C 2370 7.2 2.12 670 23.2 Table 15. Ingot Sl5 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 S l a g : 64.7% CaF , 35.3% CaTiO W Aluminium a d d i t i o n s t a r t s a t ^ = 2 . 5 ; r a t e w 1 g mm Sample Wm> 8 W m W W , g sec m -1 Amp V o l t s IS IC 2S 2C 3S 3C 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 2.18 600 590 610 615 620 620 620 22.2 22.0 22. 22. 22. 22. 22. ,4 ,3 ,2 3 ,2 - 37 -Table 16. W Ingot S16 E l e c t r o d e n e g a t i v e — below 3.6 w s w m A l t e r n a t i n g c u r r e n t — above 3.6 Closed Argon Cap S l a g : 100% CaF 2 - w i t h 2 g aluminium i n i t i a l d e o x i d a t i o n Sample w , g m ° W m W s - 1 W , g sec m Amp D . C . / A . C . V o l t s D . C . / A . C . IS 300 1.0 1.29 660 22.0 1C 450 1.5 1.27 670 22.1 2S 810 2.7 1.24 700 22 . 0 2C 960 3.2 1.20 680 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 Table 17. W Ingot S17 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 Closed Argon Cap S l a g : 64.7% C a F 2 , 35.3% C a T i 0 3 - w i t h 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 V o l t s IS 900 2.9 2.02 660 22.5 1C 1080 3.5 2.26 650 22.6 2S 1490 4 .8 2.33 660 22.5 2C . 1670 5.4 2.29 670 22.4 3S 2050 6.6 2.58 650 27.0 3C 2230 7.2 2.54 650 27.2 4S 2720 8.8 2.66 650 27.1 4C 2820 9 .1 2.54 650 27.1 - 38 -W Table 18. Ingot S18 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 I n A i r S l a g : 91.0% C a F 2 , 9.0% C a T i 0 3 Sample m W m -1 W , g sec m Amp V o l t s vT s _ 610 2.0 1.93 725 22.5 - 1190 3.6 1.81 700 22.0 _ 1360 4.5 2.19 575 22.0 — 2310 7.6 2.27 575 22.0 Table 19. Ingot S19 E l e c t r o d e p o s i t i v e In A i r S l a g : 91.0% C a F 2 , 9.0% C a T i O 3 Sample w > g m W m W s -1 W , g sec m Amp V o l t s 1000 3.1 1.97 725 21.0 - 1750 5.5 2.15 600 22.5 - 39 -I I I . 5 Remelt ing 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 . The same v a r i a b l e s have been measured w i t h the same margin of e r r o r . The q u a n t i t y of s l a g , the e l e c t r o d e and mold diameter are a l s o the same and the r a t i o of the l e n g t h of e l e c t r o d e melted to the downward t r a v e l i s t h e r e f o r e 1.26. The d e n s i t y of the s t e e l was measured to be 8.039 at room temperature . Table 20. Ingot M l E l e c t r o d e n e g a t i v e Argon B lanket S l a g : 100 wt % CaF 2 - . _ Sample W , g _m W , g sec Amp V o l t W s IS 770 2.2 1.63 690 23.8 IC .950 2.7 1.74 690 24.1 2S 1610 4.6 2.15 680 24.1 2C 1790 5 .1 2.15 680 24.0 3S 2630 7.5 2.58 690 24.0 3C 2800 8.0 2.46 690 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 S l a g : 100% C a F 2 Sample w , g m W m W s » W , g sec m -1 Amp V o l t IS 690 2.1 1.59 690 22.5 1C 850 2.6 1.55 700 22.5 2S 2140 7.0 2.10 680 22.6 2C 2290 7.5 2.18 690 22.9 3S 3640 11.9 2.20 660 22.1 3C 3790 12.4 2.14 670 21.9 Table 22. Ingot M3 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 > S l a g : 82.4% CaF^, 17.6% C a T i 0 3 Sample w , g m' B W m W s W^ , g sec •1 Amp V o l t IS 440 1.2 .93 720 23.2 1C 660 1.8 1.67 700 23.2 2S 1100 3.0 2.10 680 23.3 2C 1280 3.5 2.11 680 23.3 3S 2020 5.5 2.19 680 23.4 3C 2200 6.0 2.16 670 23.3 - 41 -Table 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 S l a g : 68.4% C a F 2 , 31.6% CaTiO Sample W , g m W , g sec Amp V o l t m' — m r W s IS 660 2.0 2.81 660 23.8 1C 840 2.5 2.58 660 24.0 2S 1330 4 .0 2.48 670 23.7 2C 1500 4 .5 2.54 680 23.3 3S 2330 7.0 2.29 680 23.4 3G 2500 7.5 2.29 680 23.4 Table 24. 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 S l a g : 100% CaF W 1 - 1 Sample W , g m W , g sec Amp V o l t m 7— m r W s IS 540 1.7 2.32 620 22.6 1C 700 2.2 2.16 640 22.6 2S 1340 4.2 2.27 630 22.8 2C 1500 4.7 2.29 640 22.7 3S 2040 6.4 2.21 630 22.4 3C 2200 6.9 2.29 630 22.4 - 42 -Table 25. Ingot M6 E l e c t r o d e p o s i t i v e Argon B lanket S l a g : 68.4% C a F 2 , 31.6% CaTiO Sample W , g _m_ W , g sec Amp V o l t W s IS 390 1.1 1.55 620 22.3 IC 570 1.6 2.16 620 22.3 2S 1330 3.7 2.30 630 22.2 2C 1490 4 .2 2.26 630 22.1 3S 2200 6.2 2.18 630 22.3 3C 2380 6.7 2.19 630 22.3 W Table 26. 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 w s Closed Argon Cap S l a g : 100% C a F 2 + 2 d e o x i d a t i o n g A l f o r i n i t i a l Sample W m W s W , g sec m Amp V o l t 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 -I I I . 6 Remelt ing 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 are s i m i l a r to those o u t l i n e d i n I I I . 4 except 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 are hot r o l l e d square s e c t i o n bars ( " g o t h i c " ) 2 approx imate ly 3.76 x 3.76 cm . 2 The c r o s s - s e c t i o n a l area i s 13.6 cm . Mold diameter i s 7.75 cm producing an average i n g o t s i z e of 7.35 cm. The c ross s e c t i o n r a t i o i s t h e r e f o r e 3 .98 , the amount of mel ted e l e c t r o d e be ing 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 the r e l a t i o n L ( - > = 1.34 L 1 — 3.98 -3 The d e n s i t y of the s t e e l i s 7.385 g cm at room temperature . 740 g of s l a g was used of which 80 g was i n the s t a r t i n g compacts. Ingots of t h i s compos i t ion have been remelted w i t h s l a g s i n the CaF2«Al202 system. T h i s i s a s imple e u t e c t i c system w i t h no s o l i d s o l u b i l i t y , the e u t e c t i c c o m p o s i t i o n i s at approx imate ly 10 wt % AlyOy (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 t h i s c o n c e n t r a t i o n of A l ^ O ^ . Dur ing a g i v e n m e l t , the o x i d a t i o n of aluminium from the i n g o t r a i s e d the alumina content of the s l a g r e s u l t i n g i n a c o n s i d e r a b l e i n c r e a s e i n the l i q u i d u s temperature of s l a g s c o n t a i n i n g more than 10% A l ^ O ^ . T h i s produced a t h i c k e n i n g of the s k i n w i t h important e f f e c t s on the thermal balance and a r a p i d decrease of the s l a g p o o l - 44 -volume. The e f f e c t on the 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 . Table 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 S l a g : 100% CaF 0 Sample W m ' 8 W m W W , g sec m ° Amp V o l t IS 1C 2S 2C 3S 3C 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 Table 28. Ingot A2 E l e c t r o d e n e g a t i v e Argon B lanket S l a g : 95% C a F 2 > 5% A 1 2 0 Three a d d i t i o n s of 100 g CaF^ have been made at W = 1070 g , 1490 g and 2900 g . Sample W ,. g _m W , g sec Amp V o l t IS 890 - 2.31 1150 21.3 1C 1240 - 2.26 1160 21.4 2S 1790 - 2.21 1140 21.5 2C 2130 - 2.26 1120 21.3 3S 3380 - 2.21 1140 21.3 3C 3740 - 2.23 1130 21.4 - 45 -Table 29. Ingot A3 E l e c t r o d e n e g a t i v e Argon B lanket S l a g : 82.5% C a F 2 > 17.5% A l ^ Sample W , g _m W , g sec Amp V o l t IS 670 1.2 3.22 1010 22.8 IC 1020 - 3.41 1040 23.2 2S 1430 - 3.72 1030 23.4 2C 1780 - 3.63 1030 23.0 2S 2390 - 3.89 1020 23.5 3C 2740 14.0 3.85 1000 23.5 Table 30. Ingot A4 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 S l a g : 100% C a F 2 Aluminiun a d d i t i o n s t a r t s at Wm = 2400 g ; r a t e : 1 g min _ _ _ _ Sample W , g _m W , g sec Amp V o l t IS 880 1.5 3.22 1080 22.0 IC 1230 (2.1) 3.32 1080 22.8 2S 1860 (4.5) 3.60 1050 21.9 2C 2210 (5.6) 3.56 1050 21.8 3C 3030 (10.2) 4.15 1050 22.2 4C 3970 17.6 4.06 1030 22.0 - 46 -I I I . 7 Remelt ing of Ferrovac E and m i l d s t e e l Ingots of Ferrovac E have been remel ted i n order 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 ther a l l o y s . Ferrovac E e l e c t r o d e s are 3.28 cm i n diameter and have been remel ted i n the 5.84 cm m o l d . The amount of e l e c t r o d e mel ted i s t h e r e f o r e 1.46 L . The d e n s i t y of the meta l i s ^ 7.8 at room temperature . Other c o n d i t i o n s are r e p o r t e d i n s e c t i o n I I I . 4 . As sampling 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 or aluminium pickup from s l a g s c o n t a i n i n g T iO^ or A ^ O ^ , o n l y the average r u n n i n g c o n d i t i o n s are r e p o r t e d . These are taken a f t e r the i n i t i a l s t a r t i n g p e r i o d . Table 31. Ingot F l Ferrovac E E l e c t r o d e n e g a t i v e S l a g : 68.6% C a F 2 , 31.4% C a T i 0 3 . _ W , g sec Amp V o l t s .m r 2.70 930 24.2 Table 32. Ingot F2 Ferrovac E E l e c t r o d e n e g a t i v e S l a g : 76.3% C a F 2 , 23.7% A l ^ W , g sec 1 Amp V o l t s 1.45 1010 22.3 - 47 -Table 33. Ingot F3 M i l d S t e e l 1018 E l e c t r o d e n e g a t i v e S l a g : 81% C a F 2 , 19% C a T i 0 3 W , g sec X Amp V o l t s m 1.37 690 23.7 Table 34. Ingot F4 M i l d S t e e l 1018 E l e c t r o d e n e g a t i v e S l a g : 65.5% C a F 0 , 34.5% CaTiO, W , g sec m -1 Amp V o l t s 1.90 670 23.5 Table 35. Ingot F5 M i l d S t e e l 1018 E l e c t r o d e n e g a t i v e S l a g : 48% C a F 2 , 52% C a T i 0 3 W , g sec m -1 Amp V o l t s 1.92 680 23.2 - 48 -I I I . 8 A n a l y s i s and sampling of i n g o t s The chemica l a n a l y s i s of the remelted i n g o t s has been performed i n two ways f o r those elements which 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 e l e c t r o n microprobe f o l l o w i n g a technique d e s c r i b e d i n appendix I V , b) The t o t a l chemica l content 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 or by a wet chemica 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 content of t i t a n i u m i n 321 S . S . and Mar 300 has proved 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 (Chromium i n 321 S .S . and 1409 A l , n i c k e l i n Maraging 300) has a l s o been measured on the e l e c t r o n probe . 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 of the probe and the p r e p a r a t i o n of the specimen. The r e s u l t i s 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 . 1 8 + 0 . 5 % f o r Cr i n 321 S . S . e t c . . . ) . The oxygen content of the i n g o t i s a l s o r e p o r t e d (see a n a l y s i s 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 to f i g u r e 17. The i n g o t 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 . One h a l f i s p o l i s h e d and etched f o r s tudy of the macrographic s t r u c t u r e . H o r i z o n t a l s l i c e s , approximate ly 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 -i n g o t s . Specimens marked ' S ' are cut near the s u r f a c e and specimens marked ! C ' are cut from the c e n t e r . The i n t e r m e d i a t e specimens marked M are kept f o r oxygen a n a l y s i s . S p e c t r o g r a p h i c a n a l y s i s i s u s u a l l y performed on the top face of the remaining s l i c e (except f o r the top s l i c e ) . Because of the curved shape of the l i q u i d meta l p o o l , the s ide , specimen corresponds to a lower m e t a l / s l a g r a t i o than the center 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 1 to 26 ( I I I . 4 to I I I . 6 ) , the d i f f e r e n c e be ing about 0.5 f o r most i n g o t s t r u c t u r e s . I I I . 9 I n c l u s i o n count and r a t i n g (16 ,17 ,19 ,20) I n c l u s i o n types have been determined w i t h the h e l p of the e l e c t r o n probe . Counts corresponding to v a r i o u s composi t ions and s i z e s of i n c l u s i o n s are g i v e n i n most cases . Reduct ion of i n c l u s i o n s i z e s and counts i s one purpose of s p e c i a l r e m e l t i n g p r o c e s s e s . 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 e l i m i n a t e s l a r g e i n c l u s i o n s , the a t t e n t i o n should be focussed on the m i c r o c l e a n l i n e s s of the 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 contents i s the ASTM standard n°E45-63 (16) . I t d i s t i n g u i s h e s f o u r g e n e r a l types of i n c l u s i o n s : A - s u l p h i d e s type B - a lumina type C - s i l i c a t e s type D - g l o b u l a r ox ides type A l l four types are separated i n t o t h i n and heavy. - 50 -The most common type found i n 321 S . S . and, to a l e s s e r ex tent i n Mar 300 i s t i t a n i u m c a r b o n i t r i d e . 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 advantageously be c o n s i d e r e d a separate t y p e . In the same s t e e l s , t i t a n i u m s u l p h i d e s are a l s o present i n the form of l i g h t s t r i n g e r s . G l o b u l a r ox ides are present i n v a r i a b l e amounts, r e l a t e d to the oxygen c o n t e n t . The i n c l u s i o n s observed i n the remel ted m a t e r i a l are almost 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 f o r s u l p h i d e s t r i n g e r s ) . The i n t e n t i o n of t h i s s tudy however was not to produce a commerc ia l ly acceptable m a t e r i a l . A d i f f e r e n t method f o r count ing 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 content and o ther chemica l c h a r a c t e r i s t i c s of the p r o c e s s : The main types cou ld be counted s e p a r a t e l y , as T i ( C , N ) , " T i S " , g l o b u l a r ox ides (D) i n three separate s i z e ranges : 1.5 to 5 u corresponding to the lowest s i z e s 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 corresponding to the l i g h t type of the ASTM standard and b i g g e r than 10 y when they are p r e s e n t . The u n i t used i s number of i n c l u s i o n s per square m i l l i m e t e r as averaged o v e r . a number of 100 or 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 present study and these are the types r e p o r t e d . Because of the r a t h e r l i m i t e d s i z e of the examined specimen, the numbers r e p o r t e d here s h o u l d o n l y be , cons idered as a q u a l i t a t i v e i n d i c a t i o n of the type (D and c a r b o n i t r i d e s ) and s i z e range represented i n a random cross s e c t i o n of the specimen. - 51 -I I I . 1 0 A n a l y s i s and sampling of the s l a g A s e c t o r weighing about 10% of the s l a g cap i s separated a c c o r d i n g to f i g u r e 17 and then powdered. S l a g s k i n s are sampled l o n g i t u d i n a l l y . The s l a g i s ana lyzed f o r the r e a c t i v e element corresponding to the meta l s t u d i e d ( t i t a n i u m or aluminium) by one of the methods d e s c r i b e d i n appendix I V . The content of the elements most l i k e l y to c o n t r o l 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 a l s o measured. These are chromium f o r 321 S . S . , 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 . For t h i s reason i t has been i m p o s s i b l e to determine s e p a r a t e l y the v a r i o u s l e v e l s of o x i d a t i o n . of i r o n and t i t a n i u m . Q u a l i t a t i v e i n d i c a t i o n of 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 ob ta ined i n most cases . 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 by the Debye-Scherrer method leads to few r e s u l t s as o n l y the p a t t e r n s 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 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 s l a g caps are p r e s e n t e d . They concern s l a g s c o n t a i n i n g at l e a s t 10% CaTiO^ i n i t i a l l y , as the X - r a y method i s not s e n s i t i v e enough to detec t c o n s t i t u e n t s i n s m a l l amounts. The dominant p a t t e r n on the Debye Sherrer photograph i s always found to be CaF„. - 52 -Table 36. Ingot Detected by X - r a y C a T i 0 3 Other (unknown) Detected c h e m i c a l l y T i 3 + S7 X X _ S8 X X X S l l X X X S13 X X X S14 X X >50% S15 X X >50% S17 X X X The chemical d e t e c t i o n of T i can be done i n two ways (appendix I V ) : a) A f t e r a shor 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 . T i i s present i f methylene b l u e i s reduced when added to the s o l u t i o n . 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 t ime of 30 minutes when T i i s to 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^ ± n t ' i e presence of an 3+ 2+ excess of ¥e^0^ f o r two m i n u t e s . T i reduces the i r o n o x i d e . Fe 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 presence of o r t h o p h e n a n t h r o l i n e i n d i c a t o r . The method i s not t r u l y q u a n t i t a t i v e 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 stages - - 3+ i s a l s o a v e r y s e n s i t i v e i n d i c a t o r f o r T i (blue to v i o l e t c o l o r a t i o n ) . Sampling of the s l a g f o r i n g o t M2 has been done at r e g u l a r i n t e r v a l s d u r i n g the m e l t . At approx imate ly two minute i n t e r v a l s , a copper rod was immersed i n the molten cap f o r a few seconds. The s k i n formed, on - 53 -copper c o n s t i t u t e d a sample weighing 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 f o r c o l l e c t i n g 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 compos i t ion of the sample c o l l e c t e d i s extremely c l o s e to that of the cap as i s proven by the a n a l y s i s of the f i n a l sample. I t should be noted that 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 i s 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 compos i t ion so long as no r e d i s s o l u t i o n i s a l lowed 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 the 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 probably r e p r e s e n t s 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 . I I I . 1 1 Composi t ion and m a t e r i a l balance of 321 S . S . i n g o t s I n i t i a l e l e c t r o d e c o n t e n t s : M a t r i x [ T i ] : 0.45% (% stands f o r wt %) T o t a l [ T i ] : 0.58% [0] : 9 ppm - 2 I n c l u s i o n s T i ( C , N ) <5 u : 160 mm 5 -* 10 u : 32 mm" 2 - 54 -Table 37. Ingot S l ( f i g . 18) F i n a l s l a g : 330 g , 1.75% T i , 0.36% C r . T i t a n i u m gained by s l a g corresponds to a 0.20% l o s s i n the 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 approx imate ly 0.30%. F l u o r i n e shows a s m a l l decrease i n the s l a g corresponding to 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 75%). I n c l u s i o n s mm 2 Sample [T i ] % [T i ] % [0] [Cr] D+Ti(C,N) D T i ( C , N ) m a t r i x t o t a l ppm % < 5 p 5 10 y 5 -> ,10 y IS 0.14 17.51 615 55 12 IM 0.25 105 1C 0.16 17.84 725 72 26 2S 0.21 17.81 560 80 28 2M - 102 2C 0.23 17.64 670 92 31 3S 0.20 17.63 3M 0.33 104 3C 0.23 17.70 4S 0.24 17.76 785 64 37 4M ''• 0.31 103 4C 0.22 18.01 560 32 22 - 55 -Table 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 gained by s l a g corresponds to 0.20% l o s s from 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 bf approx imate ly 0.30%. The F l u o r i n e content of the s l a g shows a decrease from 75.0 to 74.1% ( e q u i v a l e n t ) C a F 2 < Sample [T i ] % [ T i ] % [0] [Cr] m a t r i x t o t a l ppm % is 0.16 IM 0.23 103 IC 0.14 -2S 0.18 17.78 2M 0.34 109 2C 0.22 17.91 3S 0.22 18.11 3M 0.30 103 3C 0.21 18.16 Table 39. . 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 gained by the s l a g corresponds to a 0.21% l o s s of the 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 corresponds to a l o s s of 2.4% ( e q u i v a l e n t ) CaF 2 ( i n i t i a l : 75%). I n c l u s i o n s mm Sample [T i ] % [T i ] % [0] [Cr] D+Ti(C,N) T i ( C , N ) D M a t r i x T o t a l ppm % < 5 y 5 + 1 0 y 5 + 1 0 y IS 0.17 18.35 460 15 70 IM 0.24 80 IC 0.14 17.12 590 25 88 2S 0.22 18.14 715 25 72 2M 0.28 110 2C 0.25 17.80 680 32 120 3S 0.28 :". 17.61 575 48 94 3M 0.31 99 3C 0.24 17.92 410 12 34 - 56 -Table 40. Ingot S6 ( f i g . 19) F i n a l s l a g : 320 g , 1.96% T i , 0.91% C r . T i t a n i u m gained by the s l a g corresponds 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 -mately 0.31%. The l o s s of f l u o r i n e corresponds 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 expected from the b u i l d up of T i and; Cr o x i d e s . I n c l u s i o n s mm 2 Sample [T i ] % [T i ] % [0] [Cr] Ti(C,N)+D D M a t r i x : T o t a l ppm . % < 5 p 5 •+ 10 u IS 0.22 18.10 665 144 IM 0.25 219 1C 0.24 17.75 730 135 2S 0.20 18.19 595 160 2M 0.27 159 2C 0.26 17.54 780 170 3S 0.21 18.22 765 65 3M ' Q.27 144 17.50 3C 0.28 1240 45 - 57 -Table 41. Ingot S7 ( f i g . 20) F i n a l s l a g : 350 g ; 5.11% T i , 0.91% C r . T i t a n i u m gained i n s l a g : 5 .11-4 .44 = 0.57% corresponds to a l o s s of 0.09% i n 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 g r e a t e r than 0.15%. F l u o r i n e l o s s corresponds to a decrease i n CaF^ content of 5.0% (from 84.4 to 79.4%). Sample [T i ] % [T i ] % [0] [Cr] M a t r i x T o t a l ppm % IS 0.28 17.34 IM , : ' 0.37 151 IC 0.32 18.02 2S 0.25 18.34 2M 0.37 148 2C 0.31 17.20 3S 0.30 17.87 3M 0.35 132 3C 0.24 17.47 - 58 -Table 42. Ingot S8 ( f i g . 20) F i n a l s l a g : 355 g ; 10-30% T i , 0.77% C r . T i t a n i u m gained i n s l a g : 10 .30-8 .92 = 1.38% corresponds to a l o s s of 0.22% i n the i n g o t , agree ing w i t h the f i n a l a n a l y s i s of the m e t a l . The ba lance between m a t r i x and t o t a l t i t a n i u m corresponds to the i n c l u s i o n removal i n d i c a t e d on the graph. F l u o r i n e l o s s i s equal to a decrease i n CaT?^ ° f 4.95%. I n c l u s i o n s mm 2 Sample [T i ] % [ T i ] % [0] [Cr] D+Ti(N,C) D T i ( C , N ) M a t r i x T o t a l ppm % < 5 p 5 - > - 1 0 y 5 -> 10 u IS 0 . 2 3 17.38 840 175 12 IM 0.35 109 1C 0.26 18.60 675 145 8 2S 0.25 17.44 725 190 12 2M 0.38 100 2C 0.27 17.20 490 130 8 3S 0.30 18.03 525 95 4 3M ..; 0.34 103 3C 0.31 17.43 350 48 2 - 5 9 -Table 43. Ingot S9 ( f i g . 21) F i n a l s l a g : 347 g , 1.61% T i , 0.54% C r . The t i t a n i u m content of the s l a g corresponds to a 0.25% l o s s i n the i n g o t w h i l e the a n a l y s i s of the meta l i n d i c a t e s a 0.44% l o s s . I n c l u s i o n s mm 2 Sample [ T i ] % [T i ] % [0] [Cr] D+Ti(C,N) D T i ( C , N ) M a t r i x T o t a l ppm % < 5 y 5 + 10 y 5 + 10 y IS 0.09 17.71 IM 0.15 26 ic 0.15 17.37 360 92 25 2S 0.07 ;.. 17.63 510 85 8 2M 0.09 24 2C 0.12 17.43 310 125 65 3S 0.07 - - -3M 0.13 18 3C 0.09 18.03 180 24 2 - 60 -T a b l e ' 4 4 . Ingot S10 ( f i g . 21) F i n a l s l a g : 312 g , 2.91% T i , 0.40% C r . The t i t a n i u m gained by the s l a g 2 .91-2 .22 = 0.69% corresponds to a 0.10% l o s s i n the i n g o t w h i l e the a n a l y s i s of the meta l i n d i c a t e s a 0.42% l o s s over the l a s t 2/3 of 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 the melt r a t e c r e a t e doubts as to the s i g n i f i c a n c e of the f i r s t samples ( n ° l ) . P a r t of the s l a g was l e f t unmelted at the bottom of the i n g o t . Sample [ T i ] % [ T i ] % [0] [Cr] M a t r i x T o t a l ppm % IS (0.73) 17.42 IM 0.52 84 1C (0.32) 18.07 2S 0.07 17.94 2M 0.14 26 2C 0.12 17.21 3S 0.12 17.67 3M 0.16 21 3C 0.10 18.03 - 61 -Table 45. Ingot S l l ( f i g . 21) F i n a l s l a g : 357 g , 10.60% T i , . 0.57% C r . The t i t a n i u m gained by the s l a g : 10 .60-8 .92 = 1.68% corresponds to a 0.27% l o s s i n the i n g o t w h i l e the 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 0.41% l o s s . I n c l u s i o n s mm Sample [T i ] % [ T i ] % [0] [Cr] D+Ti(C,N) D D M a t r i x T o t a l ppm % < 5 p 5 + 1 0 y > 1 0 y IS 0.10 18.13 285 92 6 IM 0.18 40 IC 0.17 17.86 465 145 9 2S 0.10 : 17.67 305 64 12 2M 0.17 43 2C 0.18 17.49 390 100 22 3S 0.08 17.75 485 62 : 0 3M 0.15 43 3C 0.09 17.75 325 42 2 - 62 -Table 46. Ingot S12 ( f i g . 22) F i n a l s l a g : 286 g ; 2.10% T i , 0.09% C r . The t i t a n i u m content of the s l a g corresponds to a 0.24% l o s s i n the i n g o t which agrees w i t h the a n a l y s i s of the meta l (0.25% l o s s ) . The balance 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] (see g r a p h ) . I n c l u s i o n s mm Sample [T i ] % [Ti ] % [0] [Cr] D+Ti(C,N) T i ( C , N ) T i ( C , N ) M a t r i x T o t a l ppm % < 5 u 5 -> 10 y > 10 y IS 0.19 18.11 1510 25 0 IM 0.30 41 1C 0.26 18.39 600 20 0 2S 0.28 17.53 350 42 3 2M 0.39 0 2C 0.32 0.34 0 17.59 505 85 4 — 0.35 -3S 0.35 17.39 230 70 2 3M 0.42 0 3C 0.37 17.17 840 18 0 - 63 -Table 47. Ingot S13 ( f i g . 23) F i n a l s l a g : 282 g ; 12.40% T i , 0.22% C r . The t i t a n i u m gained by the s l a g 12 .40-9 .76 =2.64% corresponds t o a 0.30% l o s s i n the i n g o t which agrees w i t h the a n a l y s i s of the meta l (0.32% l o s s ) . The balance 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 account ing f o r 0.03 to 0.05% [ T i ] . -2 I n c l u s i o n s mm Sample [T i ] % [T i ] % [0] [Cr] D+Ti(N,C) T i ( C , N ) M a t r i x T o t a l ppm % < 5 p 5 -»- 10 u 0.15 IS 0.15 : 18.09 320 6 IM 0.26 7 i c 0.18 ,: 18.18 570 15 2S 0.21 18.12 230 8 2M 0.31 1 2C 0.25 17.38 400 28 3S 0.22 ,• 17.75 575 10 3M 0.30 5 3C 0.18 : 17.60 550 2 - 64 -Table 48. Ingot S14 ( f i g . 24) F i n a l s l a g : 294 g; 7.60% T i , 0.16% Cr. Aluminium additions have been made at the rate of 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 process, the slag loses 8.92-7.60 = 1.32% T i or 3.88 g which can be calculated to be the r e s u l t of a 1.56 g W gain i n the f i r s t period (— < 1.6) and a 4.44 g loss during w s the aluminium addition (see graph). The b u i l d up of titanium into the ingot i s progressive a f t e r the s t a r t of the aluminium addition. A strong discrepancy occurs between the t o t a l and matrix content of the metal. This i s probably due to the method of analysis (appendix IV). We have considered only those matrix point counts which do not exceed the average by more than 4 times while the present ingot showed a much higher s c a t t e r of measured concentrations. The o p t i c a l count for i n c l u s i o n s remains normal. A balance of matrix and t o t a l titanium i s possible and indicates that 0.27 to 0.29%[Ti] i s added to the metal i n the form of inclusions or, more probably, of l o c a l titanium r i c h regions, these could be the r e s u l t of the reduction reaction leading to p a r t i c u l a t e titanium at the slag metal i n t e r f a c e . Part of the aluminium i s vaporized before i t can reduce the slag, t h i s i s evidenced by a p r e c i p i t a t i o n of y A ^ O ^ on the electrode, the mold and the fumehood (detected by Debye-Sherrer X-ray pattern). - 65 -Table 48 (Continued) We r e p o r t i n the t a b l e be low, 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 content 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 luminium 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 e l e c t r o d e . I n c l u s i o n s mm imple [T i ] % [T i ] % [Ti ] % [0] [Cr] Ti(C,N)+D Ti(C,N)+D D M a t r i x T o t a l T h e o r e t i c a l maximum ppm % < 5 y 5 •+ 10 y > 10 _ 0.18 0.58 IS 0.24 0.58 - 840 28 0 IM 0.45 1.69 101 1C 0.23 17.60 860 155 0 2S 0.21 - 590 54 0 2M 0.70 1.66 89 2C 0.30 17.91 810 95 1 3S 0.44 -3M 0.81 1.50 81 3C 0.54 18.03 4S 0.7.8 17.84 335 325 8 4M 1.16 1.58 53 4C 1.00 17.62 250 285 32 - 66 -Table 49. Ingot S15 ( f i g . 25) F i n a l s l a g : 251 g ; 5.25% T i , 1.12% C r . Aluminium a d d i t i o n has been made at the r a t e of 1 g/min 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 , the s l a g has l o s t 9 .96-5 .25 = 4.71% T i or 11.8 g ; t h i s amount can be c a l c u l a t e d to be the r e s u l t o f a 3.9 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 aluminium 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 than i n the e l e c t r o d e n e g a t i v e case (S14) and reaches a W p l a t e a u above — = 7 . s 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 i s a l s o 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 i s probably due to 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 case . The h i g h oxygen content of approx imate ly 200 ppm can o n l y account f o r the t i e up of 0.04% [T i ] under the form of T l 2 ° 3 i n c l u s l o n s ' The t h e o r e t i c a l [T i ] l i m i t i s obta ined as e x p l a i n e d above (S14) . I n c l u s i o n s mm-2 Sample [T i ] % [T i ] % [T i ] % [0] [Cr] Ti(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 maximum ppm % < 5 y 5 -* 10 y > 10 y IS 0.14 18.41 430 35 — IM 0.50 1.82 37 IC 0.16 17.58 390 230 i : 6 - 0.68 2S 1.24 17.32 530 565 58 2M 1.53 1.59 192 2C 1.10 17.51 480 390 65 3S 1.42 17.23 670 155 2 3M 1.49 1.59 221 3C 0.99 18.04 . 560 490 10 - 67 -Table 50. Ingot S16 ( f i g . 26) F i n a l s l a g : 303 g ; 0.75% T i , 0.24% C r . The t i t a n i u m content of the s l a g corresponds to a 0.11% l o s s i n the i n g o t to be compared w i t h the chemica l a n a l y s i s of the meta l (0.12% l o s s ) . The balance of m a t r i x and t o t a l content shows a s m a l l i n c l u s i o n removal . Sample [T i ] % [T i ] % [0] [Cr] M a t r i x T o t a l ppm % IS 0.19 17.91 IM 0.34 124 1C 0.30 17.27 2S 0.38 18.06 2M 0.42 30 2C 0.35 17.84 3S . : 0.36 17.61 3M 0.41 1 3C 0.36 17.38 4S 0.36 18.21 4M Q.42 3 4C 0.33 17.53 - 68 -Table 51. Ingot S17 ( f i g . 27) F i n a l s l a g : 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 the s l a g : 10 .30-9 .26 = 1.04% corresponds to a 0.11% l o s s i n the i n g o t i n agreement w i t h the a n a l y s i s of the i n g o t (0.11 to 0.13%). The balance of m a t r i x and t o t a l [Ti ]content 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 a luminium d e o x i d a t i o n appears to have i n c r e a s e d . t h e t i t a n i u m content ; i n the lower l e v e l s of the i n g o t . I n c l u s i o n s mm 2 Sample [Ti]% [T i ] % [0] [Cr] Ti(C,N)+D T i ( C , N ) D M a t r i x T o t a l ppm % < 5 y 5 •*• 10 y 5- •+ 10 y _ 0.49 IS 0.34 17.67 1330 low (290) IM 0.44 73 IC 0.37 17.62 1040 (105) 2S 0.365 17.94 250 (220) 2M 0.44 100 2C 0.39 17.83 540 (180) 3S 0.33 18.11 705 52 3M 0.46 5 3C 0.39 17.82 725 55 2 4S 0.42 17.72 575 32 4M 0.47 5 4C 0.38 17.61 505 12 - 69 -III.12 Composition and material balance of Maraging 300 ingots I n i t i a l electrode contents: Matrix[Ti] 0.69% To t a l [Ti] 0.79% Oxygen [0] 10 ppm [Mo] 4.95% -2 Inclusions Ti(C,N)5 ->• 10 y: 76 mm Ti(C,N) >10 y : 7 mm"2 Table 52. Ingot Ml ( f i g . 28) F i n a l s l a g : 350 g, 3.12% T i ; 0.23% Fe , The titanium content of the slag corresponds to a 0.39% loss from the ingot, approximately equal to the matrix loss and showing therefore l i t t l e i n c l u s i o n removal. Inclusions mm imple [Mo] [Ti] % [Ti] % [0] [Ni] Ti(C,N)+D D D % Matrix To t a l ppm % ' < 5 y 5 -y 10 y > 10 IS 0.23 18.23 580 345 34 IM 4.93 0.36 88 1C 0.26 18.61 940 375 28 2S 0'. 33 18.41 180 65 11 2M 4-94 0.40 26 2C 0.24 18.58 295 180 18 3S 0.31 18.96 165 58 12 3M 4.88 0.41 37 3C 0.29 18.37 175 44 9 - 70 -Table 53. Ingot M2 ( f i g . 29) F i n a l s l a g : 328 g ; 4.06% T i , 0.05% Fe . The t i t a n i u m content of the s l a g i s seen to i n c r e a s e l i n e a r l y 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 occurs at a h i g h e r r a t e . The meta 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 content of the s l a g remains low throughout the e x p e r i -ment, 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 f i r s t W specimen (— = 1) hav ing a h i g h e r content of 0.24%. •. s Sample [T i ] % [Mo] % T o t a l T o t a l IM 0.48 4.90 2M 0.41 4.92 ->- 3S 0.52 4.91 3M 0.47 4.85 - 71 -Table 54. Ingot M3 ( f i g . 28) F i n a l s l a g : 370 g ; 7.12% T i , 0.07% Fe . The t i t a n i u m g a i n i n the s l a g : 7 .12-4 .44 = 2.68% corresponds to a 0.45% average l o s s i n the i n g o t , s l i g h t l y above the m a t r i x l o s s , thereby i n d i c a t i n g some i n c l u s i o n removal (approximate ly 0.05% [ T i ] ) . I n c l u s i o n s mm Sample [Mo] [T i ] % [Ti ] % [0] [Ni] Ti(C,N)+D Ti(C,N)+D % M a t r i x T o t a l ppm % < 5 p 5 + 1 0 u IS 0.33 IM 4.98 IC 0.24 2S 0.24 2M 4.88 2C 0.25 3S 0.35 3M 4.90 3C 0.32 19.02 0.33 81 18.33 18.41 0.35 88 18.88 18.64 0.42 79 18.96 380 9 700 22 560 12 635 19 400 11 490 14 - 72 -Table 55. Ingot M4 ( f i g . 28) F i n a l s l a g : 350 g ; 10.55% T i , 0.04% Fe . 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 of 0.23%, lower than i s i n d i c a t e d by the i n g o t c o n t e n t . I n c l u s i o n s mm 2 Sample [Mo] [T i ] % [T i ] % [0] [Ni] Ti(C,N)+D T i ( C , N ) D % M a t r i x T o t a l ppm % < 5 y 5 -> 10 y 5 + : 10 y IS 0.30 18.27 350 8 112 IM 4.83 0.39 81 IC 0.31 18.74 270 6 27 2S 0.29 18.39 285 18 13 2M 4.90 0.42 10 2C 0.32 18.81 150 23 8 3S 0.38 18.65 265 18 4 3M 4.89 0.57 5 3C 0.39 18.60 435 25 i 2 Table 56. Ingot M5 ( f i g . 30) F i n a l s l a g : 321 g ; 1.02% T i , 1.89% Fe . The t i t a n i u m content of the s l a g corresponds to an average l o s s of 0.15% from the i n g o t , lower than i s i n d i c a t e d by the a n a l y s i s of the m e t a l . I n c l u s i o n s mm 2 ample [T i ] % . [T i ] % [0] [Ni] Ti(C,N)+D D D M a t r i x T o t a l ppm % < 5 y 5 -> 10 u > 10 y IS 0.46 18.61 46 42 2 IM 0.62 375 IC 0.53 18.87 165 105 32 2S 0.38 18.73 340 58 14 2M 0 .77! 484 2C 0.40 18.96 88 55 20 3S 0.42 18.48 220 42 16 3M 0.59 272 3C 0.37 18.89 180 39 4 - 73 -Table 57. Ingot M6 ( f i g . 30) F i n a l s l a g : 336 g ; 8.75% T i , 0.98% Fe . 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 .92-8 .75 = 0.17%; t h i s i s i n disagreement w i t h the l o s s observed i n the i n g o t and may be due to the f a c t that p a r t of the i n i t i a l s l a g was l e f t unmelted at the bottom of the i n g o t . I n c l u s i o n s mm 2 Sample [T i ] % [Ti ] % [0] [Ni] D+Ti(C,N) D M a t r i x T o t a l ppm % < 5 y 5 + 1 0 y IS 0.46 . 18.86 280 38 IM 0.58 110 1C 0.45 18.47 245 35 2S 0.47 ' 18.71 210 46 2M 0.60 108 2C 0.46 . 18.30 490 28 3S 0.42 18.65 380 12 3M 0.56 80 3C 0.45 18.50 320 16 - 74 -Table 58. Ingot M7 ( f i g . 31) F i n a l s l a g : 282 g ; 0.73% T i , 0.18% Fe . The t i t a n i u m content of the s l a g corresponds to a l o s s of 0.08% from the i n g o t . T h i s i s i n agreement w i t h the compos i t ion of the m e t a l . Some i n c l u s i o n removal (0.03 to 0.05% [ T i ] ) i s observed as the t o t a l m a t e r i a l ba lance c l o s e s w i t h i n 0.02% [ T i ] . -I n c l u s i o n s mm 2 Sample [Ko] [T i ] % [Ti ] % [0] [Ni] Ti(C,N)+D T i ( C , N ) D % M a t r i x T o t a l ppm % < 5 y 5 -»• 10 y 5 -y 10 y IS 0.60 18.50 495 22 36 IM 4.88 0.74 6 1C 0.67 18.45 690 65 0 2S 0.67 18.62 . 385 20 0 2M 4.90 0.65 4 2C 0.61 18.38 315 32 0 3S 0.74 18.44 105 12 0 3M - 0.74 21 3C 0.67 18.97 215 4 0 4S 0.68 18.71 240 18 0 4M 4.92 0.63 27 4C 0.61 18.48 110 6 0 - 75 -I I I . 1 3 Composi t ion of 1409 A l s t e e l i n g o t s I n i t i a l e l e c t r o d e c o n t e n t s : [ A l ] 3.74% T o t a l [T i ] 0.49% M a t r i x [T i ] 0.06% Oxygen 95 ppm I n c l u s i o n s 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 -2 A l l the specimens of the remelted s t e e l are found to c o n t a i n o n l y a few ppm of oxygen. S ince n o n - m e t a l l i c a luminium should be i n the : form of alumina 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 • cons idered 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 the e l e c t r o n probe r e v e a l e d no aluminium r i c h i n c l u s i o n s i n the remel ted m a t e r i a l . i The a n a l y s i s of the s l a g f o r aluminium does not 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 reasons : a) The c o m p o s i t i o n of the 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 lumina content than the s l a g cap when the average content i s above the e u t e c t i c (15 ,18) . b) P a r t of the aluminium l o s t by the i n g o t i s swept away through 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 . Dust c o l l e c t e d on the mold , the e l e c t r o d e and the fumehood r e v e a l s the presence of . Y a lumina and a s m a l l q u a n t i t y of a a lumina (Debye-Sherrer X - r a y p a t t e r n ) . The presence of the low temperature phase i n d i c a t e s tha t ; the o x i d a t i o n of aluminium has taken p l a c e i n a r e l a t i v e l y low temperature zone of the atmosphere. - 76 -I r o n and chromium c o u l d not 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 ana lyzed i n the 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 (-0.03 to +0.05% f o r [ S i ] and -0 .07 to -0.03% f o r [Mn]) which was not cons idered to be s i g n i f i c a n t . Table 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 to 2 mm) was formed, weighing a p p r o x i -mately 350 g , w h i l e the remaining cap was o n l y 300 g ( i n i t i a l t o t a l : 660 g ) . Aluminium c o n c e n t r a t i o n i n the s l a g : 1.9% corresponding to a . 0.31% average l o s s from the i n g o t or somewhat lower than the a n a l y s i s of the meta l i n d i c a t e s _2 I n c l u s i o n s T i ( C , N ) mm Sample [Al ] [ T i ] % [T i ] % [0] < 5 p 5 + 10 y > 10 u % M a t r i x T o t a l ppm IS (3.28) 0.06 345 125 10 IM 3.28 0.52 4 IC (3.35) 0.05 425 142 8 2S (3.40) 0.04 400 60 4 2M 3.42 0.50 0 2C (3.61) 0.06 550 170 11 3S (3.51) 0.07 365 98 12 3M 3.32 0.50 1 3C (3.44) 0.13 260 85 6 - I l -l-able 60. Ingot A2 To overcome d i f f i c u l t i e s encountered w i t h o ther i n g o t s ( t h i c k s k i n and e x c e s s i v e melt r a t e ) , a d d i t i o n s of CaF^ have been done d u r i n g the 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 to a t o t a l s l a g weight of approx imate ly 1050 g . T h i s mainta ined the s l a g c o m p o s i t i o n below the 10% alumina ( e u t e c t i c ) l e v e l throughout the m e l t , but i t a l s o i n c r e a s e d the supply of i m p u r i t i e s (ox idant ) to the m e l t . Specimen [ A l ] % [T i ] % [T i ] % [0] I n c l u s i o n s M a t r i x T o t a l ppm IS 0.06 M o s t l y T i ( C , N ) IM 2.97 0.51 2 1C 0.08 2S 0.04 2M 3.30 0.51 1 2C 0.09 3S 0.05 3M 3.18 0.50 1 3C 0.05 Table 61. Ingot A3 A t h i c k s k i n formed on the sur face : of the i n g o t . T h i s caused the s l a g cap to decrease i n s i z e to the p o i n t where the e l e c t r o d e t i p was no longer immersed. A r c i n g occurred and the process had to be s topped. The f i n a l s l a g cap weighed o n l y 195 g . Sample [ A l ] % [Ti ] % [Ti ] % [0] I n c l u s i o n s M a t r i x T o t a l ppm IS (3.05) 0.07 M o s t l y T i ( C , N ) IM 3.08 0.43 2 1C (3.02) 0.03 2S (3.38) 0.04 2M 3.40 0.47 4 2C (3.53) 0.03 3S (3.42) 0.08 3M 3.38 0.49 5 3C (3.48) 0.05 - 78 -Table 62. Ingot A4 A t h i c k s k i n was formed throughout the mel t c a u s i n g the f i n a l 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 of o x i d a t i o n of the s l a g . I f the s l a g i s f u l l y d e o x i d i z e d by the s t e e l , the aluminium a d d i t i o n s s h o u l d be c o l l e c t e d i n the i n g o t ( e x c l u d i n g v a p o r i z a t i o n l o s s e s ) thereby 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 luminium, r e p o r t e d i n the t h i r d column, would be c o n s t a n t . Sample . IA1] % [ A l ] % [ T i ] % [T i ] % [0] I n c l u s i o n s Loss M a t r i x T o t a l ppm IS 0.02 IM 3.21 0.53 0.49 6 IC 0.06 M o s t l y T i ( C , N ) 2S 0.04 2M 3.28 0.46 0.47 1 2C 0.07 3M 3.35 0.78 0.47 5 3C '' 0.05 M o s t l y T i ( C , N ) 4M 3.57 0.56 0.49 6 4C : 0.09 - 79 -I I I . 1 4 Rate of o x i d a t i o n of i r o n The m a t r i x t i t a n i u m content of i n g o t s F l , F 3 , F4 , F5 has been found to be l e s s than 0.06% [T i ] or o n l y 1.5 t imes the 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 the e l e c t r o n probe . T i t a n i u m b e a r i n g i n c l u s i o n s are p r e s e n t , e s p e c i a l l y i n i n g o t F5 where they reach up to 25 u i n s i z e . The s l a g used f o r t h i s mel 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 which conta ined most ly 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 s l a g c o m p o s i t i o n has not been used f o r the other experiments i n v o l v i n g the r e m e l t i n g of s t a i n l e s s 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 content of the s l a g . Table 63. Ingot Ingot weight g S l a g weight g % Fe S lag O x i d a t i o n r a t e Wt % m e t a l F l (FVE) F2 (FVE) F3 (1018) F4 (1018) F5 (1018) 2150 1785 2225 2260 2245 350 380 370 375 370 4.12 2.30 1.63 1.89 1.97 0.67 0.49 0.27 0.31 0.33 - 80 -I I I . 1 5 S o l i d i f i c a t i o n p a t t e r n s I t has been r e p o r t e d (18) that an acceptab le 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 meta l i s i n permanent contac t 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 of the m e t a l reaches the s k i n ( f i g . 1 ) . I n c o n t r a s t , when the process 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 to a v e r y poor s u r f a c e . Our experiments c o n f i r m t h i s p o i n t e n t i r e l y . Most a l l o y s , i n c l u d i n g those used i n t h i s work, produce a d e n d r i t i c s t r u c t u r e when remelted by e l e c t r o s l a g . Among the f a c t o r s which favour the 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 are the slow s o l i d i f i -c a t i o n r a t e , 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 the d i r e c t i o n of 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 are v e r y dependent upon the t o t a l heat i n p u t s u p p l i e d to the ; p r o c e s s . The geometry of the system i s a l s o i m p o r t a n t , the 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 both the melt r a t e (power i n p u t ) and a decreas ing r a t i o of e l e c t r o d e to i n g o t diameter ( 10 ,21 ,22 ) . A sudden change i n the thermal 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 ( ingot 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 h e i g h t . In c o n t r a s t , f i g u r e 33 represents the m a c r o s t r u c t u r e of an i n g o t remelted w i t h D . C . c u r r e n t throughout (S14). No sudden change i n thermal c o n d i t i o n s has o c c u r r e d . The d e n d r i t e s are 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 . The p o o l shape i s d e l i n e a t e d by darker bands i n the e t c h i n g . A l l of our s t a i n l e s s s t e e l and Maraging s t e e l i n g o t s e x h i b i t 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 p r o f i l e . Maraging 300 produces coaser d e n d r i t e s than s t a i n l e s s s t e e l at the same power i n p u t (M3, f i g . 34 and M7, f i g . 35) . In f i g u r e 35, power has a l s o been swi tched from D . C . to A . C ; new d e n d r i t e s have n u c l e a t e d on the w a l l s when power was cut o f f . The c e n t r a l d e n d r i t e s cont inue to grow i n the same d i r e c t i o n a l though i t i s obvious tha t they do not f o l l o w the heat f l o w i n t h i s case (see f i g . 32 f o r compar ison) . T h i s demonstrates that 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 w i t h 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 i n g o t . 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 s t r u c t u r e . Because of the poor contac t w i t h the b a s e p l a t e , too l i t t l e heat was e x t r a c t e d 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 . T h i s would have a d e t r i m e n t 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 i n g o t was a l s o remel ted w i t h a, h i g h mel 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 luminium component of the. a l l o y . I t i s observed t h a t t h i s a l l o y w i l l n u c l e a t e an equiaxed s t r u c t u r e 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 , which r e s u l t e d i n a d e n d r i t i c s t r u c t u r e i n s t a i n l e s s 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 remaining 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 d e n d r i t i c s t r u c t u r e c o u l d probably have been preserved by "hot t o p p i n g " the i n g o t . I I I . 1 6 O b s e r v a t i o n of c o n v e c t i v e motions The d r i v i n g f o r c e f o r n a t u r a l thermal c o n v e c t i o n i n the meta l p o o l i s s m a l l , the h i g h e s t temperature r e g i o n be ing the top of the p o o l . 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 isotherms i n the meta 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 c u r v e d . 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 are momentum t r a n s f e r from the s l a g and the f a l l i n g d r o p , and e l e c t r o m a g n e t i c e f f e c t s . The combined r e s u l t i s l a r g e l y unknown. V i s u a l examinat ion of the s l a g and the atmosphere above the melt sheds some l i g h t on motion p a t t e r n s i n these two phases . F i l m s 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. The camera was aimed down at the s l a g s u r f a c e , c o v e r i n g one h a l f of the melt and the atmosphere above i t . G r a p h i t e p a r t i c l e s were dropped onto the s l a g s u r f a c e 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 across the s u r f a c e . The e l e c t r o d e and the s l a g emi t ted enough smoke to o u t l i n e the movements of 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 c u r r e n t of e i t h e r p o l a r i t y or a l t e r n a t i n g c u r r e n t i s used . A shor t t ime a f t e r i n t r o d u c i n g the powder (of the order of 0.05 sec) an annulus around the e l e c t r o d e 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 . At t h i s t i m e , 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 . T h i s r e g i o n 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 o u t s i d e . Some g r a p h i t e o c c a s i o n a l l y leaves the annulus to p i l e up a g a i n s t the mold w a l l ( f i g . 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, c u r r e n t s be ing g e n e r a l l y upward near the e l e c t r o d e and downward near the w a l l s , i n keeping w i t h normal thermal c o n v e c t i o n . The same p a t t e r n i s of course l i k e l y i n the s l a g bath where heat g e n e r a t i o n i s concentra ted i n the center 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 cou ld r e s u l t from the i opposing motions of c o n v e c t i o n i n the s l a g and the atmosphere ( f i g . 38) . The outward motion of the g r a p h i t e was r a p i d near the e l e c t r o d e (5 to 10 cm/sec) , 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 not be assessed w i t h a p r e c i s i o n b e t t e r than ±50%. I I I . 1 7 E l e c t r o d e temperature p r o f i l e s Ingots S4 and S5 have been mel ted i n order to e s t a b l i s h temperature g r a d i e n t s i n the e l e c t r o d e 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 c o n d i t i o n s . The steepness of the temperature g r a d i e n t near the s l a g i s e x p l a i n e d by the poor thermal c o n d u c t i v i t y of s t a i n l e s s s t e e l below 1000°C and the s i g n i f i c a n t downward motion of the e l e c t r o d e . - 84 -The p o s i t i o n of the s l a g was sensed u s i n g two W/W-26% Re thermo-couples which a l s o recorded the atmospheric temperature above the s l a g . Chromel Alumel thermocouples were used to measure the e l e c t r o d e temperature at p o i n t s 1 mm below the s u r f a c e and a t the center ( 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 ob ta ined d u r i n g these exper iments . The r e s u l t s are used to compute the r a t e of atmospheric o x i d a t i o n of 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 of p r e c i p i t a t e r e d i s s o l u t i o n to be expected i n the e l e c t r o d e . 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 at h i g h temperatures a l though the d i s s o l u t i o n r a t e i s s low below 1200°C; s o l u t i o n treatment i s , t y p i c a l l y 3 hrs at 1150°C (19) i n an a u s t e n i t i c s t e e l . The e l e c t r o d e meta l spends o n l y a few seconds above 1200°C as i n d i c a t e d by the tempera-t 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 to 1 mm/sec). A s e c t i o n cut through an e l e c t r o d e t i p has shown tha t angular T i C p r e c i p i t a t e s reach the s u r f a c e of the molten f i l m on the s u r f a c e (321 s t a i n l e s s s t e e l ) w i t h o u t s i g n i f i c a n t d i s s o l u t i o n . CHAPTER IV DISCUSSION The r e s u l t s reported i n sections III.11 to III.14 i n d i c a t e that a l o s s of reactive elements has been observed, i n a l l of our experi-ments, over and above what we would have expected from i n c l u s i o n removal. In a number of cases, the material balance written between the ingot and the slag closed w i t h i n a narrow margin. As the r e s u l t s by themselves constitute an answer to the f i r s t two questions of section 1 .2 , we s h a l l now t r y to answer the other questions regarding the influence of the atmosphere, of the slag composition, of electrochemical reactions and of the l o c a t i o n of reaction s i t e s on mass transfer rates. - 86 -I V . 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 In the i n t r o d u c t i o n ( 1 . 4 ) , mention has been made of the two p o s s i b l e sources of o x i d a n t namely the atmosphere and the s l a g . 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 or 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 mechanica l p r o p e r t i e s and chemica l s p e c i f i c a t i o n s of the remelted a l l o y s . I V . 1 . 1 Atmospheric oxygen can enter the system i n two ways: through the s l a g - g a s i n t e r f a c e or 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 balance leads 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 t a b l e : Table 64. Causes f o r l o s s M e l t i n g c o n d i t i o n s To cause 0 . 1 % l o s s of [ T i ] 0 £ from a i r Argon b l a n k e t 1% o 2 Flow of O2 through s l a g i n t e r f a c e Oxide s k i n on round e l e c t r o d e 2 g sec 2 g sec Ingot diam^ 5 .3 cm 2 g sec 3.5 ml sec of a i r 70 ml sec 1 of argon 3 -2 - 1 0.04 cm cm sec - 3 FeO t h i c k n e s s = 10 diameter Whi le 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 aforementioned 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 . The f l o w of argon tha t - 87 -we used i n the fumehood was about 80 ml sec 1 a l t h o u g h 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 motions around the e l e c t r o d e . I t seems t h e r e f o r e that the argon b l a n k e t may have c a r r i e d j u s t enough oxygen to cause the r e p o r t e d l o s s e s (around 0.3% [ T i ] ) , assuming tha 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 of 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 . At s t e e l making temperatures , 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 : h e a r t h furnace (23) . Most s t e e l making p r o c e s s e s , however, use s l a g s which are 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 orders of magnitude lower than i s the case i n h a l i d e s . Towers and Chipman (24) r e p o r t d i f f u s i o n c o e f f i c i e n t s i n the C a O * S i 0 2 • A ^ 2 ° 3 —7 —8 2 —1 system which are i n the range 3 x 10 to 10 cm sec . D i f f u s i o n c o e f f i c i e n t s i n molten 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 a v a i l a b l e data concern s e l f - d i f f u s i o n . B u r e l (15) has found 8.5 x 10~ 5 c m 2 s e c " 1 f o r A l ^ d i f f u s i n g i n CaF 2~20% A l ^ a t 1518°C w h i l e -5 2 - 1 3+ D e l i m a r s k i i and P a v l i n o v (25) g i v e 1.0 x 10 cm sec f o r Fe i n c r y o l i t e at 1000°C. When a p p r o p r i a t e l y combined w i t h the o t h e r p h y s i c a l p r o p e r t i e s of the system (k inemat ic v i s c o s i t y . . . ) , these h igher v a l u e s w i l l acount f o r the h i g h r a t e of exchange w i t h the atmosphere (see I V . 2 . 2 ) . I V . 1 . 2 O x i d a t i o n of the e l e c t r o d e above the mel t i s e v i d e n t when the c l o s e d argon cap i s not used . - 88 -Because of 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 are u s u a l l y t a r n i s h e d throughout t h e i r l e n g t h . In a l l cases , a cont inuous and t h i c k e r l a y e r of o x i d e occurs o n l y c l o s e to the molten s l a g . T h i s l a t t e r i s t h i n (< 1 u ) and adherent . Among the metals 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 temperature , i t i s a l s o the m e t a l which exper iences h i g h temperature f o r the l o n g e s t t ime 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 41 ) . 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 coats of o x i d e : FeO: k ' P F e o 0 . : k ' 3 4 P Fe„0 •. k ' 2 3 P 1.05 x I O " 2 x i 0 - 4 0 ' 5 0 ° / R T c m 2 s e c - 1 5.4 x I O " 4 x i 0 - 4 0 ' 5 0 ° / R T c m 2 s e c - 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 p r e s s u r e of oxygen i n the gas , p r o v i d e d i t s tays above 7 mm Hg (0.01 a t m . ) , has very 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 of the o x i d a t i o n of Fe i s t h e r e f o r e a p p r o x i -mated by * 2 r T f c -40,500/RT . d t . . _„ 6 = / 5.75 x e (—) dT (eq I V . 1) 600 ' where T^ i s the. meta l temperature at the s lag-atmosphere 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 at low temperatures , below the range of 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 of 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 tempera-t 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 of 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 t ime of exposure i n t o i n t e r v a l s of 1 second -or about 7.77°K d u r i n g the l a s t stages of o x i d a t i o n - . i „ n -40,500/R(600 + EATi) 6 = ± E 0 5.75 x e ° (eq I V . 2 ) where n i s the number of time i n t e r v a l s between 600°K and TR. f 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 of 6 = 1.73 x 10 cm. - 3 We have seen above that an o x i d e c o a t i n g of 10 diameter of 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 should 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 to the s l a g when i r o n i s r e m e l t e d . In the case of . s t a i n l e s s s t e e l , Maraging 300 and 1409 A l however, the r e s i s t a n c e to 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 teeper up to 1000°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; of o x i d a t i o n of F e - 1 8 C r ' 8 N i should be at l e a s t 100 t imes s lower than f o r i r o n . 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 enters the s l a g at a temperature approx imate ly 200°C lower than m i l d s t e e l ( f i g . 40 ) ; 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 -ox ide would have no measurable e f f e c t on the meta l c o m p o s i t i o n ( e q u i v a l e n t : 0.01% T i ) . In the case of s t a i n l e s s s t e e l , the e f f e c t i s a hundred times lower s t i l l . Chromium aluminium 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 at h i g h temperature and the same c o n c l u s i o n can be drawn: The o x i d a t i o n pf the 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 system. Maraging s t e e l does not have o u t s t a n d i n g o x i d a t i o n r e s i s t a n c e . I n the absence of f u r t h e r d a t a , i t can be assumed tha t i t s behaviour i s b e t t e r than or equal to that of i r o n . I V . 1 . 3 E l e c t r o l y s i s . S ince d i r e c t c u r r e n t has been used f o r many of the remelted i n g o t s , i t i s l i k e l y tha 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 these 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 M e l t i n g r a t e : 2.5 g sec x C u r r e n t : 700 Amp. - 3 - 1 = 7 . 1 0 gram-equiva lent sec 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 g of meta l of atomic weight 48 c o u l d r e a c t a t each second. T h i s represents 6.7% of the t o t a l melt r a t e . _ 1409 A l , w i t h 12.86% Cr and 3.74% A l should 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 s u r f a c e (26) . - 91 -I t should of course be p o i n t e d out that meta l which has been sub jec ted 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 w i l l be sub jec ted to the i n v e r s e anodic r e a c t i o n s when i t reaches the i n g o t . 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 these s teps of 1.5% would be s u f f i c i e n t to cause the 0.1% l o s s that we, mentioned. ; B i g u n i t s would be favoured i n t h i s respec 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 diameter r a t i o s between the i n g o t and the e l e c t r o d e which 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 aluminium i n the meta l and the s l a g w i l l p l a y an important r o l e i n the r e l a t i v e 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 . I t 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 be low when these elements are at a low c o n c e n t r a t i o n i n the s l a g , 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 f a c t o r i n t h i s case . The presence of i r o n and chromium oxides i n the s l a g s of 321 S>S. and Maraging 300 i s a l s o an i n d i c a t i o n that the l i m i t i n g c u r r e n t f o r 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 meta l has been reached. I V . 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 chemical p o t e n t i a l of the r e a c t i n g s p e c i e s . An attempt has been made (Appendix m) to measure the a c t i v i t y of - 92 -t i t a n i u m ox ides i n the CaO'CaF 2 system. The shortcoming of these 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 tha t 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 pressure was imposed by the C/CO e q u i l i b r i u m . CaF 2 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 of CaO;. The p r i n c i p a l reason f o r t h i s i s the h y d r o l y s i s of CaF^ which occurs 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 2 0 (g) • 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 2 are a l s o probably due to the presence of CaO i n the m e l t . A c c o r d i n g to the c r y o s c o p i c s t u d i e s of Koj ima and Masson (14) , the m e l t i n g p o i n t of pure CaF 2 i s depressed by ^ 2.3°C f o r every 1% CaO. S ince 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 cons idered reasonable to 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 of these oxides by assuming that they r e a c t to form the complex ox ides CaTiO^ and C a A ^ O ^ . These ox ides are then assumed to d i s s o l v e i d e a l l y i n C a F 2 . A s i m i l a r behaviour has been observed w i t h s i l i c a t e s (28) . C a F 2 ~ A l 2 0 3 (15) i s a s i n g l e e u t e c t i c 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 to Evseeb (29) . C a F 2 ~ T i 0 2 (30) has a m i s c i b i l i t y gap c o v e r i n g approx imate ly 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 of format ion (AF°) are taken f o r E l l i o t t and G l e i s e r (31) . - 93 -S lag base at 1800°K at 1800°K at 1800°K CaF 2 0.56 (Appendix I I I ) 1 1 CaF„ + CaTiO,. • CaTiO- >- CaAl„0, 1 1 3 j 2 4 AF° = -22 ,150 c a l / m AF° = -11 ,910 c a l / m CaF 0 + C a A l o 0 . >• CaTiO n • CaAl„0, 1 2 2 4 3 2 4 In the m e t a l l i c phases , the f o l l o w i n g data are a v a i l a b l e (31 ,32) . T°K R a o n l t i a n a c t . I n t e r a c t i o n parameters Henryan a c t . coef . coef . (32) (321 S . S . ) (32) o o T i T i A l A l .. £ C r , .. £ N i Y [ A 1 ] ^ [ T i ] e 0 £ T i e O e A l l o g f T i + l o g f N i 1823 1873 1923 0.020 0.063 0.033 -0.187 0.034 -0 .94 0.048 0.060 - 0 . 7 0 - 94 -I V . 1 . 5 The e q u i l i b r i u m of d e o x i d a t i o n by t i t a n i u m . I n i r o n and 321 S t a i n l e s s s t e e l , 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 the three f o l l o w i n g c a s e s : 1 . The product of d e o x i d a t i o n i s T iO^ ( h y p o t h e t i c a l ) . 2. The product of d e o x i d a t i o n i s T i ^ O ^ . T h i s i s the ox ide i observed when homogeneous d e o x i d a t i o n of s t e e l i s per formed. I n s t a i n l e s s s t e e l , T i ^ O ^ w i l l form i n pre ference to chromium ox ide (Cr^O^) when the t i t a n i u m content i s more than 0 .1 to 0.2% (32) . 3. The product of d e o x i d a t i o n i s CaTiO^, one of the major s l a g 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 are r e p o r t e d by E l l i o t t and G l e i s e r (31) 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 1800°K) . Formation of ox ides T i + 0 2 o- T i 0 2 AF° = -223,500 + 41.55 T c a l . 2 T i + 3 /20 2 • T i 2 0 3 AF° = -354,000 + 58.36 T c a l . CaO + T i 0 2 y C a O - T i 0 2 AF° = -22,150 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 T i ( s o l i d ) y T i ( l i q u i d ) A F f = A H f - T A S f = 3,700 - 1.91 T C a l . T i ( l i q u i d ) >• [T i ] (% d i s s o l v e d ) AF = RT In a ^ j " R T l n * T i N [ l % T i ] where y° i s 0.016 at 1800°K (see above) A F d = -14 ,753 - 8.83 T c a l . For the whole r e a c t i o n T i ( s o l i d ) —> [ T i ] AF° = -11 ,053 - 10.74 T. c a l . D i s s o l u t i o n of 0£ i n i r o n l / 2 0 2 • [0] AF° = -28,000 - 0.69 T c a l . - 95 -O v e r a l l d e o x i d a t i o n r e a c t i o n s : R e a c t i o n constants can be c a l c u l a t e d d i r e c t l y u s i n g AF° = -RT l n K P [T i ] + 2[0] — T i O . : AF° = AF° - AF° - 2AF° 2 6 1 4 5 K , = 1.96 x 10 7 at 1800°K 6 2 [ T i ] + 3[0] >- T i 2 ° 3 : A F 7 = A F 2 " 2 A F 4 ~ 3 A F 5 K ? = 1.76 x 1 0 1 2 [T i ] + 2[0] + CaO • C a T i O . : AF° = AF° + AF° j o b 3 K g = 9.22 x 1 0 9 These r e a c t i o n constants are 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 meta 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 c o e f f i c i e n t s . For example: (TiO ) K = 6 f 2 [ 0 ] 2 f T i [ T i ] where l o g f Q = e ° x (%[0j) + e j 1 (%[Ti]) and l o g f T i = e ^ x (%[Ti]) + e ° . (% [0]) + l o g ff± + l o g f ^ i n s t a i n l e s s s t e e l . 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 f o r t i t a n i u m and oxygen i n i r o n and s t a i n l e s s s t e e l at 1800°K are 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 of oxygen p a r t i a l p r e s s u r e s . An i n t e r e s t i n g f e a t u r e of these graphs i s to demonstrate the g r e a t e r e f f e c t i v e n e s s of d e o x i d a t i o n i n the presence of the CaO-c o n t a i n i n g s l a g . The g r e a t e r 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 remelted i n g o t s are represented on the same graphs . 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 tha t the excess c o n c e n t r a t i o n of oxygen i n s t a i n l e s s 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 n e g a t i v e . A l t e r n a t i n g c u r r e n t on the o ther 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 ; tha t i d e a l l y produced by the s l a g . The a d d i t i o n of aluminium does not seem to 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 i s important w i t h r e v e r s e p o l a r i t y . The o p p o s i t e e f f e c t of mel t p o l a r i t y i s seen w i t h Mar 300. F i g . 42 shows zones of c o n c e n t r a t i o n where the oxygen content 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 be ing 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 . I t should correspond approx imate ly to the temperature of the meta l on the e l e c t r o d e 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 products can s t i l l be removed by f l o t a t i o n . In the h i g h e r temperature reg ions 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 I V . 2 . 1 ) . . . - 97 -I V . 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 i n a s i m i l a r f a s h i o n , the product of d e o x i d a t i o n be ing 1.  A L 2 ® 3 (homogeneous) 2. C a O ' A l 0 ( s lag) E l l i o t t (31) r e p o r t s 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 i n t e r e s t : Formation of ox ides o 2A1 + 3 /20 2 • A 1 2 0 3 AF = -262,900 c a l . (1800°K) O CaO + A 1 2 0 3 • C a 0 ' A l 2 0 3 A F 1 Q = -11 ,910 c a l . (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 tandard s t a t e of pure l i q u i d a luminium, the a c t i v i t y c o e f f i c i e n t f o r [ A l ] i s g i v e n as a f u n c t i o n of the atomic f r a c t i o n : . l o g Y = - 1 . 2 0 + 2 . 2 5 X A 1 ( 1873°K) ; X A 1 < 0.2 A l -> [ A l ] (% d i s s o l v e d ) AF = -23,750 c a l / m (1800°K) D e o x i d a t i o n r e a c t i o n s 2[A1] + 3[0] y A 1 2 0 3 A F 1 2 = A F g - 3AF 5 - 2AF K 12 = 3 ' 7 7 X 1 0 + 1 5 2[A1] + 3[0] + CaO >- C a O - A l ^ O O O AF = AF + AF 13 12 10 K 1 3 = 2.46 x 1 0 1 8 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 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 data on the i n t e r a c t i o n of chromium. - 98 -A l I t should be noted tha 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 e^ i s o n l y v a l i d f o r very d i l u t e s o l u t i o n s as i t l eads to the i n c o n s i s t e n t r e s u l t that the oxygen c o n c e n t r a t i o n [0] i n c r e a s e s w i t h the aluminium c o n c e n t r a t i o n when there i s more than a few t e n t h of one percent [Al] p r e s e n t . The r e s u l t of the c a l c u l a t i o n leads to extremely low v a l u e s f o r 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 g r e a t e r than 0.01%. The oxygen p a r t i a l p r e s s u r e imposed by aluminium i s c o r r e s p o n d i n g l y lower than that imposed by t i t a n i u m as an examinat ion of the f r e e e n t h a l p i e s of the f o l l o w i n g r e a c t i o n s w i l l demonstrate : 4 / 3 [ A l ] + T i 0 2 • [T i ] + 2 / 3 A l 2 0 3 O AF^ = -25,800 c a l . (1800°K) I C . = 1200 14 and 4 / 3 [ A l ] '+ C a T i 0 3 • [T i ] + 2 / 3 C a A l 2 0 4 + l /3CaO O AF 1 5 = -11,600 c a l . K 15 = 6 1 T i t a n i u m i n the s l a g should 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 meta 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 order 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 - 3 0 A 1 2 0 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„. I V . 1 . 7 S ta te 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 . The r e s u l t s r e p o r t e d i n ( [ I I . 10) show that CaO*TiG" 2 can s t i l l be de tec ted i n a Debye-Sherrer p a t t e r n of the s l a g when aluminium i s c o n t i n u o u s l y added to i t d u r i n g the m e l t . Aluminium imposes a p a r t i a l pressure of oxygen s e v e r a l orders of magnitude lower than the s t e e l remel ted i n t h i s case , 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 . 2+ We have not cons idered the o x i d a t i o n s t a t e T i f o l l o w i n g the r e s u l t s of Rossokhin and coworkers (34) , who r e p o r t that the redox 2+ 3+ p o t e n t i a l of the system T i / T i i s n e g a t i v e w i t h respec t to T i / T i 3+ above 1100°K i n KC1 m e l t s . T i 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 data f o r the f o r m a t i o n of complex ox ides . C a O ' x T i 2 n 3 are not a v a i l a b l e . I f we assume that t h e i r s t a b i l i t y i s the same as CaO'TiO, , , 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 ox ides of T i 0 would be even h i g h e r than tha t of CaO'TiO . - 100 -I V . 2 P h y s i c a l d e s c r i p t i o n of the process I V . 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 observed and a p a t t e r n i s suggested on f i g . 38 which can be j u s t i f i e d by u s i n g the thermal 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 I I I . 1 6 ) . Campbell (35) mentions the p o s s i b i l i t y tha t a L o r e n t z f o r c e i n the f l u i d , analogous to the " p i n c h e f f e c t " w i l l f o r c e the s l a g 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 , which would oppose thermal c o n v e c t i o n . T h i s e l e c t r o m a g n e t i c f o r c e i s due to the spread of the c u r r e n t l i n e s between two e lec t rodes of unequal s i z e s . The geometry of the present e l e c t r o s l a g system leads to 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 to the c a l c u l a t i o n s of J o s h i (36) , l i m i t i n g the p o s s i b l e e f f e c t to a s h a l l o w cross 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 be ing t r a n s f e r r e d to the mold remains s m a l l when the mold i s i n s u l a t e d as i s 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 , any such c o n c l u s i o n would be m o d i f i e d by s k i n e f f e c t s ( a l t e r n a t i n g c u r r e n t ) which w i l l modify current l i n e s i n the r e g i o n of the e l e c t r o d e t i p . 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 magnetic f i e l d s produced by the power leads e t c . . . 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 meta l i n ESR u s i n g a r a d i o g r a p h i c method. They r e p o r t t h a t the drop seems f r e e f a l l i n g , i m p l y i n g tha t the extent of downward c u r r e n t s near the drop i s l i m i t e d . - 101 -The temperature e x i s t i n g at the v a r i o u s r e a c t i o n s i t e s i s an important 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 , the temperature can be es t imated from the heat f l u x be ing t r a n s f e r r e d a long the e l e c t r o d e . Immediately above the s l a g we see from f i g . 40 that the temperature g r a d i e n t i s approx imate ly 200°C cm \ S i m i l a r experiments on m i l d s t e e l e l e c t r o d e s ( f i g . 41) l e a d to approx imate ly the same v a l u e . In the l i q u i d f i l m the temperature g r a d i e n t w i l l then depend upon the thermal c h a r a c t e r i s -t i c s of the molten meta 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 to the i n t e r f a c e and o n l y take i n t o account the thermal c o n d u c t i v i t y of the l i q u i d . Schuhmann (39) r e p o r t s tha t t h i s parameter i s u s u a l l y 1/2 to 2/3 of the v a l u e i n the s o l i d f o r pure m e t a l s . We can then es t imate the temperature d i f f e r e n c e across a 0.015 cm l i q u i d f i l m ( I V . 2 . 4 ) to be o n l y 4 .5 to 6°C (300 to 400°C c m " 1 ) . Having 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 remelted (m.p. 1808°K) but w i l l l e a d to 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 of a s l i g h t l y ; d i f f e r e n t compos i t ion (17% N i , 10% Co, 4.6% Mo approxim.) 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 temperature (T 2 ) of 1727°K (40) . The m e l t i n g range of 321 S . S . i s 1672 to 1700°K (41) . The temperature at the i n g o t - s l a g i n t e r f a c e depends upon the m e l t i n g c o n d i t i o n s . Sun (22) has shown tha 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 cou ld be a c c u r a t e l y made by assuming a constant temperature across the s l a g - i n g o t i n t e r f a c e . S ince 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 = 1627°K) , he f i n d s an average meta l p o o l temperature which i s 31°K above f o r a 14 cm diameter 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) which best f i t t e d the c a l c u l a t i o n s to the exper imenta l p o o l depth was 1772°K, or 145°K above T^ f o r a wide v a r i a t i o n i n the mel t r a t e s ( A . C . power) . I f 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 exper iments , 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 of our exper iments , v a r y i n g from 1840°K w i t h 321 S . S . to 1950°K w i t h i r o n . 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 . Us ing 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 tempera-tures of 1950 to 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) . In h i s experiments 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 of 1911°K i n the s l a g cap. The problem i s f u r t h e r compl ica ted by the f a c t tha t an i n c r e a s e i n the temperature g r a d i e n t probably occurs at the s l a g - m e t a l i n t e r -f a c e s . T h i s should be e s p e c i a l l y t r u e at the p o s i t i v e s i d e of a D . C . p r o c e s s , which 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 . I V . 2 . 2 D i f f u s i o n at the s lag-atmosphere i n t e r f a c e I f o x i d a t i o n of 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 that i t i s dependent upon the l i m i t i n g d i f f u s i o n r a t e , to and from the s u r f a c e , of i o n s i n the -H- | | | 3+ 4+ systems Fe /Fe and T i / T i . 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 spec ies having the h i g h e s t o x i d a t i o n s t a t e , such as T i0„ and F e 0 o ~ : + l / 2 0 2 = 2 T i 0 3 " + l / 2 0 2 = 2 F e 0 2 " The r e s i d e n c e time of a volume element of s l a g at the 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 at the s u r f a c e as observed i n I I I . 1 6 . Assuming tha t the f l o w i s laminar 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 represented on f i g . 45. The c o n s e r v a -t i o n of f l o w through any c y l i n d r i c a l s u r f a c e (r = constant ) near the f r e e s u r f a c e r e q u i r e s 2 i r r v r = k (k = constant ) where v r = 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 other assumptions used can be t r a n s l a t e d i n t o v z = 0 (near the s u r f a c e as there i s no s l a g f l o w through the i n t e r f a c e ) 3+ 2 T i + 50 2Fe + 30 - 104 -The time of exposure to the atmosphere of a volume element of s l a g i s 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 cm. > 2 , r , - R 2 ) n . _ t = f z— dr = : - 0.25 second R k k 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 ions 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 . Furthermore , complex o x i d i z e d i o n s are a l s o l i k e l y to have the lower d i f f u s i o n c o e f f i c i e n t s when compared to „ ++ , m 3+ Fe and T. . 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 compos i t ion i s h e l d c o n s t a n t , the s o l u t i o n (composi t ion p r o f i l e ) be ing (71) : C ( z , t ) - C = (C. - C ) [ 1 - e r f ( Z ) ] (eq I V . 3 ) 1 ° irw C: c o n c e n t r a t i o n of d i f f u s i n g spec ies C : same i n the b u l k of the s l a g o & G\: same at the i n t e r f a c e (corresponding to oxygen s a t u r a t i o n ) ; D: d i f f u s i o n c o e f f i c i e n t of same - 105 -The mass f l u x through a u n i t area at time t i s g i v e n by or n i £ I - (r M / D dt " ° ^ !z-0 " ( C i " C o ) The t o t a l f l u x per u n i t t ime and over the whole area S, b e i n g , J S = f - / e (C. - c )/°- t " 1 / 2 dt t 1 O TT e o JS = 2S (C. - C ) / ^ — (eq I V . 4 ) X O 7ft ^ To apply t h i s equat ion to the present system, i t i s u s e f u l to c o n s i d e r the s imple case of a s l a g c o n t a i n i n g no T iO^ or "FeO" as a major c o n s t i t u e n t . In pure CaF^ s l a g s , used f o r r e m e l t i n g i n g o t s S6, S9, S16, M l and M5, we observed heavy element b u i l d up of 1% to 3% (Fe, T i and C r ) . I t i s t h e r e f o r e u n l i k e l y tha t C. - C would 1 o _3 exceed 1% i n the s l a g , corresponding to 0.026 g cm i n a l i q u i d 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 spec ies to 4DS 2 (C . - C ) 2 i o „ r t = - ^ 0.6 sec 6 Tr(JSr 2 2 2 i n the 58 mm mold where S = T T ( R ^ - R ) - 19.8 cm . D has been chosen equal to the d i f f u s i o n c o e f f i c i e n t f o r Al^O^ -5 2 -1 i n C a F 2 _ A l 2 0 3 mel ts repor ted by B u r e l , i . e . 8.5 x 10 cm sec (15) . - 106 -The main causes of e r r o r i n t h i s c a l c u l a t i o n are i n the v a l u e s chosen f o r D and C. - C . S ince the v a l u e p r e v i o u s l y c a l c u l a t e d f o r 1 O f j t g a l s o depends upon the p r e c i s i o n on v^ (+50%), i t can be s a i d t h a t 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 agreement. The i n f l u e n c e of temperature on D i s a l s o worth be ing n o t e d . A v a i l a b l e data concern m a i n l y d i f f u s i o n i n molten 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 the s e l f d i f f u s i o n of 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 . In NaF, G r j o t h e i m and Zuca (43) r e p o r t the s e l f d i f f u s i o n c o e f f i c i e n t of N a + to be D = 3 . 0 8 x 10 3 e 8 , 7 0 0 / R T between 1022 and 1132°C. An a c t i v a t i o n energy of 9 k c a l per ion-gram 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 of 1.27 between 1800 and 2000°K. A major assumption of the above c a l c u l a t i o n i s of course the f a c t tha t the f l o w should be laminar 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 , not because i t i s more v a l i d i n our case but because i t would remain a p p l i c a b l e to l a r g e r s i z e u n i t s . We should then dec ide whether the p e n e t r a t i o n (44) or the f i l m (45) theory f o r mass t r a n s f e r i s more adapted to our case . Toor and M a r c h e l l o (46) suggest 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 depth over which 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 cons idered to be laminar and D i s the d i f f u s i o n c o e f f i c i e n t . - 107 -The p e n e t r a t i o n theory l eads a l s o to 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 c o n s i s t e n t w i t h the p e n e t r a t i o n t h e o r y . As f o r the depth of p e n e t r a t i o n , L , we haye: t x D - 0.25 x 8.5 x 10~ 5 - 2 .1 x 10~ 5 cm 2 e Hence, L should 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 than (2 .1 x 1 0 ~ 5 ) 1 / 2 = 4 .6 x 10~ 3 cm which i s v e r y probably 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 , another 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 . Reduct ion 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 remelted meta 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 along w i t h the o thers because 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 . . i s reached. Calc ium m e t a l w i l l concentrate i n the s l a g , as Ca and CaF^ are comple te ly m i s c i b l e above 1400°C (47) and Ca i s n e a r l y i n s o l u b l e i n most f e r r o u s a l l o y s (48) . The vapour pressure of pure c a l c i u m i s one atmosphere at 1756°K w i t h a heat of v a p o u r i z a t i o n AH^ = 36,390 c a l gram-atom x (49) . We 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: p = 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 tha t CaO i s a p o s s i b l e supply 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 at the 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 . There , i t evaporates and leaves the system or a l t e r n a t e l y , e v a p o r a t e s , o x i d i z e s i n the atmosphere and condenses 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 of the mechanism would r e q u i r e at l e a s t 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 able to de tec t c a l c i u m meta 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 tha t i t e x i s t s at h i g h tempera-t u r e d u r i n g e l e c t r o l y s i s . X - r a y p a t t e r n s taken on the condensates 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 ox ide (magnetite) or aluminium o x i d e , w h i l e the presence of complex oxides such as 2Ca0'Fe0 i s p r o b a b l e . I V . 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 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 meta l p o o l i n t e r f a c e 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 lag-atmosphere i n t e r f a c 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 c u r r e n t . When d i r e c t c u r r e n t i s used , 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 s p e c i e s . J o s t (50) notes the f a c t tha 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 conduct ion and from chemical d i f f u s i o n measurements are g e n e r a l l y i n agreement, 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 g r a d i e n t : J = -UIRT'H + F n C '||] (eq I V . 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 F a r a d a y ' s c o n s t a n t . I f 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 f o l l o w i n g constants at 1800°K. -4 -3 C = 1 weight % - 5.4 x 10 mole cm ( i o n i c weight 48, s l a g d e n s i t y 2.6) (36), 8$ „ V'-„ 23 _ , 1 (_ - 1 — ~ T ~ o~7T ~ 11 '5 v o l t cm 3z i Z.U where V i s the a p p l i e d v o l t a g e across 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 -4 2 - 1 D = u RT = 10 cm sec 10~ 4 , , i n - 9 2 -1 . . . -1 »• u = d oi / Venn - o.7 x 10 cm sec mole j o u l e 8.314 x 1800 J = n (6 .7 x 1 0 " 9 x 9.6 x 1 0 4 x 5.4 x 10~ 4 x 11.5) = n x 4 .0 1 n-6 , -2 - 1 10 mole cm sec • 2 Over the s u r f a c e area of the i n t e r f a c e ( 27 cm ) , t h i s represents a -4 -1 -2 f l u x of n x 1.07. x 10 mole sec or n x 20 x 10 % T i at a melt r a t e of 2.5 g sec 1 . T h i s c o n t r i b u t i o n i s comparable w i t h the t o t a l 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 s the d i f f u s i o n of r e d u c i b l e ions toward the cathode. N e g a t i v e l y charged i o n s which are i n genera l - 110 -complexes of i o n s hav ing 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+ 4+ Fe 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 charge . P o s i t i v e l y 3+ 2+ charged ions such as T i and Fe have t h e i r d i f f u s i o n r a t e i n c r e a s e d , i n t h i s case , improving 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 -r e d u c t i o n 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 -es t imated 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 across the u n i t . I V . 2 . 4 Flow of meta l on the e l e c t r o d e S ince 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 meta l m e l t i n g on i t s s u r f a c e . We s h a l l c o n s i d e r meta 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 the 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 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 . The geometry of the system i s represented on f i g . 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 i s n e g l e c t e d , which 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 occurs on the e l e c t r o d e , 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 . T h i s e x p r e s s i o n (Eq. 2 .2 .16 of (51)) remains v a l i d i n our case and can be w r i t t e n i f we change v a r i a b l e s to f i t our p a r t i c u l a r case : - I l l -X = Z y = - x + 6 0 = 90° - B t h e r e f o r e v - S i n 9 (2y 6 - y 2 ) (eq I V . 6 ) X Zn Hm where y m i s the v i s c o s i t y of the l i q u i d m e t a l , pg the a c t i o n of g r a v i t y on the m e t a l . We s u b t r a c t the buoyancy p r e s s u r e of the s l a g by w r i t i n g P = P m " P s • The e x p r e s s i o n s f o r the maximum and mean v e l o c i t i e s remain unchanged as shown be low: 2 . pg6 s i n 0 , T T 7 v v = v . = H & „ (eq I V . 7) x,max x , i 2u m v = S ± n 9 (eq I V . 8 ) In the present case , 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 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 that p a r t of i t produced by the annulus of e l e c t r o d e o u t s i d e x . I f we c a l l t h i s f l o w Q x ( v o l u m e t r i c ) : Q = P S ^ S ± n 9 2 , r w i t h r = -xcos 0 ; , 2 u p g 6 3 s i n 0cos0 x  3 ^m Since i t m a i n t a i n s a constant geometry, the whole s u r f a c e of the cone mel t s at an equal r a t e . I f W' i s the t o t a l v o l u m e t r i c melt m r a t e - 112 -\ • K ( 1 " 4> - K  ( 1 " % ^ >  (e«  IV- 10) R 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 eads to 3y W 2 2 A r _ o / m m , x cos Q N , T 1 T n , N o = J / -75—7 \ :—« (1 o ) (eq IV.11) \/ 2ir(p -p )gx sm0 cos 0 2 \ M . . / w m s R P r o f i l e s c a l c u l a t e d from t h i s equat ion are p l o t t e d on f i g . 4 7 . T y p i c a l constants are used : 0 - 5 to 5 0 ° u = 0 . 0 5 p o i s e m . p = 7 . 0 ( l i q u i d metal) m p g = 2 . 6 (36) R = 1 .27 cm - 1 * 3 - 1 W = 2 . 5 g sec —> W' = 0 . 3 6 cm sec m m The f i l m t h i c k n e s s i s seen to v a r y from 50 to 200 u over most of the s u r f a c e of the cone i f the angle 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 take 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 . In the absence of c o n c l u s i v e evidence 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 e l e c t r o d e , 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 Mironov (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 to 2567 y . These v a l u e s are determined as a f u n c t i o n of the diameter of the e l e c t r o d e , the p r o p e r t i e s of the - 113 -s l a g and of the m e t a l , the mel t r a t e and the e l e c t r o m a g n e t i c s t i r r i n g . The t h i c k n e s s was found to decrease w i t h i n c r e a s i n g e l e c t r o d e s i z e s . Al though 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 , i t 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 constant r a d i u s X C O s 0 ( = r = constant ) are p l o t t e d , v . i s seen to decrease w i t h 9 ( f i g . 4 8 ) . Large e l e c t r o s l a g furnaces are known to have r e l a t i v e l y f l a t e l e c t r o d e t i p s where drops can form at various p o i n t s . 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 forming on e l e c t r o d e of d iameter . 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 p r e s e n t l y known w i t h s u f f i c i e n t accuracy to extend t h i s c a l c u l a t i o n . I V . 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 equat ions which d e s c r i b e d i f f u s i o n i n the meta l f o r the f l o w regime d e s c r i b e d above. In 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 , 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 or of the ox ide of the meta l which 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 . Evidence 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 working i n v a r i o u s m i x t u r e s of Ca¥^ and A ^ O ^ or 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 that 2% CaO i n pure CaF 2 i s a s u f f i c i e n t l y h i g h 2-c o n c e n t r a t i o n to permit the d i s c h a r g e of 0 from the molten 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 [ 0 ] . . I f we assume that the most l o x i d i z a b l e e lement , t i t a n i u m or aluminium i s v e r y d i l u t e , i t s i n t e r f a c i a l c o n c e n t r a t i o n w i l l tend to zero and the r a t e at which the o x i d a t i o n takes 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 of oxygen i n t o the e l e c t r o d e m e t a l . 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 supply to the system. The c o m p o s i t i o n p r o f i l e achieved ; across 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. (Note tha t the d i f f u s i v e f l u x of [T i ] must not be c o n s i d e r e d because [0] and [ T i ] r e a c t i n s i d e the m e t a l . ) 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 i s 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 to 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 . I t i s g i v e n by 10] = 10 ] , (1 - e r f C S ~ y ) ) (eq I V . 12) y ± /Tn ~ f^uTm e (equat ion 17.5-15 i n r e f (51)) where D- i s the d i f f u s i o n c o e f f i c i e n t of oxygen i n the meta l and 0,m J O t - 7 d x e n v , _ R x , i cos Q 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 , equat ion IV.12 must be i n t e g r a t e d . The t o t a l weight of oxygen t r a n s f e r r e d per u n i t t ime across the s l a g - m e t a l i n t e r f a c e - 115 -i s the same as the f l o w r a t e of oxygen at the system e x i t ( e l e c t r o d e t i p ) , which may be c a l c u l a t e d by i n t e g r a t i o n of the d i f f u s i v e f l o w through the e n t i r e area of the e l e c t r o d e cone, S. The r e s u l t i s s i m i l a r to e q u a t i o n I V .4. S / D Q 0 = / 2 [ 0 ] . / U > m dS (eq IV.13) o v e t i s c a l c u l a t e d us ing the e x p r e s s i o n ( IV .7 ) f o r v . where 6 i s e X , 1 expressed by the equat ion ( I V . 1 1 ) . T h i s l eads to the E u l e r i a n i n t e g r a l (see Appendix I ) : = r27rcos 0 , 2 / 3 f Mm , 1 / 3 , R . 5 / 3 T ( 5 / 6 ) r ( l / 3 ) e _^ 3 " , ; ^ p s i n 0 ; ^cos0 ' r (7/6) m (eq IV.14) Wi th theconstants ' used above i n e q ( l V . l l ) , t comes t o : t - 0 . 5 6 second e 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 h e l d m c o n s t a n t . I n e q u a t i o n (IV. 13) , the area of exposure dS i s dS = 2TJ-— dx r cos 0 and a f u r t h e r approx imat ion has been made when c o n v e c t i o n i n the y d i r e c t i o n i s n e g l e c t e d . I n f a c t , the r e d u c t i o n of area toward the t i p of the cone. tends to make the f l o w d i v e r g e n t w h i l e the 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 approx imat ion can n e v e r t h e l e s s be obta ined by u s i n g the area of the cone as an average v a l u e f o r S and t g as c a l c u l a t e d above. T h i s w i l l probably l e a d to a s l i g h t underest imate of the d i f f u s i v e / f l u x because the r i m of the e l e c t r o d e g i v e s the 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 , having both 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 Q = 2S[0]J - 2 i S _ s 2.4 x 10 3 g / s e c . (eq I V .15) V e u s i n g S = — — — - 5.13 c m 2 . cos 9 D . = 3 . 10 4 c m 2 s e c - 1 (va lue f o r D r t „ (54)) 0,m ^ 0 ,Fe [0]± = 0.2 wt % ( s a t u r a t i o n of Fe a t 1550°C) = 0.014 g c m " 3 The mass f l u x of oxygen i n t r o d u c e s t h e r e f o r e - j — or 0.09% [o] Wm * i n the meta l (W = 2.5 g sec ) . Converted i n t o a r a t e of o x i d a t i o n m f o r [ T i ] to T i 2 0 3 , t h i s v a l u e corresponds to 0.18%[Ti] which i s c l o s e to the observed l o s s . Q Q i s independent of t ime ( W ^ / W ^ . I t should be noted tha t the c o n t r i b u t i o n of the s u r f a c e of the drop has not yet been taken i n t o account . I t s e f f e c t should 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 assess because the extent of 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 ( ingot S15: 2.5 g ; i n g o t M5: 2.85 g ) . I f s p h e r i c a l , t h e i r diameter reaches 0.7 to 1.0 cm when they leave the e l e c t r o d e , r e p r e s e n t i n g at that t ime a s i g n i f i c a n t p o r t i o n of the exposed s u r f a c e area . * 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 which i n c r e a s e i n the order 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 ince 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 meta 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 should c e r t a i n l y be s m a l l . A drop of 1 cm diameter hanging from the t i p of the cone (0 = 40° ) 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 area of the cone w h i l e growing from zero to i t s f i n a l s i z e . I t should be remembered however tha t t h i s c o n t r i b u t i o n occurs 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 g r a d i e n t at 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 . I f we i n s e r t both the i n c r e a s e i n s u r f a c e area and i n t i n t o e q u a t i o n I V . 1 3 by , adding to t the average t ime of f o r m a t i o n of a drop (approx imate ly 1.4 s e c ) , we f i n d tha t the t o t a l oxygen p i c k up should 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 that 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 t u r b u l e n c e . I n equat ion I V . 1 3 , we see immediat ley tha 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 Q ^ , S v a r i e s o n l y s l o w l y w i t h 0 f o r 0 < 4 0 ° . t g i s p r i n c i p a l l y dependent on W which w i l l have the f o l l o w i n g i n f l u e n c e at constant geometry: t « W•.. e m . . - 1 / 2 • 1/3 Q_ <* t <* W X) e 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] - £ . i i J 2 ' 3 W m m - 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 melt r a t e should reduce the compos i t ion change. I V . 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 f i l m 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 anodic i n t e r f a c e , t h a t they no longer c o n t r o l the oxygen p o t e n t i a l . T h i s occurs when the o x i d a t i o n r a t e of r e a c t i v e elements r e q u i r e d by 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 i n excess of the l i m i t i n g d i f f u s i v e f l u x . The excess oxygen can then d i f f u s e i n t o the meta l (see p r e c e d i n g s e c t i o n ) or r e a c t w i t h one of the m a t r i x elements at the s l a g - m e t a l i n t e r f a c e to form an o x i d e (FeO, CT^>^ e t c . . ) which would d i f f u s e i n t o the s l a g . The ox ide formed i s not i n thermodynamic e q u i l i b r i u m w i t h the average compos i t ion of the metal which s t i l l c o n t a i n s 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 concentra te 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 corresponding m e t a l l i c i o n s , t o the c a t h o d i c i n t e r f a c e . T h i s e x p l a n a t i o n probably accounts f o r the presence of i r o n and chromium oxides i n the s l a g s of the Mar 300 and 321 S . S , i n g o t s remelted w i t h D . C . p o s i t i v e e l e c t r o d e . When u s i n g A . C . on the o ther hand, chromium and i r o n ox ides are 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, S13, S16, S17, M7) . Here , the e l e c t r o d e s u r f a c e i s not s a t u r a t e d i n oxygen and the k i n e t i c s of o x i d a t i o n are l e s s f a v o u r a b l e . - 119 -The presence of a major a l l o y i n g element which 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 lower 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 expressed i n the p r e v i o u s paragraph. I f the deoxidant i s d e p l e t e d , chromium w i l l be o x i d i z e d [0]^ w i l l be almost two orders of magnitude lower than w i t h pure Fe ( f i g . 4 2 ) . 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 a f f e c t e d as most of the chromium can be reduced a g a i n at the cathode. In c o n t r a s t , t i t a n i u m and aluminium w i l l not be reduced at the cathode when another ox ide i s p r e s e n t . T h i s e x p l a i n s why the l i m i t i n g r a t e of o x i d a t i o n at the anode of [T i ] and [ A l ] w i l l approach the a c t u a l r a t e of o x i d a t i o n observed i n most of the present exper iments . 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 accord ing to 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 the meta l ( I V . 2 . 5 ) . We have shown that s i g n i f i c a n t d i f f u s i o n of oxygen occurs 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 deoxidant i n h i g h c o n c e n t r a t i o n . I f 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 apply 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 p o s i t i v e ) , the equat ions change very l i t t l e . i T i ] ^ i s n e g l i g i b l e and, i f we put [ T i ] 0 equal to the i n i t i a l c o n c e n t r a t i o n of [ T i ] (or IA1 ] e t c . . . ) , we o b t a i n the p r o f i l e : 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 of oxygen i n t o the m e t a l . Here [ T i ] = I T i ] e r f o 6 -(eq IV.16) - 120 -t Is the same, l e a d i n g to the approximate f l u x of [T i ] Q T i = - 2 S [ T i ] o ( e q I V . 1 7 ) which i s independent of t ime or W /W . m s The main problem here 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 2 -1 c o e f f i c i e n t f o r t i t a n i u m . J o s t (50) g i v e s D„. _ = 1 0 cm sec a t S i , F e -4 2 - 1 1560°C w h i l e D... „ i s g i v e n as 1.22 x 10 cm sec i n r e f . (30) a t A l , F e ° -4 2 - 1 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 cm sec i n the same c o n d i t i o n s by r e f . (55) . -4 2 -1 We s h a l l t h e r e f o r e s e l e c t D m . = 1 0 cm sec T i , m _3 In s t a i n l e s s s t e e l 321, [ T i ] Q = 0.45 wt % = 0.0315 g cm (p - 7 . 0 ) , l e a d i n g t o : Q T i = 3.0 1 0 " 3 g s e c " 1 or Q T i = 0 . 1 2 weight % Wm T h i s i s somewhat l e s s than 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 which has been mentioned i n ( I V . 2 . 5 ) a p p l i e s h e r e , namely i n the c o n t r i b u t i o n of the hanging d r o p . A l s o , 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 . In the case of Maraging 300 s t e e l , approx imate ly 0.35% j T i ] i s l o s t ( f i g . 30, i n g o t s M5, M6) . I Til 0 I s somewhat h i g h e r , at 0.68 wt % - 3 or 0.048 g cm , l e a d i n g to the c a l c u l a t e d v a l u e : Q T i - r ^ * 0.18 wt % W™ wm - 121 -I V . 2 . 7 D i f f u s i o n i n the meta l at the s l a g - i n g o t i n t e r f a c e ( d i r e c t p o l a r i t y ) S ince c o n v e c t i o n p a t t e r n s i n the meta l p o o l are p r e s e n t l y unknown, Klyuev (52) assumes that each drop spreads across the top s u r f a c e of the i n g o t , the s u r f a c e exposed be ing renewed every second (reduced area = area /mel t r a t e ) . T h i s l eads 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 which i s two to three t imes lower than the i n g o t . 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 exper ienced 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 at which 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 es t imated f o r d i r e c t p o l a r i t y c o n d i t i o n s . I f un i form r a d i a l c o n v e c t i o n and a p e n e t r a t i o n model are assumed, equat ion ( IV .4 ) d e r i v e d i n ( I V . 2 . 2 ) a p p l i e s : J = 2(C. - C ) / - £ -i o i r t 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 of [T i ] to a d e p l e t e d i n t e r f a c e ( [ T i ] ^ = 0, see I V . 2 . 2 ) . 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 ( 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 t a b l e : Table 65. Ingot A[Ti ]% : [T i ] % W 1 0 4 x J t P e r i o d of drops ° m - 1 -2 -1 6 M a t r i x M a t r i x g sec g cm sec sec sec S6 0.22 0.23 1.69 1.49 1.48 0.95 M l 0.37 0.32 2.15 3.18 0.63 1.02 M2 ^ 0 . 4 0 " 0.29 2.10 3.44 0.46 0.83 t i s e c a l c u l a t e d from J -4 u s i n g D - 10 2 - 1 cm sec - 122 -We see tha t the r a t e of renewal of the s u r f a c e i s comparable w i t h 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 c rea te 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 meta l a g a i n s t the i n t e r f a c e . I f t 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 tha 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 to renew the whole s u r f a c e . I f , on the c o n t r a r y t i s s m a l l e r t h a n ; t h e 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 i n renewing the s u r f a c e . T h i s model w i l l be v a l i d i f the depth at which the meta l i s renewed L » ( t x D ) 1 / 2 (see I V . 2 . 2 ) or L » 0.2 to 2 x 10~ 2 cm. I V . 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 i n t e r f a c e s 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 of chromium or i r o n oxides i n the s l a g i s n e g l i b i b l e . We have d i s c u s s e d p r e v i o u s l y tha t these components should be present 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 a l l o y i n g elements i s not h i g h enough to accommodate 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 [A1 ] q > 3.74% as : here and I A I ] ^ remains >> 0 . The r a t e c o n t r o l l i n g s tep here i s most l i k e l y to be the r a t e of supply 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 of o x i d a n t a t the boundary and the : t r a n s f e r occurs i n the s l a g phase. - 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 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 t i s f e l t however that 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 meta l a t i t s t e r m i n a l v e l o c i t y or 5 to 10 cm sec ^ . The d i f f u s i o n c o e f f i c i e n t of A ^ O ^ i n C a F 2 ~ A l 2 0 3 i s known to be approx imate ly 8.5 x -5 2 - 1 10 cm sec (15) or s l i g h t l y lower than f o r a l l o y i n g elements i n l i q u i d m e t a l s . Whi le 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 order 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 expect tha 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 which we have c a l c u l a t e d f o r the d i f f u s i o n pf [T i ] or [0] i n ( I V . 2 . 5 ) and ( I V . 2 . 6 ) , a l though 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 f i e l d between D . C . and A . C . o p e r a t i o n ( I V . 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 s t e a d i l y as the o x i d a n t of the s l a g i s exhausted, p r o v i d e d that the system i s c l o s e d . Us ing d i r e c t p o l a r i t y , the anodic i n t e r f a c e i s the i n g o t - s l a g boundary. At t h i s boundary, mass t r a n s f e r i n the s l a g should be of the same order of magnitude as at the o ther i n t e r f a c e . A l l the mass; t r a n s f e r c a l c u l a t i o n s made so f a r apply to d i r e c t c u r r e n t o p e r a t i o n . The main reason f o r t h i s i s the f a c t that the i n t e r f a c e i s known to be at an oxygen p o t e n t i a l grea ter than the chemica 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 d r i v i n g f o r c e f o r d i f f u s i o n . When a l t e r n a t i n g c u r r e n t i s used , on the o ther hand, the i n t e r f a c e i s at an i n t e r m e d i a t e oxygen 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 i n b o t h phases i n contac t i s - 124 -to be c o n s i d e r e d . The f l u x per u n i t area w i l l then be lowered 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 area (both the e l e c t r o d e and the i n g o t sur faces i n s t e a d of one of them). Al though t r a n s f e r of c u r r e n t across the s l a g - m e t a l i n t e r f a c e must be F a r a d a i c , 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 + ne = [M] (where M can be C r , T i , Fe , e t c . . . ) . I t would appear that t h i s 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 assymetry i n the c u r r e n t wave form at constant RMS v o l t a g e and hence no net r e l e c t r o l y s i s occurs (27) . I V . 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 meta 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 r e s i d e s i n the p o s s i b i l i t y of r e v e r s i o n of the mass t r a n s f e r process at the o ther e l e c t r o d e . The f o l l o w i n g 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 case , but r e m a i n s . v a l i d w i t h d i r e c t p o l a r i t y . Once oxygen has d i f f u s e d i n t o the meta l on the e l e c t r o d e f i l m , i t 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 . I f there i s a s u f f i c i e n t amount of deoxidant i n the m e t a l , as i s the case h e r e , homogenizat ion f o l l o w e d by p r e c i p i t a t i o n of ox ides takes p l a c e . A p a r t of the d e o x i d a -t i o n products w i l l be removed from e i t h e r the drop or the meta 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 extremely ; 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, there 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 i n t e r f a c e where i t can be e l e c t r o l y t i c a l l y removed. Thus, the f l u x of oxygen i n t o the meta l i s not e l e c t r o l y t i c a l l y r e v e r s i b l e . - 125 -When a l l o y i n g elements concentra te i n the s l a g , on the o ther hand, they r a p i d l y reach a c o n c e n t r a t i o n of a few tenths of a p e r c e n t . T h e i r r a t e of r e d u c t i o n at 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 compos i t ion gradient and i n an e l e c t r i c f i e l d ) i s s u b s t a n t i a l . 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 experiments performed. Ingot M2 i s a good example. At the e a r l y stages 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 to 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 I I I . 1 2 ) . 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 tha 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 of 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 cont inued to concentrate i n the s l a g . I V . 2 . 1 0 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 apply 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 ( i . e . of the i n g o t l e n g t h ) . T h i s has been 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 c u r r e n t under an oxygen c o n t a i n i n g atmosphere, whether t h i s i s a i r or 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 decrease 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 p l a c e : a) A product 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 system, making - 126 -o x i d a t i o n thermodynamical ly l e s s f a v o u r a b l e ; b) The o x i d a n t becomes deple ted i n the system. We have observed such a decrease i n the f o l l o w i n g cases : - when u s i n g A . C . power as i n i n g o t s S12, 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 both cases the accumulat ion of r e a c t i o n products was comparable w i t h tha t of mel t s produc ing i n g o t s showing no such e f f e c t . The reason i s t h e r e f o r e 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 of i m p u r i t i e s such as s i l i c a (which has been d e t e c t e d : I I I . 3 ) and the amount of m e t a l ox ides (FeO e t c . . ) forming at the s t a r t of a g i v e n r u n . I t can a l s o be TiO^ i f t h i s element i s a c o n s t i t u e n t of the s l a g . A l l w i l l p r o g r e s s -i v e l y become deple ted i f there i s no e x t e r n a l supply of oxygen. The former w i l l d i sappear much f a s t e r than the l a t t e r owing to the i n i t i a l c o n c e n t r a t i o n s . 3 ( S i 0 2 ) + 4 [ T i ] y 3 [ S i ] + K T i ^ ) 3 (FeO) + 2 [ T i ] y 3[Fe] + ( T i ^ ) 3 ( T i 0 2 ) + [ T i ] y 2 ( T i 2 0 3 ) 3 ( F e 2 0 3 ) + 2IT1] y ( T i ^ ) + 6 (FeO) P o s s i b l e " . regenerat ion" r e a c t i o n s b e i n g 2FeO \+ l / 2 0 2 or T i 2 0 3 ' + l / 2 0 2 ++•' ' -Ca + 2e —y 2 T i 0 2 •y Ca° (vapour) a t the cathode (see I V . 2 . 2 ) . - 127 -I V . 2 . 1 1 Macroscopic assessment bf the process (oxidant i n l i m i t e d 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 of 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 t r a n s f e r 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 s u p p l y . E . Steinmetz (56) g i v e s the mathemat ica l treatment f o r the exchange of m a t e r i a l between a mobi le and a s t a t i c phase. E l e c t r o -s l a g furnaces can be c l a s s i f i e d w i t h the processes where the contac t i s " t r a n s i t i o n a l " ( s l a g s t a t i o n a r y , meta l f l o w i n g ) , w i t h o u t homogenizat ion of the meta l phase subsequent to the r e a c t i o n (see s e g r e g a t i o n i n I V . 2 . 1 2 ) . Whi le 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 aminar f l o w , the treatment of the problem g i v e n here r e s t r i c t s the mass t r a n s f e r r e s i s t a n c e to a boundary l a y e r near the i n t e r f a c e , hence i t i s 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 i s concentra ted on the s l a g s i d e of the i n t e r f a c e (note however that 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 case , the v i s c o s i t y u g be ing r e l a t i v e l y -h i g h a t ^ 0.75 p o i s e (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 occur 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 rans fer c o e f f i c i e n t s are constant over the area of r e a c t i o n . - 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 area per u n i t time f o r a component X i s t h e r e f o r e expressed a s : - 128 -j = K ((X) - (X)) in the slag boundary layer S 1 o r j = K m (W ~ i-n t n e m e t a l boundary layer (X) and [X] be ing b u l k c o n c e n t r a t i o n s of the phases , w h i l e [X]^ and (X)^ are 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. , < are mass t r a n s f e r c o e f f i c i e n t s i n the s l a g and m e t a l s m ° l i m i t l a y e r s . The "process e f f e c t i v e n e s s ' 1 to, taken here as the amount of removal of 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 (56) (X) nW p KAt o , m r s •U /'111 O v u = oo" e x p CTTT7"w— > nW +p KA s m s ( T r a n s i t i o n a l contac 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 through 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 of X [ X ] . w m Henrian e q u i l i b r i u m constant: n = area of contact between phases melt r a t e 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 . . In our case , the r e a c t i o n s of 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 to rfrl^. The equat ion becomes : - p K At/W (X) = ( X ) o e S 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 to t i m e : - 129 -p K A -p xAt/W (X) = -(X) -4— e S S o W s The rate of change of (X) or (X) i s r e l a t e d to the rate of oxidation of the a l l o y i n g element, through the exchange reaction (written here for titanium): n(X) + m[Ti] • m(Ti) + n[X] A material balance per unit time requires M_ W (X) [Ti] = [Ti] + — rjr—• — (Eq IV. 18) 'o m ML^  W m where M represents the atomic weight of the corresponding element (concentrations are expressed i n weight f r a c t i o n ) . Replacing (X), t h i s equation becomes [Ti] * [Ti] O O w m n : M T i with (Ti) = — — — (X) i s the stoichiometric (Ti) equivalent of the e m M. o . oxidant i n i t i a l l y i n the slag. W We can further observe that i f the melt rate i s constant t = -— W p <A Wm m s and by c a l l i n g Y = — and <j> = — ; — the equation becomes s Wm r , (Ti) -<i>Y [ T i ] ' = 1 " TTir *e ( E q- I V , 1 9 ) 0 : o - 130 -T h i s equat ion 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 at an a p p r e c i a b l e r a t e . Ingot S12 i s i n f a c t the o n l y one which f u l f i l s these requirements because our i n g o t s remelted 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 remelted w i t h a s l a g r i c h i n o x i d a n t (T±0^), ( I V . 3 . 2 ) . A few curves of the f a m i l y eq ( IV.19) are represented on F i g . 52. They show that a shor t t a n s i e n t w i l l be ob ta ined when <j> i s i (T i ) h i g h and the o x i d a n t (or f T . -•— ) l o w . o These curves 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 ther elements which are l i k e l y to be exchanged w i t h the s l a g : a . 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 ,10,57) or v i c e v e r s a ( 7 , 5 8 ) . The behaviour of carbon i s a l s o dependent upon the a c t i v i t y of Si02 i n the s l a g (10) , a c c o r d i n g to the p o s s i b l e r e a c t i o n : ( S i 0 2 ) + 2[C] = 2(CO) + IS1] The c o m p o s i t i o n p r o f i l e of [Mn], J S i ] , [Cj a long 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 eq . ( IV.19) 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 Holzgruber (57) , t h i s r e l a t i o n s h i p i s observed f o r (CaO)/(Si02) between 0.4 and 2 f o r the r e a c t i o n of [ S i ] and [Mn] and ( C a O ) / ( S i 0 2 ) between 0.3 and 1.2 f o r the r e a c t i o n of [ C ] . Outs ide these v a l u e s , 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 of 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 ( I V . 2 . 3 ) to ( I V . 2 . 9 ) . b . Sulphur has a l s o been the s u b j e c t of a number of s t u d i e s and a l though the amount of d e s u l p h u r i z a t i o n observed i s known to i n c r e a s e * w i t h the b a s i c i t y of the s l a g ( i n f a c t , the a c t i v i t y of CaO ) ( 1 0 , 5 8 , 5 9 , 6 0 , 6 1 ) , one of the mechanisms by which 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 of SO^ by the atmosphere (62 ,63,57,58) making the l o s s approximate ly constant even i n s h o r t i n g o t s of 100 mm or so i n l e n g t h (21 ,64) . I V . 2 . 1 2 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 mel t r a t e appears i n the m e c h a n i s t i c equat ions proposed e a r l i e r . 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 to the i n f l u e n c e of 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 melt r a t e can be the r e s u l t of v a r i o u s causes which a l l tend to i n c r e a s e the heat f l o w going up the e l e c t r o d e : ; 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 of the gap between the e l e c t r o d e and i n g o t (h igher 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 : g e n e r a t i o n ) , - a r c i n g . it C h o u d h u r y and a l . (58) r e p o r t that 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 constant b a s i c i t y . - 132 -When only an increase i n power i s considered, the process i s run at a higher temperature which means that the mass transf e r processes are accelerated i n the slag and at the ingot surface. The electrode, by melting, provides i t s own temperature regulation and mass transf e r per unit time i s f a i r l y constant i n the metal f i l m . There-fore, increasing the melt rate should reduce composition changes when the rate c o n t r o l l i n g step i s i n the metal f i l m of the electrode. Closing the gap between electrode and ingot w i l l c e r t a i n l y l i m i t the extent of the hot zone, thereby decreasing exchanges with the atmosphere. Variations I n the melt rate r e a d i l y influence the ingot structure. An excessive melt rate w i l l , f o r example, deepen the pool and have an adverse e f f e c t on dendrite o r i e n t a t i o n . I f v a r i a t i o n s are pronounced, the volume pf the pool and the rate of s o l i d i f i c a t i o n are affected which may lead to segregation. We have so far accepted the fact that the ingot composition r e f l e c t s the composition of the metal melted at a p a r t i c u l a r time without considering homogenization of the pool. The absence of segregation when the f i n a l pool volume s o l i d i f i e s supports t h i s point: We could not detect a terminal composition transient by electron probe analysis. A zone r i c h e r i n solute i s found to be confined to the l a s t ^ 100 u near the surface and may correspond to the l i m i t layer for d i f f u s i o n during s o l i d i f i c a t i o n . I f we r e t a i n t h i s value pf the l i m i t layer, 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 at the end of a run should be (65): - 133 -k = — = 0.996 (Eq IV. 20) e 1-k fjS \ i / 1 + (-jJ-2-) e - tD o _2 i f f =6.10 cm/sec (assuming that a pool 1.8 cm deep s o l i d i f i e s i n 30 sec) 6 = I O - 2 cm (100 y) -4 2 D = 10 cm /sec k Q ( T i i n Fe) = 0.4 (35) •, k g i s very high and e x p l a i n s why we do not observe l o n g i t u d i n a l segregation at the end of a run. The value chosen f o r <5 i s r e l a t i v e l y low. Chalmers (65) gives values from 0.1 cm f o r n a t u r a l convection -3 i n metals to 10 cm f o r vigorous s t i r r i n g . The o v e r a l l p i c t u r e of a small s o l i d i f y i n g e l e c t r o s l a g ingot i s therefore t h a t of a n e a r l y constant composition i n the ingot and the metal pool 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 the s o l i f i c a t i o n i n t e r f a c e . 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 that major a l l o y i n g elements w i l l ever segregate to a high extent when the pool i s not 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 ) that e l e c t r o s l a g u n i t s of a wide s i z e range are i n r e g u l a r production use. I t i s t h e r e f o r e of i n t e r e s t to ponsider the i n f l u e n c e of u n i t s i z e on the c h a r a c t e r i s i t i c s of the process. - 134 -If we consider the s i z e v a r i a t i o n of several remelting parameters we can q u a l i t a t i v e l y draw the following o u t l i n e : a) Big units usually operate at a higher voltage, reaching i n some cases 45-50 V or more. As the current density necessary to melt the metal and maintain a s a t i s f a c t o r y pool geometry i s approximately inversely proportional to the diameter of the electrode (10), there i s an increase i n electrode-ingot gap with s i z e i f Ohm's law applies: I = j B X s (Eq IV.21) where I i s the ingot-electrode gap, V the voltage applied across the bath, I the current and B the e f f e c t i v e cross-section of the slag bath and A g i t s conductivity. As inductive (A.C.) or r e s i s t i v e losses i n the long power leads p a r t l y account f o r the high voltage applied to b i g u n i t s , we observe that I w i l l be approximately proportional 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^). X g i s r e l a t i v e l y constant because the average temperature of the slag does not vary with the s i z e of the unit provided the same metal i s remelted (27). Thus, the influence 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 ions i n the,slag w i l l decrease with (eq. IV.5) and hence with the increase i n s i z e of the u n i t . b) From the preceding paragraph, i t i s apparent that a comparable geometry w i l l be found i n most ESR units.. This follows i f the metal pool i s to remain shallow when compared to the mold diameter (2R^) , thus maintaining a small angle between the d e n d r i t i c structure and the mold wa l l . - 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 e l e c t r o d e t i p p r o f i l e . The angle 9 ( I V . 2 . 4 ) at 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 reach the p o i n t where s e v e r a l drops can form at d i f f e r e n t p o i n t s of the t i p . I t i s not ye 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 e l e c t r o d e s . c) Holzgruber (66) r e p o r t s that 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 (according to which the depth of the s o l i d i f i c a t i o n f r o n t , which i s here 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 square r o o t of t ime) 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 immediate ly deduced t h a t the p o s s i b l e melt r a t e i s p r o p o r t i o n a l t o and the r i s e of the i n t e r f a c e i n v e r s e l y p r o p o r t i o n a l to R^. In s m a l l u n i t s , l i k e the one p r e s e n t l y u s e d , i t seems tha t lower melt 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 achieve 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 than a s h a l l o w • k p o o l . I n f a c t , the exponent of R^ i n the r e l a t i o n s h i p a R^ i s of the order of 1.75 i n most cases . d) Reviewing now the mechanisms proposed i n S e c t i o n s I V . 2 . 2 tp I V . 2 . 1 1 , the main assumption w i l l be tha 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 not occur when s i z e i s i n c r e a s e d . i 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 t h a t 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 ; i n g o t s . e) The r e a c t i o n s w i t h the atmosphere ( I V . 2 . 2 ) were found to f o l l o w a r e l a t i o n s h i p of the t y p e : - 136 -JS oc — f o r the mass f l o w of oxygen JS. 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 * - l - 1 / 2 JS W cc SW t ' m m e The area of contact i s the annulus exposed to the atmosphere which v a r i e s w i t h the " f i l l r a t i o " R/Rj^ = f• Assuming t to be • 1 75 constant and to v a r y as , JS w; 1 - a - f2> P^ 0- 2 5 f i s o f t e n c l o s e r to 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 s l a g 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 proper 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 f i l m ( I V . 2 . 3 - I V . 2 . 4 ) causes a compos i t ion change A[0] or A[M] « R 2 t ~ 1 / 2 W ~ 1 e m * - 2 / 3 ^5/3 , ' _1.75 as t a w R and W cc R e m m We f i n d A10] or A M cc R 7 / 6 W M _ 2 / 3 cc f 7 / 6 - 137 -Here , 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 f i l l r a t i o (W may a l s o change w i t h f, and t depends upon the geometry of the e l e c t r o d e cone) . g) 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 leads to the same r e s u l t s as paragraph (e) when a p p l i e d to the whole c r o s s - s e c t i o n ; A[0] or A[M] <* R M 0 , 2 5 h) The macroscopic model used f o r A . C . ( I V . 2 . 1 1 ) g i v e s : • _ i - P s K A t / W s AIM] <x KA W„ e M 2 * 1.75 I f A « R^, .W j^. = R^ " and K i s independent of s i z e , A F M 1 ' 0.25 " P s K A t / W s A [M] cc e which i s a f u n c t i o n of R^ the maximum of which depends upon the v a l u e of the exponent, < 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 . g) 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 that the order of magnitude of the t o t a l l o s s observed i n our u n i t i s comparable to or h i g h e r than the r e p o r t e d 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 or d e o x i d a t i o n of the s l a g has been t a k e n . Holzgruber r e p o r t s l o s s e s of TMn] t [ S i ] 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 which are of the order 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) r e p o r t e d a combined l o s s of [ C ] , I S i ] and [Mn] amounting to 0.20% i n a low chromium s t e e l (0.9% C, 1.17% C r ) . - 138 -IV.3 Review of the experimental r e s u l t s with respect to composition changes IV.3.1 Influence of the atmosphere Ingots S l and S3 have been remelted under an argon blanket which lowers the oxygen p a r t i a l pressure without eliminating the supply of oxygen. Ingot S2 has been remelted i n a i r , but i n other respects, with the same conditions. A l l three ingots have undergone a comparabl loss of [ T i ] , having almost i d e n t i c a l composition p r o f i l e s ( f i g . 18). By contrast, ingots remelted under pure argon (S16 and 17, f i g . 26 and 27) show a loss of titanium which i s s u b s t a n t i a l l y lower. We conclude that the p a r t i a l pressure of oxygen i n the atmosphere bears l i t t l e r e l a t i o n to the amount of titanium l o s t from 321 s t a i n l e s s t e e l , while the actual rate of supply of oxygen controls the l o s s , when t h i s i s below a c e r t a i n value. The same conclusion applies to Mar. 300 when ingots Ml (argon , blanket), M2 (air) and M7 (pure oxygen) are compared. The rate at which the atmosphere supplies oxygen to the system as derived from the rate of oxidation of i r o n observed with the ingots F l to F5 i s somewhat higher than would account f o r the losses usually observed. IV.3.2 Influence of the slag composition on 321 S.S. ingots When the system i s covered by an o x i d i z i n g atmosphere, t h i s f actor has a; v a r i a b l e influence, depending mainly upon the melting - 139 -p o l a r i t y . Ingots S l to S6 have been remelted 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  n e g a t i v e . Whi le the s l a g used f o r i n g o t S6 was pure CaF2» S l to S5 were melted u s i n g 25% C a O A ^ O ^ . We have seen that c a l c i u m a luminate cou ld 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 run ( 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 yet present i n the s l a g . T h i s should e x p l a i n why the l o s s of T i i n S l , S2, S3 decreases w i t h time ( f i g . 18) to reach 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 remelted 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^. D e s p i t e t h i s f a c t , the m a t r i x l o s s of [T i ] i s s l i g h t l y lower (0.20 to 0.15%). 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 mel t r a t e however (see I V . 3 . 6 ) , i n which 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 l o s s . The e q u a t i o n proposed i n s e c t i o n ( I V . 2 . 7 ) ( D i f f u s i o n i n the anodic meta l at the s l a g - m e t a l i n t e r f a c e ) can r e a d i l y account f o r the approximate magnitude of the l o s s and the f a c t tha t the l o s s remains approximate ly constant throughout the m e l t . N e v e r t h e l e s s , the t o t a l 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) as evidenced by the c o n c e n t r a t i o n of these ox ides i n the s l a g remains dependent upon the r a t e of oxygen p i c k up from the atmosphere. Ingots S9-S10 and S l l have been remelted 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 ( f i g . 21 ) . 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 c o m p o s i t i o n . I t i s probable tha t h e r e , 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 f o r i T i ] ( I V . 2 . 6 ) was reached i n the e l e c t r o d e f i l m . 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 of (T i ) at the cathode which remained n e g l i g i b l e i n the presence of about 0.5% (Cr) i n the s l a g . 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 meta l on the e l e c t r o d e , combined w i t h the equat ion of S e c t i o n ( IV.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 i n t h i s r e g i o n can r e a d i l y account f o r the approximate magnitude and constancy of the observed l o s s e s , the t o t a l r a t e of o x i d a t i o n remaining dependent upon the r a t e of p i c k up of oxygen from the atmosphere as i n the preceding case . Ingots S12-S13 have been remel ted w i t h a l t e r n a t i n g c u r r e n t . Ingot 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 i n ( I V . 2 . 1 1 ) . 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 i s p l o t t e d , , which shows that the f l u x of [T i ] obeys e q u a t i o n ( IV.19) where the f o l l o w i n g constants have been used ( f i g . 22) : 2 Area of r e a c t i o n ( ingot + e l e c t r o d e t i p + d r o p ) : 33 cm , - 3 - 1 Mass t r a n s f e r c o e f f i c i e n t : K = 3.16 x 10 cm sec , Oxidant i n the s l a g : ( T i > e = 1.90%. . • The melt r a t e was almost constant d u r i n g the r u n , W = 2 .1 g sec which g ives the parameters of the equat ions § ~ 0.13 and ( T i ) e / [ T i ] Q -4 . 2 . The i n i t i a l amount of 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 v a l u e f o r a s l a g , such as the one u s e d , which has not been d e o x i d i z e d . The va lue of 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 : K - - —'-—7. g i v e s 6 of approximate ly 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 of exchange w i t h the atmosphere c a l c u l a t e d i n ( I V . 2 . 2 ) . 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 the atmosphere would then 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 i n s t a n c e ) (see I V . 2 . 2 ) . W. Holzgruber and a l . (11) r e p o r t an i n g o t compos i t ion p r o f i l e where the s i l i c o n decreases at the s t a r t of the m e l t . I f we take the p a r t of 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 , a f t e r \ s u b s t r a c t i n g the constant component of the l o s s (make [ S i J Q - 0.12% i n s t e a d of 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 equat ion ( S i ) e ( IV . 19) w i t h the f o l l o w i n g c o n s t a n t s : , • — = 4.7 ( e q u i v a l e n t o x i d a n t : l b l J o 0.56% S i ) and <(> = 0 .16. The o n l y dimension reported : i s the diameter of the i n g o t , 43 cm and we can assume the area of contac t , A -2 ' 2200 cm and the melt r a t e (57) W = 0 . 4 3 t o n / h a u r . The mass t r a n s f e r m c o e f f i c i e n t comes then to be remarkably c l o s e to the v a l u e we have - 3 - 1 c a l c u l a t e d : < - 3 . 3 6 x 10 cm sec Ingot S13 was remelted 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 obvious on f i g . 23. T h i s i s to be expected as ( T i ) e / [ T i ] Q i s a t l e a s t 7 . 2 , c o u n t i n g T i 4 + as the 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 to e q u a t i o n ( IV.19) which i s t h e r e f o r e meaning l e s s f o r the beg inning of the m e l t , ( f i g . 47 ) . Here 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 at the v a r i o u s s l a g m e t a l i n t e r f a c e s and t h e r e f o r e should be n e a r l y constant w i t h t i m e . 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, which were remelted under argon. The l o s s i s s m a l l but remains - 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 s t a r t i n g c o n d i t i o n s . The d e o x i d i z e d s l a g which contains no CaTiO^ s t i l l causes a h i g h e r l o s s of T i at 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 27) . 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 . I V . 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 Maraging 300 s t e e l The resu l t s obta ined 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 s t e e l . Ingots M l , M3, M4 ( f i g . 28) have been remelted 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 n e g a t i v e , they e x h i b i t comparable l o s s e s of m a t r i x t i t a n i u m w h i l s t the molybdenum content 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 compos i t ion 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 these 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 s t a i n l e s s s t e e l ( I V . 3 . 2 ) , we f i n d that the magnitude of 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 of [T i ] i n the s t e e l . T h i s could i n d i c a t e that the r a t e l i m i t i n g step i s a l s o d i f f u s i o n i n the meta l at the s l a g - i n g o t i n t e r f a c e ( I V . 2 . 7 ) . The i n c r e a s e i n A [T i ] i s g r e a t e r than that of [ T i ] because of v a r y i n g p r o p e r t i e s of 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 , m e l t i n g temperature ) . Ingots M5 and M6 remel ted 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 other on the b a s i s of s l a g c o m p o s i t i o n . The l o s s of T i i s lower than i n the e l e c t r o d e n e g a t i v e case and lower even than that obta ined w i t h S . S . 321 i n comparable c o n d i t i o n s . In 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 , the 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 - 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 cases . In these c a s e s , 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 cone around the drop a r e a . T h i s i m p l i e s that 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 f o r 321 S .S . and Mar. 300 were h e l d c o n s t a n t , the l o c a l heat f l u x 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 cases . T h i s would l e a d to 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 area between the two cases , Mar . 300 be ing 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 of S e c t i o n ( I V . 2 . 6 ) as does S . S . 321, w h i l s t l e a d i n g to a lower melt l o s s of t i t a n i u m . 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 t i t a n i u m deple ted s u r f a c e , l e a d i n g to the observed h i g h content of (FeO) i n the s l a g . I V . 3 . 4 I n f l u e n c e of the s l a g compos i t ion on 1409 A l s t e e l Al though the melt 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 ther s t e e l s , i t i s g e n e r a l l y apparent tha t the s l a g composi-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 . T h i s was expected because 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 r a t e 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 (or supply) of oxygen through the d e o x i d i z e d s l a g . I n these c o n d i t i o n s , we would observe a constant r a t e of l o s s (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 igher than tha t observed f o r T i w i t h the o ther s t e e l s (see r e s u l t s i n I I I . 1 3 ) . - 144 -I V . 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 Ingots F l to F5 have been remel ted under v a r i o u s s l a g s w i t h d i r e c t p o l a r i t y . With i r o n , the r a t e of t r a n s f e r of oxygen through the s l a g should be r a t e l i m i t i n g . The presence 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 evidenced by i n g o t s F2 to F 5 , i n that order ( I I I . 1 4 ) . The o x i d a t i o n 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^ was h ighes t (52%) than was the case f o r pure CaF2« I t i s l o g i c a l tha t the presence of a redox couple i n the s l a g should i n c r e a s e the r a t e of t r a n s f e r w i t h the atmosphere ( I V . 2 . 2 ) . A constant 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 ( I V . 2 . 1 ) . Ingot F l (Ferrovac E) shows a h i g h e r r a t e than F4 (1018 m i l d s t e e l ) w i t h v i r t u a l l y the same s l a g . The o x i d a t i o n of [ C ] , [ S i ] and [MnJ may have p layed a r o l e i n the f i n a l m a t e r i a l b a l a n c e . I V . 3 . 6 I n f l u e n c e of the mel 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 f o l l o w i n g s e r i e s of exper iments : a) Ingots S l , S2, S3 , S6, S7, S8, a l l remel ted 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 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 composi-t i o n on A l T i ] . F i g . 53 shows t h i s same parameter A [ T i ] p l o t t e d a g a i n s t the melt r a t e at v a r i o u s l e v e l s of these i n g o t s . The curve p l o t t e d i s A [ T i ] x W = k which 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 at - 145 -the s l a g - i n g o t i n t e r f a c e ( I V . 2 . 7 ) or at the s lag-atmosphere i n t e r f a c e ( I V . 2 . 2 ) . k has been found from the l e a s t square f i t r e l a t i o n s h i p on the s t r a i g h t l i n e A [ T i ] = k x i -W m The f a c t tha 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 at the h i g h temperatures a s s o c i a t e d w i t h h i g h mel t r a t e s . The bottom s e c t i o n s of i n g o t s S l , S2, S3 have been exc luded because the e q u i l i b r i u m r a t i o of T i / A l was not achieved i n the s l a g ( I V . 3 . 2 ) . 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 leads to 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 s c a t t e r . c) Ingots S9, S10, S H show no i n f l u e n c e of the mel t r a t e on the l o s s of 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 ( I V . 2 . 7 ) suggest that A [ T i ] should v a r y as W ' (see m I V . 2 . 5 ) . 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 that p a r t of the e l e c t r o d e f i l m was deple ted 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 ; ' - 2 / 3 some I T i ] . I n t h i s case the l o s s would then be A [ T i ] = k + k ' , k be ing l a r g e and k ' be ing concealed by the s c a t t e r on A l T i ] . I V . 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 of d e o x i d i z i n g the s l a g i s most obvious when redox couples are p r e s e n t . The continuous 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 i n the [T i ] content of the i n g o t s above the i n i t i a l v a l u e . 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 the 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 tha t the c a p a c i t y 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 reduced. 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 tha t the l o s s of [Al ] tends to decrease w i t h t i m e . T h i s i s c o n s i s t e n t w i t h the 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 having l i t t l e t r a n s p o r t c a p a b i l i t i e s f o r oxygen. E v a p o r a t i o n of p a r t of the aluminium 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 . I V . 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 occas ions (Sec t ions I V . 3 . 1 to I V . 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 or r e v e r s e p o l a r i t y ( D . C . ) (see I V . 3 . 2 - I V . 3 . 3 ) . The i n f l u e n c e on the l o s s of t i t a n t i u m and the oxygen c o n t e n t , however, was reversed when pass ing from 321 S . S . 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 the two metals 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 c o n c e n t r a -t i o n ( I V . 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 , the observed l o s s i s s i g n i f i c a n t l y lower w i t h Maraging 300 s t e e l (M5, M6). The oxygen content on the o ther hand i s l a r g e r . Delayed p r e c i p i t a t i o n f o r ox ides 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 f i l m ( I V . 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 meta l i s c a t h o d i c a l l y p r o t e c t e d . When u s i n g A . C . power, i t i s found ( i n g o t s S12-S13) that the observed o x i d a t i o n i s more c o n s i s t e n t w i t h an o x i d a t i o n mechanism which does not i n v o l v e p o l a r i z a t i o n of the e l e c t r o d e s ( I V . 2 . 1 1 - I V . 3 . 2 ) . With 1409 A l i n g o t s , 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 . p o s i t i v e and n e g a t i v e e l e c t r o d e was not found to be s i g n i f i c a n t , presumably because v o l a t i l i z a t i o n of aluminium i s the predominant t r a n s f e r mechanism, which would be independent of p o l a r i t y . I V . 3 . 9 I n c l u s i o n and oxygen contents I t has been suggested (67,68) that i n c l u s i o n s are 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 . T h i s c o n c l u s i o n i s a r r i v e d at by comparing the v e l o c i t y of r i s e of the i n c l u s i o n s , c a l c u l a t e d from S t o k e ' s l a w , w i t h the 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 . T h i s 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 e l e c t r o s l a g i n g o t (67 ,69) . Other workers suggest however that i n c l u s i o n s are most l i k e l y to be removed when they come i n contac t w i t h the s l a g , i n the l i q u i d f i l m on the e l e c t r o d e or i n the drop (15 ,52 ,69 ) . For a s t e e l having v e r y h i g h counts of c a r b i d e or c a r b o - n i t r i d e i n c l u s i o n s (1409 A l ) , we f i n d tha t o n l y l i t t l e removal i s a c h i e v e d . The i n i t i a l s i z e of the i n c l u s i o n s i s n e a r , but not c l e a r l y above a t y p i c a l c u t - o f f s i z e f o r f l o t a t i o n . T h i s would be i n the range 15-20 u f o r a normal E . S . melt (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 t ime 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 can 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 of diameter u . D i f f u s i o n c o e f f i c i e n t s i n l i q u i d meta ls are of the order -4 2 - 1 of 10 cm sec and the l i f e of a 10 u i n c l u s i o n s h o u l d not exceed a f r a c t i o n of a second. In s h o r t , 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 m e t a l b e f o r e r e a c h i n g the p o o l . The oxygen content of 1409 i n g o t s was n e g l i g i b l e and no ox ide i n c l u s i o n could be d e t e c t e d , i n agreement w i t h the d e o x i d a t i o n c a l c u l a t i o n s ( I V . 1 . 6 ) . The r a t e at which oxygen d i f f u s e s i n t o the s t e e l and the k i n e t i c s of ox ide p r e c i p i t a t i o n are the p r i n c i p a l f a c t o r s c o n t r o l l i n g the f i n a l count of ox ide p a r t i c l e s found i n the m e t a l . In both 321 S . S . and Mar. 300, the amount of oxygen found i n the s t e e l remains low (y 100 ppm) when compared w i t h t i t a n i u m content (2000 ppm) and does not s t o i c h i o m e t r i c a l l y account f o r the 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 which i s thus l a r g e l y represented by the c a r b i d e c o n t e n t . The range of oxygen c o n c e n t r a t i o n s observed i n the i n g o t s i s compared w i t h the 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. In a l l cases , the c o n c e n t r a t i o n i s lower than that imposed by the m a t r i x (see chromium l i n e on f i g . 42 ) . Having argued that the m e t a l s u r f a c e was not complete ly deple ted i n deoxidant 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 should 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 content of the i n g o t r e s u l t s from a d i f f u s i v e f l o w of oxygen. T h i s i s observed on f i g . 42 (321 S . S . ) but not on f i g . 43 (Mar. 300). 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 i n d i c a t e - 149 -that there 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 or 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 cases , there was a s i g n i f i c a n t decrease i n the T i ( C , N ) c l a s s f o r s i z e s > 5 y , the extent of which 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 used , 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 thermal h i s t o r y of the . ingot . A number of workers have suggested a c o r r e l a t i o n between s l a g compos i t ion ( c o n c e n t r a t i o n and type of o x i d e components) and the nature and count of i n c l u s i o n s ( 6 7 , 4 ) . In the present system, 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 that 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 content of the metal w i t h an i n c r e a s e i n A ^ O ^ content i n CaF^-CaO-Al^O^ s l a g s 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 . The b a s i c i t y of the s l a g , on the o ther hand, 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 ob ta ined w i t h 321 S .S . i n g o t s 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 . F i g . 54 represents the i n f l u e n c e of the A l ^ O ^ content of 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 ^ content i n the system CaF2~CaTi0 3 ; both ox ides cause a s i m i l a r r e d u c t i o n of the oxygen l e v e l i n the i n g o t , which cannot be accounted f o r by a change i n ,the melt r a t e . - 150 -I V . 3.10 Importance of 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 turned our a t t e n t i o n toward both the m a t r i x content of r e a c t i v e elements (microprobe a n a l y s i s ) and the t o t a l content ( s p e c t r o g r a p h s or wet chemica l a n a l y s i s ) . This d i s t i n c t i o n i s important because of the need to d i f f e r e n t i a t e between the a l l o y i n g elements which are conta ined i n i n c l u s i o n s and those which 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 e t c . . . ) . 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 c a r b i d e p r e c i p i t a t i o n s i n c e there would be c a r b i d e p r e c i p i t a t e s i n a s i z e range which would r e g i s t e r as a m a t r i x content i n microprobe a n a l y s i s . 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 both the e l e c t r o d e and the i n g o t represent an e q u i l i b r i u m p r e c i p i t a -t i o n r e a c t i o n , 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 content and the; c a r b i d e i n c l u s i o n content bear a constant r e l a t i o n s h i p . I t appears that i n most i n g o t s the amount of element ( t i t a n i u m ) t i e d up i n ox ide 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 ) . Carbides are u s u a l l y i n h i g h e r c o n c e n t r a t i o n s w i t h a r e l a t i v e l y constant s i z e d i s t r i b u t i o n from sample to sample. The same m e c h a n i s t i c c o n c l u s i o n s can be drawn from the t o t a l element a n a l y s i s , as from m a t r i x t i t a n i u m a n a l y s i s . T h i s c o n s t i t u t e s an answer to one of the o r i g i n a l q u e s t i o n s , the t i t a n i u m l o s s does not represent s imply a removal of ox ide 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 i n g o t i s not l i k e l y to represent 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 merely d i f f e r e n t p r e c i p i t a t i o n c o n d i t i o n s . CHAPTER V CONCLUSIONS The present o b s e r v a t i o n s c o n f i r m that a l o s s of r e a c t i v e elements f o r an a l l o y s t e e l can occur 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 . I n the experiments per formed, t h i s l o a s was between <v> 5 and 80% of the o r i g i n a l content which was 0.5 to 4 wt %. In many cases , the v a r i a t i o n of c o m p o s i t i o n f e l l o u t s i d e the range which would be r e q u i r e d by commercial s p e c i f i c a t i o n s . 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 lower 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 of 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 % of 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 . In 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 system, a m a t e r i a l ba lance 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) or e x c e s s i v e s e g r e g a t i o n of 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 balances are observed. 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 which can account f o r the observed r a t e of t r a n s f e r across the f o l l o w i n g i n t e r f a c e s . These a r e : ; - Atmospheric o x i d a t i o n of the e l e c t r o d e . - 152 -- D i f f u s i o n of oxygen and a l l o y i n g elements i n the l i q u i d f i l m of the e l e c t r o d e ( e l e c t r o d e p o s i t i v e ) . - D i f f u s i o n of oxygen and o x i d i z a b l e spec ies at the s lag-atmosphere i n t e r f a c e . Models i n v o l v i n g unknown parameters are proposed f o r the f o l l o w i n g : - D i f f u s i o n through the s l a g - i n g o t i n t e r f a c e . - 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 supply ( A . C ) . 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 e lements . The f i r s t one i s the presence of an o x i d a n t , FeO, S±0^ or H O 2 w h i c h , be ing i n l i m i t e d supply w i l l cause a d e c r e a s i n g l o s s w i t h t ime 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 3+ 4+ 2+ 3+ s t a t e s , such as T i / T i or Fe /Fe which ac t s as a c a r r i e r f o r atmospheric oxygen. 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 ox ide to i n i t i a t e such a p r o c e s s . The c o n v e c t i v e mass f l o w of oxygen brought to the mel t r e g i o n by the atmosphere i s the most important f a c t o r i n the o x i d a t i o n r a t e . S ince both o x i d a t i o n of the e l e c t r o d e and of the s l a g occur at t h e i r l i m i t i n g r a t e s f o r very low p a r t i a l pressures at the atmospheric i n t e r f a c e , the e q u i l i b r i u m oxygen p a r t i a l pressure of the o x i d a t i o n r e a c t i o n s 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 processes p l a y an important r o l 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 , i n that anodic p o l a r i z a t i o n i s achieved by p r o g r e s s i v e s a t u r a t i o n i n ox ide components at the s l a g s i d e of the anode, e v e n . i f o n l y a s m a l l c o n c e n t r a t i o n of ox ide ions i s present i n the s l a g . A 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 achieved at the anode, which 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 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 processes i n the m e t a l . At the cathode, r e d u c t i o n of the l e a s t s t a b l e ox ides of the s l a g occurs at a r a t e which i s d i m i n i s h e d by atmospheric o x i d a t i o n of the s l a g . Both these e l e c t r o c h e m i c a l processes r e s u l t i n a net l o s s of r e a c t i v e e lements . In most cases , the s l a g c o n c e n t r a t i o n of l e s s r e a c t i v e elements 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 t r a n s f e r toward the cathode. When a l t e r n a t i n g c u r r e n t (60 Hz) i s u s e d , t r a n s f e r through the s l a g meta l i n t e r f a c e s occurs at a lower r a t e which seems to be ; independent of process c u r r e n t d e n s i t y . As mass t r a n s f e r occurs i n the same d i r e c t i o n over the whole area of s l a g - m e t a l c o n t a c t , 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 which decreases w i t h the d e p l e t i o n of i n i t i a l o x i d a n t . 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 compos i t ion 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 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-able mass t r a n s f e r c o e f f i c i e n t s . The a d d i t i o n of (TiC^) to the s l a g , as has been suggested ( 9 ) , does not seem, per s e , 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 the 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 (TiC^) 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 at which atmospheric oxygen i s accepted by redox r e a c t i o n s i n the s l a g . - 154 -Appendix I . Flow of m e t a l i n the e l e c t r o d e f i l m 1. C a l c u l a t i o n of the exposure t ime i n the f r e e f l o w case ( I V . 2 . 4 ) t = f° d x e _ v . R x i cos9 Equation (IV.7) g i v e s v . = P g , s l n 9 S 2 4 r XI 4y 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 ( IV. 11) • o , 3p W „ . 2 2„ 0 = Pg s l n 6 3 / ( m m . 2 1 x cos 6 . 2 x i 2u \/ 2irpg s i n e cos 0 -) —J U y " ; m V K S x R T h e r e f o r , t . - 2 V — i 1 ' 3 ' , x 2 ' 3 < l -m cos 9 1^  I f we r e p l a c e - = A , the equat ion becomes cos 0 ' M t = 2 ( . . . ) 2 / 3 ( . . . ) 1 / 3 f° x2/3a - 4 r 2 / 3 dx A A nr- r - 9 f . 2 / 3 / ' . 1 / 3 A 4 / 3 , A 2 / 3 , .2 2 , - 2 / 3 , or t - -2 ( . . . ) ( . . . ) A / x (A - x ) dx o 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 : / . . . d x = 1 / 2 A 1 / 3 B ( 2 / 3 2 + 1 , - 2/3 + 1) = l / 2 A 1 / 3 B ( 5 / 6 , l / 3 ) o . 1 / 3 r (5/6) r d / 3 )  1 , l k r (7/6) r i s a t a b u l a t e d f u n c t i o n (70). - 155 -T h e r e f o r e : , 2 T T C O S 6 , 2 / 3 f y m ) l / 3 ( R 1 5 / 3 1-129 x 2.678 e 3 ^ pg s i n B } l c o s Q } 0.9027 m 2. 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 w i t h the s l a g . See f i g u r e ( A l . l ) f o r a d e s c r i p t i o n of the geometry of the system. L e t us c o n s i d e r a s m a l l annulus of l e n g t h dx around the cone at d i s t a n c e x from the t i p , t h e t h i c k n e s s of the f i l m b e i n g 6 ^ , the o r i g i n of y can be taken as the m e t a l - s l a g i n t e r f a c e . a) The meta l f i l m obeys the c l a s s i c a l p a r a b o l i c law f o r the p r o f i l e of v e l o c i t i e s , e q u i v a l e n t to the r e l a t i o n used f o r i n I V . 2 . 4 but r e w r i t t e n f o r the f o l l o w i n g boundary c o n d i t i o n s : / V x d y 2 2 Q v . .6 = — 7 6 = W (1 - " ° ) (The t o t a l f l o w i s the X f A m r,2 / dy R , . mel t r a t e at x) o T h i s l eads to v = f u n c t i o n of v . , y , W and the v a r i o u s x x , i m g e o m e t r i c a l and p h y s i c a l constants ( 6 , p , u ^ . . . ) and an e x p r e s s i o n f o r 6 : FT (v . , 6 , x , W ) = 0 1 x , i ' ' ' m - 156 -b) The s l a g develops a laminar boundary l a y e r , over the e l e c t r o d e , i n the case represented, v i s d i r e c t e d upwards and 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); we may s e l e c t f o r example: 3 v -v . = v (3/2n - l / 2 n ) where n = y/d x X , l » .' d i s a f u n c t i o n of the p h y s i c a l constants of the s l a g ( u g , 6, v ,..) and x, l e a d i n g to the r e l a t i o n F 2 ( v x j i , d, x) - Q c) At the slag-metal boundary, we have already w r i t t e n (v .) -= ( v x ^)g^ ag> i n 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 conservation of momentum between the phases. I f we c a l l T x t h i s shear f o r c e per u n i t area ( T x j L ) s l a g = (xx i ) m e t a l Sv ... .3v X I X | ° r y s l y " ! = - »m W I . < y—K) y—>• 0 This leads to a t h i r d equation a l s o c o n t a i n i n g the v a r i o u s p h y s i c a l constants: F 3 ( v i ) . , d, 6 , x, Wm) - 0 - 157 -d) The three equat ions 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 melt 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 0 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 : v . = f ( x ) x , i 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 t ime ; ^ — = t R V x , i cos 8 E q u a t i o n 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 . , d , 6 , and x . They can be s o l v e d n u m e r i c a l l y , x , 1 The unknown constants i n the equations are p r i n c i p a l l y v ^ , which 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 a g a i n s t the e l e c t r o d e and u g which depends upon temperature g r a d i e n t s . The f a c t tha t the i n t e r f a c e i s not everywhere convex w i l l c e r t a i n l y 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 data on v , n u m e r i c a l c a l c u l a t i o n s are CO not attempted h e r e . - 158 -Appendix I I . Segregat ion i n ESR We are concerned here w i t h gross l o n g i t u d i n a l s e g r e g a t i o n as our d i s c u s s i o n i s based on the assumption t h a t no m i x i n g occurs between the b u l k of the l i q u i d p o o l of m e t a l and the s o l i d i f y i n g l i q u i d . L e t us c o n s i d e r a s o l u t e X hav ing p a r t i t i o n c o e f f i c i e n t k Q , be ing the r a t i o of c o n c e n t r a t i o n s i n the 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 the s o l i d i f i c a t i o n i n t e r f a c e : k = s o [X] 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 (72) 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 (72) o I n p r a c t i c e , 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 to the h i g h e r c o n c e n t r a t i o n i n the l i q u i d 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 (see I V . 2 . 1 2 ) . I f the l i q u i d i s homogeneous, the "wors t ' case" of l o n g i t u d i n a l s e g r e g a t i o n i s encountered. Dur ing a r u n , the c o m p o s i t i o n of the l i q u i d meta l would i n c r e a s e from the average c o m p o s i t i o n of the I X ] o e l e c t r o d e : [ X j Q and tend toward — — at i n f i n i t y . The e l e c t r o s l a g e process would then 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 to the f i r s t pass of a zone r e f i n i n g o p e r a t i o n (73) : k. W 1X3. e s = 1 - (1 - k ) e \ (A2.1) 1 ^ e where W and W. are the weight of the s o l i d i f i e d meta l and of the s £ - 159 -l i q u i d zone r e s p e c t i v e l y . W g I f we assume tha t a steady s t a t e i s achieved (— = °°) £ [ X ] o ™ s = ^ o > wt -n^ Let us now s i m u l a t e a sharp v a r i a t i o n i n the melt 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 . to A . C . w i t h our u n i t . 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 the e l e c t r o d e s tops 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 s o l i d i f i e s (0 < g < 1 ) . The compos i t ion 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 (73) : IX] = [X] (1 - g ) k _ 1 (A2.2) s o At the end of t h i s s t a g e , g = G. In the second s t a g e , the power i s r e s t o r e d and we assume tha t 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 l i q u i d i s r e e s t a b l i s h e d . 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 meta l of the compos i t ion IX] . At the end of t h i s stage I X j o k - 1 IX] = (1 - G) + GjX] (A2.3) e ° 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 occurs a g a i n and we are i n the i n i t i a l t r a n s i e n t s i t u a t i o n , the o r i g i n f o r Wg b e i n g the b e g i n n i n g of r e s o l i d i f i c a t i o n . M u l t i p l y i n g (A2.3) by k g i v e s the i n i t i a l compos i t ion of the s o l i d at the b e g i n n i n g of t h i s s t a g e : - 160 -k -1 [ X ] a = [ X ] Q (1 - G) + G k e [ X ] Q (A2.4) When the process c o n t i n u e s , [X] v a r i e s a c c o r d i n g to (A2.1) s r e w r i t t e n f o r the new o r i g i n : W S = 1 + [(1 - G) K e + G k - 1] e 6 W £ [x]q - - e 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 represented g r a p h i c a l l y on f i g u r e ( A I I . l ) . The maximum c o n c e n t r a t i o n s tep occurs 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 to the v a l u e of G: S o l u t e M a t r i x k G [X] A r x ] „ o A B T i Fe 0.40 0.2 1.14[X] .995[X] o o T i Fe 0.40 0.5 1.51[X] .958[X] o o N i Fe 0.83 0.5 1.12[X] .978[X] o o In p r a c t i c e , G = 0.2 would c o n s t i t u t e an important 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 step 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 Mar. 300 s t e e l . - 161 -Appendix I I I . A c t i v i t y of TiC>2 i n CaF 2~CaO A 3 . 1 D e s c r i p t i o n of the experiment CaF 2~CaO s l a g s prepared from the pure chemicals C a F 2 ( B r i t i s h Drug House Co. ) 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 c a r b i d e (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 between 1700 and 1900°K. The atmosphere was carbon monoxide a t atmospheric p r e s s u r e . The c r u c i b l e ( 6 . 3 mm of i n s i d e diameter and c o n t a i n i n g two to three 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 which acted as a susceptor f o r a 12 KW R . F . set ( f i g . A 3 . 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 pressed p e l l e t which remained s o l i d through the experiment and c o u l d be e a s i l y separated from the s l a g at the end of the r u n . The time of 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 at the lowest temperature and, as no compos i t ion d i f f e r e n c e was observed , subsequent mel t s were kept at 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 thermo-c o u p l e . 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 then analysed 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% C a F 2 , which i s s l i g h t l y o u t s i d e the range of the a n a l y t i c a l e r r o r . I i - 162 -A3.2 R e l a t i o n to ESR The experiment was designed to measure the a c t i v i t y of t i t a n i u m oxides 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 r e m e l t i n g as t h i s 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 meta l and s l a g (see I V . 1 . 5 ) . To c ircumvent 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 of t i t a n i u m , the system had to be kept at an oxygen p o t e n t i a l comparable to that 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 [ T i ] . The 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 through the r e a c t i o n . 2C(gr) + 0 2 ( g ) = 2CO ( g ) ( A 3 . 1 ) AF° at 1700°K = -125,000 c a l AF° at 1900°K = -133,000 c a l hence the oxygen p a r t i a l p r e s s u r e under one atmosphere of CO i s ; P 0 2 = 1 0 ~ 1 6 * ° atm at 1700°K = 1 0 ~ 1 5 - 3 atm at 1900°K By comparison, an i r o n a l l o y c o n t a i n i n g 0.5% [ T i ] i n e q u i l i b r i u m w i t h T i 0 2 at an a c t i v i t y of 0.01 w i l l impose: 0 2 ( g ) + [ T i ] ( s ) = ( T i 0 2 ) ( s ) AF° = -150,000 c a l / m at 1800°K _ AF° K a T i Q ? 0.01 RT _ 18.2 p ~ o 0.03 x 0.005 x pO U Y T i l T i ] P 0 2 or pO. - 10 atmosphere. - 163 -A 3 . 3 React ions ot T i C Since oxygen (CO) can be t r a n s f e r r e d through the s l a g , the f o l l o w -i n g r e a c t i o n s w i l l l e a d to the d i s s o l u t i o n of t i t a n i u m i n t o the s l a g . T i C + 2C0 y T i 0 2 + 3C (A3.2) 2TiC + 3C0 y T i 2 ° 3 + 5 C (A3.3) T i C + CO >• TiO + 2C (A3.4) In the exper iments , the a c t i v i t i e s of T i C and C were u n i t y , as the s l a g was i n contact w i t h the pure s o l i d s , a n d the p a r t i a l pressure of CO was a l s o u n i t y . The f r e e energy change of the above r e a c t i o n s w i l l t h e r e f o r e l e a d to the a c t i v i t y of the t i t a n i u m ox ides through the r e l a t i o n AF° = - RT l n a T i 0 : x Table ( A . l ) g i v e s the v a l u e of f r e e energ ies of f o r m a t i o n of the compounds of i n t e r e s t at v a r i o u s temperatures : Table A . l Free e n t h a l p i e s of f o r m a t i o n ( 7 4 ) , c a l / m o l e T°K CaO CaF 2 T i 0 2 T i 2 ° 3 TiO(31) T i C CO ( s o l i d ) ( s o l i d ) ( s o l i d ) ( s o l i d ) (gas) 1600 -110,320 -226,900 -157,250 -260,600 -88,450 -39 ,850 -60,600 1700 -110,000 - 2 2 2 , 1 0 0 - 1 5 3 , 1 0 0 -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 energ ies of r e a c t i o n s A3 .2 to A3.4 i s s t r a i g h t f o r w a r d and leads to the diagram of f i g u r e ( A I I I . 2 ) r e p r e s e n t i n g 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 ox ides w i t h r e s p e c t to temperature . I n 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 ( T i 4 + ) ^ ° 2 ( T i 3 + ) 2 a T i 2 ° 3 e t c . Z ( T i n + ) a T i 0 2 + 2 a T . Q ^ + a T i Q E ( T i n + ) a T i 0 2 + 2 . . . T h i s i s represented 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 c o e f f i c i e n t s 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 decreased . T i a l s o forms complex 2-ipns w i t h 0 a l though 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+ 2 -T i should 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 charge of t h i s i o n . A3.4 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. The m e l t i n g p o i n t of pure CaF 2 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 t h e r e f o r e 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 5 + 2 % CaO i f CaO i s the o n l y i m p u r i t y (14) . We s h a l l n e v e r t h e l e s s c o n s i d e r i t to 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 composi t ions used i n the exper iments : - 165 -Table A . 2 S lag % CaO mole f r a c t i o n CaO range of temperatures i n v e s t i g a t e d °K 1 0 0 1727 - 1901 2 9.35 0.125 1693 - 1935 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 constant at one atmosphere. A3.5 A c t i v i t y c o e f f i c i e n t of TIO^ A f t e r the exper iments , the s l a g s were analysed f o r t i t a n i u m . 3+ 4+ 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 by Ce ) The r e s u l t s were then converted i n t o mole f r a c t i o n of T i 0 2 i n the s l a g [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 of T i 0 2 c a l c u l a t e d a c c o r d i n g to the formula o a T i 0 9 ^TiO, j — 2 -( T i 0 2 ) a^,^Q i s known from f i g u r e A I I I . 2 . The composi t ions measured are r e p o r t e d i n Table 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 . A I I I . 4 . I t can be r e a d i l y seen that the i n t r o d u c t i o n of CaO i n the - 1 6 6 -Table A . 3 S lag (Table A .2 ) Temperature °K T i content % N T i 0 2 X X ° 2 as of % T i o Y T i 0 2 1 1727 .162 .264 7.2 1 1733 .072 .120 13.1 1 1773 .570 .930 .785 1 1851 .366 .596 .324 1 1863 .336 .550 .291 1 1901 .126 .210 .440 2 1693 0.318 .500 7.2 2 1701 0.498 .780 3.87 2 1718 1.89 2.99 .703 2 1798 1.35 2.12 .217 2 1828 1.05 1.65 .170 2 1893 .618 .970 .105 2 1935 .747 (11.7) (.005) 3 1726 2.04 3.10 .580 3 1743 3.01 4.58 .284 3 1813 1.90 2.87 .125 3 1828 1.49 2.24 .125 3 1893 1.62 2.44 .042 3 1901 1.70 2.57 .036 4 1728 1.03 1.49 1.15 4 1728 1.81 2.62 .656 4 1738 2.62 3.80 .376 4 1778 3.17 4.60 .144 4 1803 1.58 2.29 .186 4 1875 3.00 4.35 .031 5 5 5 1723 1782 1901 2.00 2.24 1.13 2.84 3.18 1.60 .674 .192 .058 - 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 2-forming a complex w i t h 0 . When a l a r g e excess of CaO i s used (0.125 or 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 i s CaO 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 curves pass from y° > 1 to 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 that the v a l e n c e s t a t e of T i passes from a form which has a p o s i t i v e d e v i a t i o n (at low temperature) 2-to another form which tends to be complexed, presumably by 0 (at h i g h tempera ture ) . I f there was no change i n the s t a t e of o x i d a t i o n of T i , y° would be expected to tend toward u n i t y as the temperature i n c r e a s e s . 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 which i s most 4+ s t a b l e at low temperature ( T i , see f i g u r e A I I I . 2 ) i s a l s o known to 2-form complexes w i t h 0 - 168 -A3.6 D i s c u s s i o n The r e s u l t s ob ta ined are of l i m i t e d v a l u e m a i n l y because the s t a t e 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 Sherrer p a t t e r n c o u l d not be i d e n t i f i e d . CaF^ i s always the dominant p a t t e r n . Specimens c o n t a i n i n g 0.39 and 0.47 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 (hydrated CaO) w h i l e a compound of TiO,, cou ld sometimes be detec ted i n the few samples where there was 3% or more T i . T h i s compound was e i t h e r 4Ca0-3T i0 2 or C a O ' T i 0 2 . Another problem a r i s e s from the presence of carbon . The f r e e energy of the r e a c t i o n CaO(s) + 3C(gr) • C a C 2 ( s ) + C0(g) i s 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 K p = 1.35 x 1 0 ~ 3 = 8,100 at 1900°K K = 0.117 P CaC„ , o r -1/1—3 . i-7rt«oT o r , f o r pCO = 1 atm. 2 = 1.35 x 1 0 " " . a t 1700°K a . = 0.117 at 1900°K The f o r m a t i o n of CaC 2 i n the s l a g w i l l occur to some e x t e n t , e s p e c i a l l y at h i g h temperature . Carbon a n a l y s i s of the specimen i n d i c a t e s that a v a r i a b l e amount (0.1 to 3%) w i l l be p i c k e d up i n the s l a g . I t i s - 169 -however i m p o s s i b l e to separate tha t p a r t of the carbon which i s a c t u a l l y r e a c t e d w i t h the components of t h e . s l a g from the p a r t s which r e s u l t from the r e a c t i o n s of T i C w i t h CO or from s t r a i g h t mechanica l e r o s i o n 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 A3 .2 shows, the p a r t i a l p r e s s u r e of 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 present exper iments . The r a t i o s of va lence s t a t e s of t i t a n i u m should 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 though 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 which i s a r t i f i c i a l l y a t t r i b u t e d to Ti02 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 present case . . - 170 -Appendix I V . A n a l y t i c a l methods A 4 . 1 A n a l y s i s of s l a g s A l l slags were f i r s t crushed to a f i n e powder b e f o r e be ing f u s e d . I t has been found tha t an extremely 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 t i m e . 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 ( I I I . 1 0 ) . A 4 . 1 . 1 j 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 per forming an a c i d f u s i o n (75) . Two methods were used w i t h comparable r e s u l t s : a) To a sample weighing Q . l to 0.5 g (the sample should be l a r g e r i f both i r o n and t i t a n i u m are to be determined from the same s o l u t i o n ) add 10 ml concentra ted H^SO^. Fuse f o r at 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 or 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 fuse 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 or s i l i c a u n t i l a deep red 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 fuse 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 ) . The l i q u i d should be c l e a r . 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 tand f o r 48 hr d u r i n g which t ime 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 then 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 analysed immediate ly 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 i r o n f o u r drops of methylene b l u e and 1 gram H^BO^ i s passed through 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 which has been f l u s h e d w i t h CO^ to 3+ prevent 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 at 60°C (76) -3 u s i n g a s tandard s o l u t i o n of C e r i c s u l f a t e 5 x 10 N) u n t i l the 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 c o l o r . The t i t r a t e d s o l u t i o n i s then cooled to room temperature . E i g h t drops of o - p h e n a n t h r o l i n e f e r r o u s complex are added and the t i t r a t i o n i s cont inued 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 i r o n . A b l a n k v a l u e i s taken f o r both p a r t s of the a n a l y s i s . The c e r i c s u l f a t e s o l u t i o n i s s t a n d a r d i z e d a g a i n s t a weighed amount of a rsen ious ox ide (77) . b) Spectrophotometr ic d e t e r m i n a t i o n of t i t a n i u m : 1.5 ml ^2®2 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 ml H^PO^ should a l s o be added i f the b lank ( 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 . The s o l u t i o n i s then d i l u t e d to 50 ml w i t h 2 N . H^SO^. Spectrophotometry i s performed at 410 my. The c o l o r a t i o n i s s t a b l e (75) . c) Spectrophotometr ic d e t e r m i n a t i o n of F e : T h i s d e t e r m i n a t i o n has been c a r r i e d out at 500 my u s i n g 1-10 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 f o r Fe i n an ace ta te b u f f e r (2 M sodium aceta te ) (75) . • Both T i and Fe can be determined from known amounts of the same f u s i o n s o l u t i o n . - 172 -A 4 . 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 to 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 Cr( IV) i n the presence of d i p h e n y l -c a r b a z i d e ; wavelength : 540 my (64 ,65) . A 4 . 1 . 3 F l u o r i n e A sample c o n t a i n i n g about 0.2 g CaF^ i s fused 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 N a 2 C 0 3 . The product of f u s i o n i s e x t r a c t e d i n an a c i d s o l u t i o n of 1:2 H C l . I t i s d i l u t e d w i t h 350 ml H 2 0 and 50 ml of 0 .1 M EDTA i s added. The volume i s brought to 500 m l and the s o l u t i o n i s a l l o w e d to s tand f o r one h o u r . Immediately a f t e r the pH has been brought to 9 w i t h s a t u r a t e d NaOH, the a c t i v i t y of F i s read u s i n g a s a t u r a t e d ca lomel e l e c t r o d e 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 (Orion Research I n c . , model 94-09) . Measuring i n s t r u m e n t : p o t e n t i o m e t r i c e l e c t r o m e t e r , 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 dry CaF2» ( f i g . A I V . l ) . At l e a s t one r e f e r e n c e specimen i s analysed 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 . T h i s a l l o w s the .{ exper imenta l e r r o r to be reduced from c i r c a 10% to 1%. R e a c t i o n constants (75) : 10 .3 ( s o l u b i l i t y p r o d u c t ) . 10.7 10.3 6.2 Oa** ..+ 2F~ = CaF 2 : pK = Ca"1"*" + Y 4 " = CaY 2 ~ (EDTA): p K Q = + ; 4- 3 -H + Y = HY : p K x = H + + H Y 3 " = H Y 2 " : pK = - 173 -2-Hence, the apparent pK f o r the complex CaY i s about 8.6 a t pH 9 (75) , a s s u r i n g the s o l u b i l i t y of CaF . A4.2 A n a l y s i s of the s t e e l s A 4 . 2 . 1 T o t a l m e t a l l i c 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 a c c o r d i n g to the o f o l l o w i n g t a b l e ; some samples were d u p l i c a t e d between l a b o r a t o r i e s . Table A . 4 S t e e l Element Method Done by 321 S . S . T i Mn S i T i Spec t rographic S p e c t r o g r a p h i c S p e c t r o g r a p h i c Wet chemica l ( U n i v e r s a l Cyclops JSpecialty S t e e l s D i v . (. ( 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 . ) Mar. 300 T i T i Mo Spec t rographic S p e c t r o g r a p h i c S p e c t r o g r a p h i c Vasco ( L a t r o b e , P a . ) j l n t . N i c k e l Co. 1 (New York) 1409 A l A l T i S i Mn S p e c t r o g r a p h i c U n i v e r s a l Cyclops S p e c i a l t y S t e e l s D i v . A 4 . 2 . 2 M a t r i x m e t a l l i c elements Microprobe : . JEOL, model JXA-3A. Microprobe 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 s t e e l and Maraging 300 s t e e l . In a d d i t i o n , the main a l l o y i n g element of each s t e e l (Cr and N i r e s p e c t i v e l y ) was analysed on the second channel - 174 -of the probe . K emiss ion was detec ted f o r each e lement . r a _g The e l e c t r o n beam had an average i n t e n s i t y of 8 x 10 Amp and was a c c e l e r a t e d under a 25 KV p o t e n t i a l . The c o u n t i n g t ime was 10 seconds. Aluminium and chromium were analysed 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 of 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 tep be ing 1udiamond paste f o l l o w e d by a l i g h t pass on 0.05 u a l u m i n a . T i t a n i u m : 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 es t imated 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 [T i ] = k - E g s i s assumed to be v a l i d at the peak of i n t e n s i t y of the K^ l i n e of t i t a n i u m : N = counts measured on specimen, p B = counts measured on T i f r e e m a t r i x . N g = counts measured on pure T i . I f the r e l a t i o n s h i p between the t o t a l content [ 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 versus —^g f o r a grea t number s of specimens, i t i s found that 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 best specimens show k = 0.9 and t h i s v a l u e has been r e t a i n e d f o r the c o r r e c t i o n to be a p p l i e d (see f i g . A 4 . 2 ) . The curve 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 . A 4 . 2 ) . T h i s program computes the c o r r e c t i o n s f o r background, a b s o r p t i o n , c h a r a c t e r i s t i c f l u o r e s c e n c e , 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 leads o n l y to approximate v a l u e s f o r low c o n c e n t r a t i o n s , the e x p e r i m e n t a l curve was p r e f e r r e d . T h i s choice bears l i t t l e consequence to the g e n e r a l c o n c l u s i o n s of the work , however. I n order 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 l e a s t 50 p o i n t counts were taken at random from each specimen and those counts which exceeded twice the average v a l u e were r e j e c t e d . The percentage of r e j e c t i o n was u s u a l l y below 10%. Al though t h i s procedure gave coherent r e s u l t s f o r most i n g o t s , i t 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 content was reduced from the s l a g . E x c e s s i v e s e g r e g a t i o n i s suspected to have defeated the method i n t h i s case , as an e x c e s s i v e d i f f e r e n c e i s 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 which i s known p r o v i d e d one s tandard 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 response , 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 taken on each specimen. A 4 . 2 . 3 Oxygen Each r e p o r t e d v a l u e represents the average c o m p o s i t i o n of three adjacent specimens weighing 0.5 to 3 g . The Leco Oxygen A n a l y s e r which 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 mel ted 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 approx imate ly 2000°C; CO ! - 176 -i s produced, swept away by a c a r r i e r gas ( h e l i u m ) , converted to CC^ and trapped i n a m o l e c u l a r s i e v e . CO^ i s then r e l e a s e d and ana lysed by a chromatographic method. - 177 -BIBLIOGRAPHY 1. Duckworth W.E. and Wooding P . J . , Trans . Vacuum M e t a l l u r g y C o n f e r -ence, Am. Vac . S o c , N . Y . (1968). 2. Roberts R . J . , Trans . Vacuum M e t a l l u r g y Conference , Am. Vac . S o c , ( V I , 1969). 3. Duckworth W.E. and Hoyle G . , E l e c t r o - s l a g R e f i n i n g , Chapman and H a l l ( }969). 4. Holzgruber W . , P r o c . F i r s t 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 , P i t t s b u r g h ( V I I I , 1967). 5 . Holzgruber W . , Pe tersen K. and Schneider P . E . , Trans . Vacuum M e t a l l u r g y C o n f . , Am. Vac . S o c , N . Y . (1968). 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 , 1967). 7. H l i n e r y J . and Buzek Z . , Sbornik Ved. P r a c i V . S . B . O . (Ostrava) 11 , 483 ( I I I , 1965). 8. Yuassa G . , P r o c Secon4 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 , 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 ta te S c i e n t i f i c and Techn. P u b l . House of L i t e r a t u r e on Ferrous and Non-Ferrous M e t a l l u r g y , Moscow (1963). 10. H o l z g r u b e r . W . , D o c t o r a l D i s s e r t a t i o n , Mont. Hochschule Leoben, A u s t r i a ( V I , 1967). 11. H o l z g r u b e r . W . , Machner P . and P l o e c k i n g e r E . , Trans Vac . Met . C o n f . , Am. Vac . S o c , N . Y . ( V I , 1969). 12. Decker , R . F . , Seminar on Maraging S t e e l s h e l d by I n t . N i c k e l Comp. P i t t s b u r g h 1962. 13. Beynon G . , Unpubl ished r e s e a r c h , Department of M e t a l l u r g y , U . B . C 14. Koj ima H . and Masson C . R . , Can. J o u r n a l of C h e m i s t r y , 47, 4221 (1969). 15. M i t c h e l l A . and B u r e l B . , M e t a l l u r g i c a l T r a n s . , _1, 2253 ( V I I I , 1970); and B u r e l B . , M . A . Sc . T h e s i s , Department of M e t a l l u r g y , U . B . C . ( V I I , 1969). 16. A . S . T . M . Standart n° E45-63 (1969) book of s tandards p a r t 31. 17. J e r n k o n t o r e t Research O r g . , Examinat ion 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). - 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 . , 205, 947 ( I X , 1967). 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 . , 360 ( I V , 1966). 21. W h i t t a k e r D . A . , P h . D . t h e s i s , McMaster U n i v e r s i t y ( V I I I , 1967) . 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 ( IX , 1969). 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, (1963). 24. Towers H. and Chipman J . , Trans A . I . M . E . , 209, 769 (1957). 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 Science and Technology, Dec 1970 (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 , 1967). 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 , 12 , ! 47, ( IX , 1969). 30. H i l l e r t L . , A c t a Chemica S c a n d . , 19 , 1516 ( V I , 1965) . 31. E l l i o t t J . and G l e i s e r M . , Thermochemistry f o r S t e e l making, Addison-Wes ley , 1960. 32. Segawa K. and Tsunetomi E . , Trans I . S . I . J . , 9_, 89 (1969). 33. H o l f e r t C , Lambrecht J . and Praske W . , F r e i b u r g e r Forschung. , 122, 155 (1966). 34. Rossok in B . G . , Smirnov M . V . and Loginov 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 , Baraboshkin N . A . and P a l ' g u e v ( e d i t o r s ) , Consul tant Bureau, N . Y . (1967). 35. Campbell J . , J o u r n a l of M e t a l s , 22, 23 ( V I I , 1970). 36. J o s h i S . , unpubl i shed r e s e a r c h , Department of M e t a l l u r g y , U . B . C . 37. Cameron J . , E t ienne M. and M i t c h e l l A . , 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 1 , 1839 ( V I , 1970). - 179 -38. Panin V.V., Borovskiy O.B. and a l . , Russian Metallurgy and Mining, 52 (I, 1964). 39. Schuhmann R., M e t a l l u r g i c a l Engineering Vol I, Addison-Wesley (1952). 40. Int. 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% Ni Cast Maraging Steel (W.A.K. 7-1-1966). 41. Metals Handbook Vol I, 8th e d i t i o n , Am. Soc. Met., (1967). 42. M i t c h e l l A. and Joshi S., Observations on the e l e c t r i c a l and thermal properties of the slag-skin region i n ESR., Accepted by M e t a l l u r g i c a l Transactions, no. 69-358-C. 43. Grjotheim and Zuca, c i t e d by D i f f u s i o n Data, Vol 2, no. 3-4, 358, (1968). 44. Whitman W.G., Chem. & Met. Eng., 29^ , 147 (1923). 45. Danckwerts P.V., Ind. Eng. Chem.,43, 1460 (1951) and A.I.Ch.E. Journal JL . 456 (1955). 46. Toor H.L. and Marchello J.M. A.I.Ch.E. Journal 4^, 97 (1958). 47. Brebig M.A. i n Molten Salt Chemistry, Blander M. ( e d i t o r ) , Interscience (1964). 48. Sponseller D.L. and F l i n n R.A., Trans. A.I.M.E. 230, 876 (VI, 1964). 49. Hultgren R.R., Selected Values for the Thermodynamic Properties of Metals and A l l o y s , Cal. Min. Res. Lab., U. of C., Berkeley, (1956). 50. Jost W., D i f f u s i o n , Academic Press, N.Y. (1960). 51. Bird R.B., Stewart W.E. and Lightfoot E.N., Transport Phenomena John Wiley, (1965). 52. Klyuev M.M. and Mironov Yu.M. , S t a l i n English, 480 (VI, 1967).. 53. M i t c h e l l A., Proc. Second Int. Symp. on ESR Technolgoy Vol I, Mellon IN s t i t u t e , (IX, 1969). 54. Darken L.S. and Gurry R.W., Physical Chemistry of Metals, McGraw H i l l , (1953). 55. Wanibe Y. and Sano K., c i t e d by D i f f u s i o n Data, Vol I, no. 3, 60 (1967). 56. Steinmetz E., Archiv. fur Eisenhuttenwesen, 36, 421 (VI, 1968). 57. Holzgruber W. and Ploeckinger E., Stahl and Eisen, 88, 638:(1968). - 180 -58. Choudhury A . , K l i n g e l h o f e r H . J . and Wahls ter 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 , 1969) . 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 , 1966). 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 . , Sbornik Ved . P r a c i V . S . B . O . (Ostrava) '11 , 477 ( I I I , 1965). , 62. H l i n e r y J . and Buzek Z . , i b i d . , 1 1 , 505 ( I I I , 1965) . 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 (1967). 64. H l i n e r y J . , Chmelar I . and Buzek Z . , Sborn ik Ved . P r a c i V . S . B . O . (Ostrava) 11 , 471 ( I I I , 1965). 65. Chalmers B . , P r i n c i p l e s of S o l i d i f i c a t i o n , John W i l e y (1964). 66. Holzgruber W. , P r o c . Second 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 ( IX , 1969). 67. L a t a s h Y u . V . , Maksimovich B . I . and Medovar B . I . , Avtom. S v a r k a , n o . 9 , 17 (1960). 68. Madono 0 . , P r o c . Second 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 ( 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 , 1969). 70. Handbook of Mathemat ica l F u n c t i o n s , U . S . N a t . Bureau of S tandards , A p p l i e d Mathematics S e r i e s No. 55 , Wash. ( V I , 1964). 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 (1963). 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. Pfann W . G . , Zone M e l t i n g , J . W i l e y (1966). 74. Glassner A . , The Thermochemical P r o p e r t i e s of the Oxides F l u o r i d e s and C h l o r i d e s to 2500°K. , Argonne N a t . L a b . , U . S . Atomic Energy Commission (1957). 75. C h a r i o t G . , Les methodes de l a ch imie a n a l y t i q u e , Analyse 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., 21 , 698 ( V I , 1949). - 181 -77. V o g e l A . I . , Q u a l i t a t i v e I n o r g a n i c A n a l y s i s , J . W i l e y , (1960). 78. Microprobe A n a l y s i s Genera l I n t e n s i t y C o r r e c t i o n , F o r t r a n 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 - 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 r e m e l t i n 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 . U . B . C . u n i t . - I S 3 t A T U M M M»CTO»\ RAM »0»ITIOM V rtlOIMK U N I T / CMttHtftO^. ELtCTMM %U>*<*1\ CVLINMM X mom CONTACT eucTRooc> RCTUtM BU»j> Figure 3. Commercial u n i t (Consarc Corporation - 185 -cu rH rH 0 ) O CO QJ H •H ft 4-1 JH W a cd o > o C •H u 4-1 OJ rH ai o s CU Pi o o o CM > E 4j .C O 00 LO F i g u r e 5. S c h e m a t i c d r i v e motor c i r c u i t . - 186 -F i g u r e 6. E l e c t r o d e h o l d e r . Copper basep la te F i g u r e 7. Basepla te arrangement. (1 :2 .5 approx imate ly ) - 188 -F i g u r e 9. Fume hood ( 1 : 2 . 5 ) . Rubber b e l l o w s Water cooled stub (copper) F i g u r e 10. Atmospheric s h i e l d (1 :2 .5 a p p r o x . ) . Figure 11. Powder feeder. m - , ^ -i), tr- ,, , . & ' f i g u r e 14. View of the l a b o r a t o r y . E l e c t r o d e h o l d e r Water coo led copper e x t e n s i o n Mold E l e c t r o d e S lag F i g u r e 12. Non consumable e l e c t r o d e VO F i g u r e 13. Sampling of the s l a g . T I H-OQ C f-i Bus bars Electrode leads CO n n> 3 tu rt H-n o fD o c Exchangeable connections DC - AC switch DC bias - 193 -D . C . V o l t s 0 200 400 600 800 1000 D . C . Amperes F i g u r e 16. Hobart Welders . Output C h a r a c t e r i s t i c s . - 194 -7 — / / 2S 2M 2C / I / / / IS IM IC F i g u r e 17. Sampling of i n g o t and s l a g . - 195 -- 196 -- 197 -F i g u r e 20. Ingots S7, S8. E l e c t r o d e n e g a t i v e , CaF„ + CaTiO 198 -(0 d d ro d CM d 0^ r - , F i g u r e 21, Ingots S9, S10, S l l . E l e c t r o d e p o s i t i v e . 0.6 Wt % [ T i ] rnrrrnrr Inclusions removed ' ' I I I I I I I i [ T i ] o tot. 111711 [ T i]o mtx. 0 .4 SI2 total ^ ^ 0 3 matrix (calculated) 0.2 0.1 1_ DC | AC I _L ± 6 « m / V V Y 8 0.6 °z Wt % [Ti]o tot. [Til m n - T T - T T - r - r T T - T y Inclusions removed. 0.4 SI3 [TiJo mtx. 0.3 total 0.2 0.1 -DC } AC I J L wm/w s 8 - 201 -- 202 -a l u m i n i u m added. F i g u r e 26. Ingot S16. E l e c t r o d e n e g a t i v e / A . C . CaF . Argon . - 204 -p iq <fr ro CM d •-• d d d d F i g u r e 27. Ingot S17. E l e c t r o d e n e g a t i v e / A . C . CaF„ + CaTiO . A r g o n . - 205 -F i g u r e 28. Ingots M l , M3, M4. E l e c t r o d e n e g a t i v e . - 206 -wt % cm 0.8 0.6 0.4 0.2 0.0 M2 A [Ti] from slag composition. • [Ti] as analysed (total). o t m u / / i ' A 1 /-/ / — - ~ x w r l 0 2 4 6 8 F i g u r e 29. Ingot M2. E l e c t r o d e n e g a t i v e . CaF, 16 (Ti) in slag 12 10 8 10 12 w / w "nrr w s - 207 -CTi ]o total ( • ) %[Ti] [ T i ] o matrix 0.6 0.5 • 0 total • A 0.4 A fi A A matrix o O A A O 0.3 — 0.2 • O M 5 0.1 A A M 6 — 1 1 1 1 1 1 0 1 2 3 4 . 5 6 wm F i g u r e 30. I n g o t s M5, M6. E l e c t r o d e p o s i t i v e . % [Ti] [Ti]o total • o • Inclusions removed [Ti]o matrix o O o o 0.6 O 0 * matrix O • o 0 5 M7 — 0.4 • total O matrix D C J A C — 0.3 1 _ 1 1 1 i i ! i i 1 8 r- 2Q9 -F i g u r e J 2 . macrograph, i n g o t S 1 7 . F i g u r e 3 3 . Macrograph, i n g o t Slh. - 210 -F igure 3^. Macrograph, ingot M5. F igure 36 . Macrograph, ingot A l - 211 -F i g u r e 3 5 . Macrograph, ingot M7 D . C . e l e c t r o d e p o s i t i v e . A . C . D . C . e l e c t r o d e n e g a t i v e . Figure 37 . P a t t e r n of g r a p h i t e p a r t i c l e s on the s l a g (the e l e c t r o d e i s shaded). F i g u r e 39. Thermocouple l o c a t i o n s on the e l e c t r o d e . - 214 -0 20 4 0 6 0 80 100 Figure 40. Distance mm. - 215 -T 1 1 1 r Figure 4 i . Distance mm. - 217 -- 218 -log p 0 2 - 12 -13 -14 -15 -16 -17 % T i + 0 2 (g) f T i 0 2 T i 2 ° 3 CaTiO, { S S - T i O , F e - T i 0 2 S S - T i 2 0 3 Fe- T i 2 0 3 S S - C a T i O , Fe -CaTiO. J L J L O.OI F i g u r e 44, O.I 1.0 D e o x i d a t i o n e q u i l i b r i a o f Fe and 321 S . S , as a f u n c t i o n o f pO . Wt % [Ti] - 2 1 9 -F i g u r e 46. F l o w on t h e e l e c t r o d e t i p . - 220 -- 2 2 1 -concentrations [ML electode. Oxygen ( O ' l slag, reaction plane. Figure 49. Oxidation of an alloying element in small concentration (electrode positive). 0 10 2 0 3 0 4 0 Q o 5 0 Figure 50. t as a function of e - 222 -g u r e 51 . O x i d a t i o n o f a m a j o r a l l o y i n g e l e m e n t i n t h e e l e c t r o d e f i l m ( e l e c t r o d e p o s i t i v e ) . - 223 -ure 52. Equation ( I V .19). - 224 -321 S.S. - 225 -F i g u r e 54. I n f l u e n c e of the s l a g compos i t ion on the oxygen c o n t e n t . - 226 -- 227 -CX] [ X ] , k-1 D Q S = C X ] 0 + C X 3 0 C ( I - G U Gk - I] e " k ( W s / V V W. F i g u r e A 2 . 1 . Composi t ion p r o f i l e due to melt r a t e d i s c o n t i n u i t y . I atm. CO O o o o o o o o o o o 4 o o 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. G r a p h i t e s u s c e p t o r . 3. G r a p h i t e f e l t i n s u l a t i o n . 4. Quartz tube . F i g u r e A 3 . 1 . E q u i l i b r a t i o n apparatus (1 :1 approx imate ly ) - 229 -- 230 Ratio of activities of titanium ions 1700 C / C O (I atm.) 1800 1900 F i g u r e A 3 . 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 . - 231 -- 232 -ppm CaR L . 6 0 0 . 4 0 0 2 0 0 • I . I • 2 0 3 0 4 0 m V F i g u r e A 4 . 1 . 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 versus s a t u r a t e d ca lomel e l e c t r o d e . Range of c a l i b r a t i o n s . Total Ti % - 233 -0 .2 .4 .6 .8 1.0 F i g u r e A 4 . 2 . C a l i b r a t i o n curves f o r microprobe a n a l y s i s of m a t r i x t i t a n i u m . 

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