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The isothermal decomposition of austenite in the bainite region Morgan, David William 1949

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THE ISOTHERMAL DECOMPOSITION OF AUSTENITE IN THE BAINITE REGION by David William Morgan  A thesis submitted i n p a r t i a l fulfilment of the requirements for the degree of MASTER OF APPLIED SCIENCE i n the department of MINING AND METALLURGY  The University of B r i t i s h Columbia A p r i l , 1949  ABSTRACT  The isothermal decomposition of austenite i n the bainite region has been examined.  The progress  of the transformation i n several hypoeuteetoid and eutectoid steels was investigated metallographically from a qualitative point of view.  A survey: was made  of the information available on the i n i t i a t i o n , course, and end product of the transformation. The factors entering into the transformation were examined separately, their temperature-dependence and interactions investigated.  A theory of the decom-  position of austenite was proposed, and this theory examined i n the l i g h t of the phenomena associated with the isothermal and anisothermal progress of the transformation.  INEEX  I II III  INTRODUCTION. • . .•  1  EXPERIMENTAL METHODS  3  DISCUSSION (a) The I s o t h e r m a l T r a n s f o r m a t i o n o f A u s t e n i t e i n the B a i n i t e Region. 5 (b) The Stear Mechanism o f B a i n i t e Formation. . . 12 (c) L a t t i c e Coherency 12 d) The E f f e c t s o f R e s i d u a l S t r e s s e s 14 e) Behaviour o f t h e Carbon. . . . . . . . . . . 13 ( f ) The E x t e n s i o n o f F e r r i t e Regions Without Shear. 17 (g) The Energy Change i n B a i n i t e Formation . . . 19 (h) N u c l e a t i o m 20 ( i ) Growth * 22 ( j ) The Process o f R e a c t i o n i n t h e Upper Temperature Region 24 (k) The Process o f R e a c t i o n i n the Lower Temperature Region 2j? (1) E f f e c t o f G r a i n S i z e 26 (m) A n i s o t h e r m a l Behaviour . . . . . 27  i  IY SUMMAHr. Y ACKNOWLEDGEMENTS VI BIBLIOGRAPHY  32 36  37  THE ISOTHERMAL DECOMPOSITION OF AUSTENITE IN THE BAINITE REGION  I ~ INTRODUCTION  Although there has been a great deal of i n f o r m a t i o n published on the theory o f the demomposition of a u s t e n i t e i n the b a i n i t e r e g i o n , and many t h e o r i e s have been advanced t o e x p l a i n d i f f e r e n t phenomena a s s o c i a t e d w i t h t h i s  decomposition,  there has been no recent summary o f the a v a i l a b l e knowledge i n this field.  The object of the present i n v e s t i g a t i o n i s t o  b r i n g together and c o r r e l a t e t h e e x i s t i n g data, w i t h a view t o a s s i s t i n g i n p r o v i d i n g a b e t t e r understanding o f the mechanism of the r e a c t i o n . The nature of the transformation o f a u s t e n i t e i n the b a i n i t e r e g i o n renders i t s study p a r t i c u l a r l y d i f f i c u l t . interdependent  Two  mechanisms of transformation are a v a i l a b l e ;  (a) p r e c i p i t a t i o n and growth by d i f f u s i o n , and (b) phase change by • m a r t e n s i t i c shear.  Since both are temperature-dependent.,  the complications of the transformation are reduced i f the process i s c a r r i e d out i s o t h e r m a l l y r a t h e r than during continuous c o o l i n g . As some of the f a c t o r s involved,- such as v a r i a t i o n i n carbon concentration on a micro s c a l e , and  2  d i s t r i b u t i o n of internal stress, may not be d i r e c t l y observed, t h e i r importance must be deduced from the observable phenomena and results associated with the reactions occurring during transformation and other reactions of a similar nature.  This  r e s t r i c t i o n prevents the quantitative evaluation of the rate and course of the transformation from basic principles but, from a knowledge of the reaction behaviour at different tempe- • ratures of transformation, the r e l a t i v e importance of the different factors may be estimated. In t h i s investigation the different reactions have been examined i n d i v i d u a l l y from a thermodynamic and k i n e t i c viewpoint.  Their v a r i a t i o n with temperature has been indicated.  The interaction between the individual reactions has been investigated and a theory proposed, for the decomposition process. This theory has been applied to the experimental data a v a i l a b l e . Where the terms bainite and f e r r i t e are used i n many places interchangeably throughout t h i s report, bainite generally i s taken to refer to the l a b i l e aggregate of carbide plus f e r r i t e (possibly supersaturated with carbom), and f e r r i t e to refer to the body-centered cubic form of i r o n , whether supersaturated with carbon or not, and whether formed by shear or by a diffusive growth.  3  II «. TJiYPgRiMENTAIi METHODS  The s t r u c t u r e s of a number o f l o w - a l l o y hypo:-eutectoid and e u t e c t o l d s t e e l s have been examined m i c r o s c o p i c a l l y a f t e r having been p a r t i a l l y transformed  i s o t h e r m a l l y a t various tempe-  r a t u r e s i n the b a i n i t e r e g i o n . The i n i t i a l stages of isothermal transformation have been i n v e s t i g a t e d i n a s e r i e s o f s t e e l s w i t h 0.55% Carbon, w i t h and without  0.3,5%  Molybdenum and w i t h v a r y i n g N i c k e l content.  The preparation of these a l l o y s has been, described  elsewhere )* 1  A s i m i l a r s e r i e s w i t h higher carbon a n a l y s i s (up t o 0.80 weight percent), was i n v e s t i g a t e d i n the region o f b a i n i t e formation. Various l o w - a l l o y commercial s t e e l s have been examined t o v a r y i n g extents. The specimens were prepared as f l a t d i s c s approximately 0.0.5 inches t h i c k .  A wire was attached t o each t o f a c i l i t a t e  handling during heat treatment. for  15  minutes a t  1600°  The specimens were a u s t e n i t i z e d  P. i n a n e u t r a l s a l t bath, quenched t o ,  and held f o r a measured time a t , the isothermal transformation temperature i n a s a l t bath, and immediately brine-quenched t o room temperature.  The t r e a t e d specimens were ground on emery t o  remove any p o s s i b l e surface e f f e c t s and t o prepare f o r p o l i s h i n g The specimens were p o l i s h e d e l e c t r o l y t i e a l l y using a mixture o f p e r c h l o r i c and a c e t i c a o i d s , 18^ ml. p e r c h l o r i c a o i d , s p e c i f i c g r a v i t y 1 . 6 l gm./e.c, 165 ml. a c e t i c a c i d , 10 ml. water, w i t h some aluminum introduced i n t o the s o l u t i o n ) . 2  The p r i n c i p a l  ©tenant used was 2% n i t al<. c o n t a i n i n g 1% Zephiran C h l o r i d e . * This p r e p a r a t i o n of the specimen t o be examined was found to produce a h i g h l y - d e t a i l e d undisturbed s u r f a c e . S t r u c t u r e s shown i n the accompagnyimg micrographs were obtained by t r e a t i n g commercial SAB 1 0 8 0 s t e e l c o n t a i n i n g 0 . 7 5 % Carbon*  The micrographs were taken using an oil-immersion  o b j e c t i v e of N.A.  1.J52.  I n i n t e r p r e t i n g these micrographs i t should be noted that the m a g n i f i c a t i o n s are s u f f i c i e n t l y l a r g e that the observed s t r u c t u r e i n the f i e l d shown may not be t r u l y r e p r e s e n t a t i v e o f the degree o f transformation throughout the specimen. I n the f o l l o w i n g d i s c u s s i o n , the r e s u l t s of t h i s metallographic i n v e s t i g a t i o n are given together w i t h a survey of the published r e s u l t s of other i n v e s t i g a t i o n s .  Leading references  have been given f o r r e s u l t s drawn from the l i t e r a t u r e .  I n some  oases references have been c i t e d f o r evidence supporting the results of t h i s i n v e s t i g a t i o n .  8 $ aqueous s o l u t i o n , d i s t r i b u t e d by Winthrop Chemical Company, Inc., New York, N.Y.  5  I H - DISCUSSION  (a)  The Isothermal Transformation of A u s t e n i t e i n the B a i n i t e  Region. . I n the e a r l y stages of formation, b a i n i t e oceurs as lamellae nucleated on the g r a i n boundaries^) ( F i g . I ) .  These  lamellae c o n s i s t of f e r r i t e , p o s s i b l y supersaturated with]: carbon at lower temperatures^ ), and carbide p a r t i c l e s , p r e c i p i -  t a t e d , on the f e r r i t e - a u s t e n i t e i n t e r f a c e i n the e a r l y s t a g e s ^ ) . Svidence has been given that the h a b i t plane of the b a i n i t e i n r e l a t i o n to the parent a u s t e n i t e changes w i t h temperature of t r a n s f o r m a t i o n ^ ) , and that the o r i e n t a t i o n of the f e r r i t e i n b a i n i t e i s independent of temperature of formation^), being the same as proeutectoid f e r r i t e .  The cementite i s i n a f i n e s t a t e  of d i s p e r s i o n , X-ray l i n e i n t e n s i t i e s being considered comparable w i t h those of tempered m a r t e n s i t e ^ ) .  The r e s u l t s of magneto-  metric i n v e s t i g a t i o n s have been i n t e r p r e t e d to i n d i c a t e that i n a l l o y s t e e l s , the carbides tend to be c l o s e r to the simpler  Fe^G  composition as the temperature of transformation i s lower!9). During transformation i n the upper temperature regions the lamellae when i n i t i a l l y formed are i r r e g u l a r , o f t e n o c c u r i n g i n groups of lamellae w i t h s i m i l a r o r i e n t a t i o n ( F i g . X ) .  These  lamellae grow as bloeky formations ( F i g . IX) o r , sometimes, i n l e n s shapes which are more concave at higher temperatures (Fig. I I I ) .  N u c l e a t i o n apparently stops soon a f t e r the i n i t i a l  p e r i o d , and the lamellae agglomerate by side-growth.  The  transformation a t high temperatures goes t o completion by the agglomeration  of e x i s t i n g p l a t e s followed by the extension o f  regions so formed, r e s u l t i n g i n an aggregate of f e r r i t e and cementite.  B a i n i t e formed i n the lower temperature range i s  f i n e r i n s t r u c t u r e , l e s s i r r e g u l a r i n c r o s s - s e c t i o n , and more uniform ( T i g . 17). As the temperature o f transformation i s lowered the tendency o f the lamellae o f the same o r i e n t a t i o n t o group together becomes l e s s apparent^) ( F i g . 17). The decompos i t i o n o f the a u s t e n i t e i n the lower range goes t o completion by the formation o f new l a m e l l a e , apparently nucleated by e x i s t i n g plates. A n a l y s i s o f the o v e r - a l l transformation r a t e s 7 ) has shown a progress from three-dimensional towards two-dimensional growth as the temperature of isothermal transformation i s lowered. There i s e v i d e n c e 3 » 8 ) that carbon enrichment o f the untransformed a u s t e n i t e occurs w i t h the formation o f b a i n i t e . This has "been shown t o be thermodynamically l i k e l y ? ) .  The  enrichment o f the a u s t e n i t e by carbon from the b a i n i t e i s oountered by carbon d e p l e t i o n during carbide f o r m a t i o n ^ ) . 1  Figure I SAE 1080, p a r t i a l l y transformed isothermally at 700° F. 2000x.  Electropolished. Etched i n 2% n i t a l with Zephiran Chloride.  The two micrographs above show the appearance of the i n i t i a l high-temperature bainite formation.  The bainite i s i n groups  of irregular similarly-oriented lamBllae. o r i g i n of the bainite may heisbe observed.  The grain-boundary  Figure I I SAE 1080, p a r t i a l l y transformed isothermally at 2000 x .  800° F .  Electropolished. Etched i n 2% n i t a l with Zephiran Chloride.  This micrograph shows an extreme form of high-temperature bainite.  The growth i s acicular, but the rate of agglomeration  of the lamellae by sidewise growth i s rapid, therefore only the advancing edges of the plates i n a group are separate. This specimen was cooled to the isothermal transformation temperature slowly enough to permit the formation of some nodular p e a r l i t e .  Figure I I I SAE 1080, p a r t i a l l y transformed isothermally at 600° F . 2000 x .  Electropolished. Etched im 2% n i t a l with Zephiran Chloride.  These micrographs i l l u s t r a t e the intermediate-temperature bainite growth. early stages.  P a r a l l e l lamellae occur i n groups i n the The lamellae thicken as they grow, often  becoming lens-ehaped as i l l u s t r a t e d .  Few new bainite plates  appear i n the l a t e r stages of growth, the going to completion by agglomeration.  transformation  (a)  <b) Figure 17  SAE 1080, p a r t i a l l y transformed isothermally at (a) 4J?0° F . , and (b) 500° F . 2000 x .  Electropolished. Etched i n 2% n i t a l with Zephiran Chloride. Low-temperature bainite i s finer and more regular than  that formed at higher temperatures. lamellae to occur i n  The tendency for p a r a l l e l  groups i s l e s s , as i s here shown.  New plates are formed throughout the course of the reaction.  11  Figure V SAE 1080, p a r t i a l l y transformed isothermally at 700° F . 2000 x.  Electropolished. Etched i n 2% n i t a l with Zephiran Chioride. The above structure follows a grain boundary.  The  irregular agglomerated structure with associated lamellae i s t y p i c a l of the high-temperature reaction i n the early stages of growth.  12  (b)  The  Shear Mechanism o f B a i n i t e Formation. From c r y s t a l l o g r a p h i c . a n d s t r u c t u r a l c o n s i d e r a t i o n s  b a i n i t e i s generally considered  t o be formed, i n the  stages a t l e a s t , by a l a t t i c e s h e a r i n g p r o c e s s , martensite  formation.  early  comparable t o  Such a mechanism i s t o be expected a t  temperatures where the s e l f - d i f f u s i o n r a t e o f the i r o n i s  low;  the shear p r o c e s s r e q u i r e s o n l y a s m a l l movement o f atoms from the p o s i t i o n s i n the parent phase to the p o s i t i o n s i n the phase, and  hence w i l l take p l a c e e a s i l y  1 1  ).  The  formation  b a i n i t e takes p l a c e as a time-dependent growth p r o c e s s , opposed to m a r t e n s i t e  formation,  new of  as  which, d i s r e g a r d i n g r e l a x a t i o n  e f f e c t s , i s . ; e s s e n t i a l l y independent o f time.  Martensite  for-  mation i n t h i s r e s p e c t , resembles mechanical t w i n n i n g ) .  Since  1 2  b a i n i t e formation  i s a shear-type r e a c t i o n , i t w i l l produce  r e s i d u a l shear s t r e s s e s .  A l s o , s i n c e the s p e c i f i c volume o f  aire  b a i n i t e i s g r e a t e r than t h a t o f a u s t e n i t e , shear s t r e s s e s s e t up by the formation volume.  The  o f b a i n i t e because o f t h i s i n c r e a s e i n  r e l a t i o n - s h i p o f b a i n i t e t o the parent  austenite  i s such t h a t coherency o f the l a t t i c e s a t the i n t e r f a c e s e x i s t i f the b a i n i t e i s compressed and/or the a u s t e n i t e  may stretched  w i t h i n l i m i t s o u t l i n e d i n the subsequent d i s c u s s i o n .  (c)  L a t t i c e Coherency. The  problem o f f o r c e d l a t t i c e coherency has  been  i n v e s t i g a t e d i n the case o f p r e e i p i t a t i o m o f l a m e l l a r s t r u c t u r e s from s o l i d s o l u t i o n ? ) . 1  The  a p p l i e d to b a i n i t e formation  reasoning  and  r e s u l t s may  be  t o y i e l d a rough estimate o f  the  13  maximum s i z e o f b a i n i t e which may be coherent w i t h the parent austenite*  The p l a t e thickness increases u n t i l the s t r a i n  energy i s equal to that required f o r the formation o f a disordered i n t e r f a c e . This thickness i s o f the order o f 100/d atom diameters, where d !  !  i s the percentage m i s f i t between the  two l a t t i c e s on the i n t e r f a c e plane.  I n a d d i t i o n to the assump-  t i o n s used i n reference 13),(namely: that the m a t e r i a l i s i s o t r o p i c ; that a l l the s t r a i n i s taken up i n the p r e c i p i t a t e ; that the e l a s t i c equations o f a continuous medium may be a p p l i e d ; that Hooked Law w i l l hold over the l a r g e s t r a i n s i n v o l v e d ) , we have neglected i n our a p p l i c a t i o n the e f f e c t o f the shear stresses associated w i t h b a i n i t e formation and the e f f e c t , probably not s m a l l , of carbon i n s o l u t i o n i n the b a i n i t e . This estimate i s s u f f i c i e n t to i n d i c a t e , however, that coherency w i l l be probable only i n those regions wherein the a u s t e n i t e s t r a i n i s l e s s r e s t r i c t e d ^ a s a t the g r a i n , boundaries, and near the advancing edge of a b a i n i t e p l a t e .  We may  expect the advancing edge i t s e l f to be coherent w i t h the a u s t e n i t e , since the movement o f an atom from Coherent I n t e r f a c e  i t s p o s i t i o n i n the austenite t o  Disordered Interface  i t s p o s i t i o n i n the b a i n i t e i s small enough (approximately 1/3 o f the interatomic distance) that  Figure VI  14  l o c a l d i s t o r t i o n w i l l take up the d i s c o n t i n u i t y without breaking coherency. A g r a p h i c a l r e p r e s e n t a t i o n of the s t r a i n s produced near the growing edge of a b a i n i t e p l a t e because of l a t t i c e coherency i s given i n F i g u r e VI.  (d) The E f f e c t s of Residual  Stresses.  Assuming a badnite p l a t e to have formed, the shear . stresses thereby produced w i l l oppose a s i m i l a r shear r e a c t i o n w i t h the same orienta^on,, and a s s i s t shear r e a c t i o n s  i n other 14\  s p e c i f i e d complementary d i r e c t i o n s / . Considering Figure V I I , i f the b a i n i t e a r i s e s from a shear i n the d i r e c t i o n of the dotted arrows i t w i l l produce shear  stresses  i n the m a t r i x as shown by f u l l arrows.  the  Since the shear  may  occur i n one d i r e c t i o n o n l y ^ ) , 1  the r e s i d u a l stresses act so  as  to oppose, any b a i n i t e formation Figure VII  of s i m i l a r o r i e n t a t i o n .  however, i n any c r y s t a l of austenite along which the b a i n i t e formation may tJae shear stresses may different direction.  There are,  several planes and take p l a c e , and  directions  therefore  a s s i s t the formation of b a i n i t e along a The  formation of b a i n i t e of such a comple-  15  mentary o r i e n t a t i o n would serve as a means o f r e l a x a t i o n f o r the  s t r e s s e s set up by the f i r s t p l a t e .  A l s o , the s t r e s s e s  produced by a f o r e i g n p a r t i c l e o r phase i n a m a t r i x are s t r o n g e s t near the p a r t i c l e , as may be deduced from the phenomenon o f s t r e s s - r e l i e f by agglomeration i n a l l o y s having a h i g h l y dispersed p r e c i p i t a t e .  The f a c t that p a r a l l e l p l a t e s do not  form so r e a d i l y a t lower temperatures can be accounted f o r by t h i s process of reasoning.  (e)  Behaviour of the Carbon. I t i s considered that d u r i n g the formation o f b a i n i t e  whole groups o f atoms may move simultaneously from the o l d t o the new phase, entrapping the carbon atoms9).  This method o f  growth i s t o be expected, where p o s s i b l e , since the growth r a t e i s f a s t e r and the a c t i v a t i o n energy i s lower than f o r an ordered individual diffusion ^). 1  I t does not, however, exclude the  advancement o f the b a i n i t e by a proaess o f growth wherein the carbon i s not trapped, but d i f f u s e s away o r p r e c i p i t a t e s as carbide. Any carbon atoms entrapped i n b a i n i t e w i l l have a higher r a t e o f d i f f u s i o n i n the b a i n i t e than i n the a u s t e n i t e . Taking a c t i v a t i o n energies o f 18,000 oal/mol f o r carbon i n f e r r i t e ^ ) , and 3>2,000 cal/mol f o r carbon i n a u s t e n i t e ! 7 ) , and 1  the average time between basic a c t s o f d i f f u s i o n as being given approximately by  H/TR t - h N e'  h - P l a n c k s constant H - a c t i v a t i o n energy for a diffusion movement N - Avogadro*s number R - gas constant T - temperature 1  16  we may  o a l c u l a t e that at a temperature of 400° C. a carbon  atom i n f e r r i t e i s l i k e l y t o have about 10,000 times as many ohanges of p o s i t i o n i n any given time as a carbon atom i n austenite.  This unbalance of d i f f u s i o n r a t e s , coupled w i t h  the l a r g e f r e e energy change of carbon between a u s t e n i t e and f e r r i t e w i l l r e s u l t i n a r a p i d increase i n carbon concentration i n the a u s t e n i t e adjacent to a newly-formed block of b a i n i t e . There i s considerable s t r a i n i n the l a t t i e e near the i n t e r f a c e , and i f the b a i n i t e has l o s t coherency w i t h the a u s t e n i t e we  may  expect a high m o b i l i t y of d i f f u s i n g atoms at the i n t e r f a c e * * ) . 1  The heat of r e a c t i o n w i l l a s s i s t a higher l o c a l m o b i l i t y .  Such  c o n d i t i o n s promote r a p i d n u c l e a t i o n of c a r b i d e , and hence w i l l tend to p r e c i p i t a t e the carbide i n a very f i n e form.  The  fine  s i z e of the carbide w i l l r e s u l t i n i t having a lower a l l o y content than the e q u i l i b r i u m c o n d i t i o n s at that temperature r e q u i r e , as has been observed and f u l l y d i s c u s s e d ? ) . 1  carbide o f the simple Fe^C  That the  s t r u c t u r e should p r e c i p i t a t e i n  preference to a more complex a l l o y carbide may a l s o be deduced from c o n s i d e r a t i o n of the r e l a t i v e p r o b a b i l i t i e s of forming  .  c r i t i c a l s i z e n u c l e i of the a l t e r n a t i v e carbides. The problem o f carbide p r e c i p i t a t i o n i s rendered more complex by the i n e q u a l i t i e s of concentration, that i s , i f by d i f f u s i o n from a region of r a p i d l y formed b a i n i t e  p  a high-  carbon r e g i o n i s formed, under the c o n d i t i o n s of r a p i d n u c l e a t i o n o u t l i n e d above most of the n u c l e i w i l l be of c r i t i c a l or neare r i t i c a l s i z e f o r that concentration of carbon.  The concentration  o f carbon i n the region where the carbides are p r e c i p i t a t e d i s  17 reduced by d i f f u s i o n towards the unreacted a u s t e n i t e , which i s of lower carbon content, and by p r e c i p i t a t i o n onto the carbides* Wow, as the carbon concentration i s reduced, the c r i t i c a l s i z e of the carbide nucleus, below which the nucleus i s unstable, increases.  At temperatures o f r a p i d d i f f u s i o n , i t i s qiuite  conceivable that the c r i t i c a l s i z e o f nucleus could increase a t a greater r a t e than the increase in. s i z e by growth o f the carbids p a r t i c l e s present, and so could exceed the s i z e o f many o f these, rendering them unstable.  The unstable carbides would  then r e d i s s o l v e . The r e s u l t would be an increased carbon conc e n t r a t i o n i n the unreacted a u s t e n i t e . The carbide p a r t i c l e s remaining a f t e r the carbon conc e n t r a t i o n has become more uniform continue t o grow i n a normal fashion.  The growth o f carbides i m p l i e s a r e d u c t i o n i n the  carbon concentration o f the surrounding a u s t e n i t e .  This process  acts simultaneously w i t h the carbon-enrichment by b a i n i t e f o r mation.  A t any one temperature o f r e a c t i o n , whether o r not the  unreacted a u s t e n i t e i s enriched o r depleted w i t h regard to carbon w i l l depend upon the r e l a t i v e r a t e s of the b a i n i t e and the carbide r e a c t i o n s . i  One may expect that carbon-enrichment w i l l  occur when the b a i n i t e r e a c t i o n i s more r a p i d , d e p l e t i o n when slower.  (f)  This i s i n d i c a t e d experimentally8,10).  The Extension o f f e r r i t e Regions Without Shear. I f an a u s t e n i t e - f e r r i t e i n t e r f a c e i s considered a f t e r  i t has l o s t coherency, i t may be seen that i n d i v i d u a l i r o n atoms could jump from p o s i t i o n s i n the a u s t e n i t e t o more s t a b l e positions  18  i n the f e r r i t e without i n c r e a s i n g the shear s t r e s s e s associated w i t h the shear mechanism of b a i n i t e formation. Such a method of phase growth has been shown to be dependent upon the d i f f u s i o n r a t e of carbon away from the i n t e r f a c e ) . 9  As a r e s u l t o f the  concentration gradient caused by b a i n i t e formation, d i f f u s i o n may increase the carbon concentration i n the unreacted a u s t e n i t e . The amount of the enrichment w i l l be i n f l u e n c e d by the degree o f carbide p r e c i p i t a t i o n .  Therefore three f a c t o r s w i l l l a r g e l y  c o n t r o l the r a t e of transformation o f a u s t e n i t e to f e r r i t e i n t h i s manner:  the carbon concentration i n the a u s t e n i t e remote  from the i n t e r f a c e , the a c t i v a t i o n energy o f i r o n t r a n s f e r i n g across the i n t e r f a c e , and the d i f f u s i o n r a t e of carbon.  The  carbon concentration i s dependent upon the degree of transform.-*a t i o n , i n c r e a s i n g i n the upper b a i n i t e range, showing l i t t l e change a t lower temperatures.' The temperature-dependence  o f the  rate of growth a t t r i b u t a b l e t o the i r o n and carbon a c t i v a t i o n requirements w i l l be o f the' form ezp(-A/RT), where A i s proport i o n a l to the a c t i v a t i o n energies.  Since the carbon d i f f u s i o n  rate i s the predominant.factor i n such a r e a c t i o n ) , the r a t e o f 9  growth w i l l decrease approximately e x p o n e n t i a l l y w i t h temperature.. Growth i n t h i s manner w i l l f a c i l i t a t e the segregation of any a l l o y i n g elements, those such as n i c k e l and manganese which have lower f r e e energy when d i s s o l v e d i n a u s t e n i t e tending to d i f f u s e away from the boundary so as to stay i n the a u s t e n i t e , and those elements such as chromium which have lower f r e e energy when d i s s o l v e d i n f e r r i t e tending to enter the b a i n i t e .  Such  segregation has been advanced as an explanation f o r the abnormally  19  l o n g times f o r completion o f t r a n s f o r m a t i o n i n the upper bainite region i n certain a l l o y  (g)  steels^).  The Energy Change i n B a i n i t e Formation. A phase change i s p o s s i b l e o n l y i f the f r e e energy i s  decreased by the r e a c t i o n .  C o n s i d e r i n g b a i n i t e formed by shear,  w i t h entrapped carbon, f o u r f a c t o r s determine the f r e e  energy  changer, the f r e e energy change of the i r o n i n g o i n g from the f a c e centered s t r u c t u r e to the body-centered, dG|, ; the d i f f e r e n c e i n Q  f r e e energy between carbon i n a f a c e - c e n t e r e d c u b i c l a t t i c e , carbon i n the body-centered c u b i c l a t t i c e dG^;  and  the change i n  entropy o f the carbon, dS; and the change in. s t r a i n energy a s s o c i a t e d w i t h the t r a n s f o r m a t i o n , dU.  T h i s may  he w r i t t e n a s ) : 9  dG = dGpe • CdGc - CTdS * dU. ' C - carbon c o n c e n t r a t i o n T - temperature There i s no need to c o n s i d e r s u r f a c e energy i f the l a t t i c e s a r e assumed coherent a t the time o f t r a n s f o r m a t i o n . In any t r a n s f o r m a t i o n t h e r e i s a c e r t a i n a c t i v a t i o n energy which c o n t r o l s the r a t e o f r e a c t i o n .  Tor martensitic-  shear r e a c t i o n s w i t h s m a l l movements t h i s i s i n s i g n i f i c a n t , however, and may  be n e g l e c t e d .  Thus assuming t h a t the t r a n s f o r m a t i o n w i l l occur whenever the f r e e energy change i s n e g a t i v e , we w i l l examine the f a c t o r s i n v o l v e d t o account f o r the time-dependence  o f the  bainite reaction. dGj, , dG , Q  c  and dS a r e independent o f time, but C, the  carbon c o n c e n t r a t i o n , and dU, the s t r a i n energy change,  will  20  f l u c t u a t e w i t l i time and w i t h the degree of transformation.  The  carbon concentration w i l l f l u c t u a t e by chance d i f f u s i o n , by the e f f e c t .of nearby b a i n i t e formation, by p r e c i p i t a t i o n of carbides as described, and by v a r i a t i o n s i n i n t e r n a l s t r e s s c o n d i t i o n s . References 1 8 , 2 0 , 2 1 ) .  The i n t e r n a l s t r e s s w i l l be very h i g h  under the i n i t i a l e f f e c t of quenching, and w i l l be r a i s e d by b a i n i t e transformation.  I t w i l l r e l a x at an appreciable r a t e i n  the temperature range o f b a i n i t e formation.  The r e l a x a t i o n r a t e  i s temperature dependent e x p o n e n t i a l l y , of the form exp(-B/T), where B i s dependent upon the amount o f i n t e r n a l s t r e s s . I f we consider a s i n g l e b a i n i t e p l a t e growing edgewise by shear, we may see that the r a t e w i l l be r e s t r i c t e d by the r e l a x a t i o n r a t e of the opposing r e s i d u a l s t r e s s e s . As the temperature of transformation i s lowered the decrease i n f r e e energy by the change of i r o n from the face-centered cubic to the bodycentred cubic form increases r a p i d l y , p a r t i a l l y o f f s e t t i n g the decrease i n the r e l a x a t i o n r a t e of the i n h i b i t i n g s t r e s s e s .  (h)  Nucleation. Metallographic examination has shown that b a i n i t e  tends t o nucleate p r e f e r e n t i a l l y on the g r a i n b o u n d a r i e s ? > ) . 22  This i s supported by the examination of proeutectoid f e r r i t e , which has a c l o s e l y - r e l a t e d mode of formation, and which, because of i t s l e s s e r tendency towards l a m e l l a r growth, shows more c l e a r l y i t s region of n u c l e a t i o n .  There i s evidence that n u c l e a t i o n and  growth of b a i n i t e are g r e a t l y a s s i s t e d by p l a s t i c f l o w ? ) . 2  N u c l e a t i o n o f b a i n i t e i n the g r a i n boundaries i s to be expected.  21  The  c o n t r i b u t i o n to the s t r a i n energy by volume change i s  s m a l l e r a t the g r a i n b o u n d a r i e s ^ ) . 1  Amongst the  disordered  m a t e r i a l there i s a higher p r o b a b i l i t y of f i n d i n g s i t e s u l a r l y s u i t e d to n u c l e a t i o n ^ ) .  Regions having a  2  i n t e r n a l s t r e s s f o r n u c l e a t i o n may  partic-  favourable  be expected because o f  the  c o n s t r a i n i n g e f f e c t o f the g r a i n boundaries, the v a r i a t i o n i n i n t e r n a l s t r e s s b e i n g p a r t i c u l a r l y great d u r i n g the t<ime when quenching s t r e s s e s are o p e r a t i v e .  Conditions  of p l a s t i c  produce both a l a r g e number o f n u c l e a t i o n s i t e s and variation i n internal stress.  flow  a wide  B e t t e r d e f i n e d evidence o f  the  e f f e c t o f s t r e s s c o n d i t i o n s i s found i n the ease o f t r a n s f o r mation below the M martensite adjacent  line -?). 2  g  needles,  Here, b a i n i t e n u c l e a t e s  the t r a n s f o r m a t i o n  t o the m a r t e n s i t e  proceeding  on  the  more q u i c k l y  than i n the untransformed  matrix.  From t h i s evidence i t would be expected t h a t g r a i n s i z e would g r e a t l y a f f e c t the number o f s i t e s o f p o s s i b l e n u c l e a t i o n .  In  g e n e r a l , the p r o b a b i l i t y t h a t a p r e f e r r e d s i t e o f n u c l e a t i o n w i l l transform  i n t o a nucleus i s dependent i n some manner upon  the degree o f t r a n s f o r m a t i o n the b a i n i t e t r a n s f o r m a t i o n factors.  i n the surrounding  material ^).  In  2  the p r o b a b i l i t y i s a f f e c t e d by  three  F i r s t l y , when the s t e e l i s quenched to the t r a n s f o r -  mation temperature the s t r e s s e s produced by the quenching w i l l promote n u c l e a t i o n where they a r e , by chance, so o r i e n t e d as a s s i s t the b a i n i t e shear on any  plane under c o n s i d e r a t i o n .  f a c t o r w i l l be e f f e c t i v e o n l y a t the b e g i n n i n g Secondly, any  of  This  transformation.  i n c r e a s e i n carbon c o n c e n t r a t i o n caused by  b a i n i t e formation w i l l i n h i b i t n u c l e a t i o n .  to  previous  This f a c t o r w i l l  be  22  of greater importance at higher temperatures, and w i l l  be  dependent upon the amount of b a i n i t e and p r e c i p i t a t e d carbide near to the n u c l e a t i o n s i t e .  T h i r d l y , s t r e s s e s set up by the  previous formation of b a i n i t e w i l l i n h i b i t n u c l e a t i o n of b a i n i t e of a s i m i l a r o r i e n t a t i o n , as described, but may a s s i s t n u c l e a t i o n of b a i n i t e of a complementary o r i e n t a t i o n * * ^ ) . 1  2  The e f f e c t of t h i s l a s t f a c t o r w i l l increase w i t h the amount of transformation.  I t w i l l have i t s greatest e f f e c t i n the same  regions, those near to p r e v i o u s l y formed l a m e l l a e , i n which the carbon-enrichment f a c t o r w i l l be strongest, but since s t r e s s r e l a x a t i o n decreases w i t h temperature, the s t r e s s f a c t o r w i l l increase w i t h decreasing temperature. i t may  From these considerations  be seen that the r a t e of formation of n u c l e i w i l l be  very  high i n i t i a l l y , but w i l l decrease r a p i d l y , the r a t e of decrease being l e s s at lower temperatures,  f u r t h e r , the r a t e o f n u c l e a t i o n  w i l l be a f f e c t e d by a change i n the number of s i t e s of p o s s i b l e n u c l e a t i o n , as by a change im the amount of g r a i n boundary material.  D i r e c t observation of n u c l e a t i o n r a t e s of b a i n i t e i s  not p r a c t i c a l , but i t i s to be noted that at higher temperatures the r e a c t i o n tends to go t o completion by agglomeration, whereas at lower temperatures i t progresses by the formation of  new  lamellae throughout the r e a c t i o n p e r i o d .  (i)  Growth. The isothermal transformation of a u s t e n i t e i n the  b a i n i t e region must be considered to take place by t w o c d i f f e r e n t processes,; one m a r t e n s i t i c i n nature, the other d i f f u s i v e .  The  23  former i s s t r o n g l y a f f e c t e d by the r a t e o f s t r e s s r e l a x a t i o n , and both are a f f e c t e d by the behaviour o f the carbon. The m a r t e n s i t i c - t y p e r e a c t i o n i s nucleated on the grain, boundaries.  During growth i t creates high r e s i d u a l s t r e s s e s  which i n h i b i t the growth o f s i m i l a r l y - o r i e n t e d l a m e l l a e , and promote growth along c e r t a i n other planes. growth i s high, favouring continued  The edgewise r a t e o f  extension i n the same plane.  The s t r e s s c o n d i t i o n s may be expected t o be analogous to those set up by martensite formation, and i t may thus be assumed t h a t the r a t e o f transformation through shear i s i n t h i s way c o n t r o l l e d by the r a t e of s t r e s s r e l a x a t i o n . A r e l a t i o n s h i p has been suggested between the i n d u c t i o n period of b a i n i t e and the creep strength o f the austenite27).  The formation o f b a i n i t e causes  r a p i d carbon p r e c i p i t a t i o n and i f the temperature i s i n the upper range r a i s e s the carbon concentration o f the unreacted austenite.  This increase i n carbon i n h i b i t s the transformation?). The d i f f u s i o n r e a c t i o n  may be considered as the a d d i t i o n  of s i n g l e atoms o f i r o n to a nucleus o f f e r r i t e across a d i s r u p t e d interface.  The r a t e o f a r e a c t i o n dependent upon d i f f u s i o n has  been shown.to decrease e x p o n e n t i a l l y w i t h temperature.  A t any  one temperature of r e a c t i o n the r a t e o f transformation o f austen i t e by t h i s process w i l l depend upon the area o f f e r r i t e - a u s t e nite interface.  The f l a t p l a t e s formed by the shear r e a c t i o n  provide a l a r g e i n t e r f a c e area, and so i t i s evident that the amount o f transformation by d i f f u s i o n , increases r a p i d l y as the amount o f sheared product increases.  I  24  (j)  The Process of Reaction i n the Upper Temperature Region, At higher temperatures the r a t e of d i f f u s i o n and the  r a t e of s t r e s s r e l a x a t i o n are g r e a t e r . The change i n f r e e energy of the i r o n i n going from the austenite t o the f e r r i t e i s l e s s , and carbon enrichment o f the unreacted a u s t e n i t e i s more l i k e l y . Afeer the i n i t i a l quenching stresses have relaxed the v a r i a t i o n i n s t r e s s conditions i n the m a t e r i a l w i l l be very l i m i t e d , s i n c e relaxation i s rapid.  For t h i s reason sheared b a i n i t e i s i n i t i a t e d  upon quenching which, since the s t r e s s c o n d i t i o n s promoting nucl e a t i o n w i l l o f t e n p r e v a i l over . d i s t a n c e s of a s i z e  comparable  to g r a i n r a d i i , w i l l tend t o form i n groups of p a r a l l e l lamellae. Any p l a t e w i t h i n such a group w i l l not i n h i b i t independent  growth  of adjacent p l a t e s by reason o f r e s i d u a l stresses,except when very close toggther^because o f the r a p i d r e l a x a t i o n r a t e . As i n d i c a t e d under N u c l e a t i o n , page 20, the number o f n u c l e i formed a f t e r the i n i t i a l stages w i l l be very l i m i t e d .  The p l a t e s w i l l  extend edgewise hy a continued shear a c t i o n and sidewise by a diffusion reaction.  The edgewise growth w i l l stop a t such discon-  t i n u i t i e s as g r a i n boundaries, F i g u r e I . The amount o f a u s t e n i t e transformed by sidewise growth increases as the a u s t e n i t e - f e r r i t e I n t e r f a c e area increases by the shear r e a c t i o n , and decreases as the i n t e r f a c e area i s decreased by the decrease o f unreacted austenite,  fhe r a t e o f growth sidewise decreases as the carbon  content o f the a u s t e n i t e i s increased by r e a c t i o n  The type  of growth curve i n d i c a t e d by t h i s process i s i n accordance w i t h those observed7).  25  (k)  The Process of Reaction i n the Lower Temperature Region. At lower temperatures d i f f u s i o n and r e l a x a t i o n r a t e s  are lower.  The f r e e energy change of the i r o n transformation  i s greater, and l e s s carbon enrichment of the a u s t e n i t e i s likely.  Because of increased f r e e energy change the shear r e -  a c t i o n i s more l i k e l y to occur, and l a r g e r blocks of atoms may be expected to transform at one time, r e s u l t i n g i n g r e a t e r entrapment of carbon*).  The s t r e s s e s set up w i l l be l a r g e r as the  temperature i s lower since the r e l a x a t i o n r a t e i s lower, and there higher s t r e s s e s w i l l i n h i b i t nearby growth of a s i m i l a r orientation, thereby both reducing the tendency to form .groups of s i m i l a r l a m e l l a e , and a s s i s t i n g growth of a complementary nature, which a c t s to r e l i e v e the s t r e s s , r e s u l t i n g i n a c r i s s - c r o s s p a t t e r n . Sidewise growth i s l i m i t e d ; the thermal energy being lower, the d i f f u s i o n r e a c t i o n w i l l be i n h i b i t e d by i t s a c t i v a t i o n i . energy. Sidewise growth by shear i s i n h i b i t e d by the c o n t r a r y r e s i d u a l stress.  The edgewise growth i s such as to tend to keep the  plates f l a t •  The growth r a t e curve w i l l be ofatwo-dimensional  nature, as has been experimentally indicated*?). This theory of growth explains s a t i s f a c t o r i l y the s t r u c tures observed m e t a l l o g r a p h i c a l l y during the isothermal transforma t i o n of a u s t e n i t e :  i n the upper temperature range i r r e g u l a r  coarse b a i n i t e , showing groups of p a r a l l e l lamellae ( F i g s . I , I I I ) tending to t h i c k e n as they grow, transforming only p a r t i a l l y by formation of p l a t e s , and completing transformation by a t h i c k e n i n g , d i f f u s i v e process;  as the temperature of transformation i s  lowered the b a i n i t e becomes more r e g u l a r , f i n e , s i m i l a r l y - o r i e n -  26  -.r.ted lamellae are l e a s l i k e l y t o occur i n groups ( F i g . I T ) , r e a c t i o n proceding t o completion by the continued  formation o f  new p l a t e s .  (1)  E f f e c t o f Grain S i z e . The e f f e c t o f g r a i n - s i z e variance on the behaviour o f  the t r a n s f o r m a t i o n * 8 ) 22  of decomposition.  2  i s i n agreement w i t h the proposed theory  Grain s i z e i n the lower range has no a p p r e c i -  able e f f e c t , in. the upper range has very l i t t l e e f f e c t on the i n i t i a l stages o f transformation but tends t o a c c e l e r a t e the l a t e r stages o f transformation.  The l a m e l l a r s t r u c t u r e o f b a i n -  i t e i s b e t t e r defined when formed i n a u s t e n i t e o f l a r g e g r a i n size *). 2 0  The number o f n u c l e i which grow w i l l be determined by  the s t r e s s conditions.- s e t up by t h e i r growth, not by the number of p o s s i b l e s i t e s o f n u c l e a t i o n .  Therefore, although more s i t e s  may be a c t i v a t e d during the i n i t i a l stage before the quenching stresses have been r e l a x e d , many of these w i l l become i n o p e r a t i v e because o f the growth o f nearby p l a t e s , and the growth r a t e o f a l l p l a t e s w i l l be reduced i f the d e n s i t y o f lamellae i s greater. The o v e r a l l transformation by shear w i l l a l s o be retarded by the c o n s t r a i n i n g a c t i o n o f the g r a i n boundaries ?)• 2  Some more n o t i -  ceable a c c e l e r a t i o n of the transformation occurs towards the £pper temperatures ^). 2  The 1% transformation time i s not a p p r e c i -  ably a l t e r e d , since the products appearing up t o that stage are n e a r l y a l l sheared, but that f o r 99% transformation i s reduced, since the sheared products are spread out over more g r a i n boundary area, and hence give a greater a u s t e n i t e - f e r r i t e i n t e r f a c e area.,  27  a l l o w i n g more r a p i d transformation by the d i f f u s i o n mechanism.  (m) Anisothermal Behaviour, Much u s e f u l information may be gained from a study o f the e f f e c t o f p a r t i a l transformation a t one temperature on the transformation at a d i f f e r e n t temperature level »?°»?!). 22  P a r t i a l transformation to f e r r i t e ( l e s s than 1%) a c c e l e r a t e s beyond additivity?~subsequent b a i n i t e formation {!%).  It is  i n d i c a t e d that holding a f i x e d time i n the f e r r i t e range a c c e l erates formation of b a i n i t e by a percentage which i s independent of the temperature a t which the b a i n i t e f o r m s ? ) . 0  Considering temperatures above and below the nose o f the f e r r i t e C* curve when the f e r r i t e transformation i s d i s t i n c t n  from the b a i n i t e transformation, h o l d i n g a t a higher temperature i n the f e r r i t e r e g i o n has a greater e f f e c t than h o l d i n g a t a lower temperature f o r which the time f o r 1% transformation i s the same.  An explanation of t h i s has been put f o r t h ? ) i n terms  of n u c l e a t i o n .  0  Because, as has been shown, the transformation  to bainite. i s l e s s c o n t r o l l e d by the number o f n u c l e i than by the s t r e s s c o n d i t i o n s , i t i s f e l t that t h i s evidence should be discussed w i t h regard t o carbon behaviour and s t r e s s c o n d i t i o n s * . The proeutectoid f e r r i t e r e a c t i o n may be considered as an extended b a i n i t e r e a c t i o n i n which the carbon migrates away from A r e a c t i o n i s considered a d d i t i v e f o r a given amount o f t r a n s formation i f that amount o f transformation oecurs when the t o t a l f r a c t i o n a l time i n t e g r a t e d over the various temperatures of the r e a c t i o n i s equal t o one, where the f r a c t i o n a l time a t a temperature i s defined as the a c t u a l time a t the temperature d i v i d e d by the time necessary a t that temperature to produce the given amount o f transformation.  X  28  the f e r r i t e into the austenite.  Because of the relationship of  the f e r r i t e to the austenite some stress w i l l arise from volume expansion and possibly from shear formation of f e r r i t e .  Consid-  er .a. fraction of 1% transformation at three temperatures, two i n the f e r r i t e region, at. a higher and lower temperature where the time for 1% transformation i s the same, and one i n the bain- . i t e region.  ThB:, structures w i l l be comparable i n that they  consist of f e r r i t e having a similarly-determined orientation with regard to the parent austenite.  The bainite w i l l have  associated with i t residual stresses, some entrapped carbon and a carbon-enriched region surrounding i t containing precipitated carbides.  The f e r r i t e w i l l have associated residual stresses,  much smaller, and less at the higher temperature, few p r e c i p i tated carbides, and a carbon concentration i n the surrounding austenite much less above that of the remainder of the matrix, since diffusion rates are higher i n the f e r r i t e range.  When the  f e r r i t e i s produced at the upper temperature there w i l l be least localized increase In carbon.  From this i t may be seen that, i f  austenite, p a r t i a l l y transformed to f e r r i t e i s quenched into the bainite range we may consider the f e r r i t e as b a i n i t e , without the high associated stresses and l o c a l increase i n carbon concait r a t i o n , and hence the reaction w i l l go to 1% i n a time less than that required by isothermal transformation at the bainite temperature alone. P a r t i a l bainite formation (less than 1%) accelerates subsequent f e r r i t e formation (lf») but the effect i s less than that corresponding to a d d i t i v i t y ^ ) . This i s explained i n the 0  29 preceding paragraph, since the stresses and carbon enrichment associated with the bainite w i l l reduce the rate of f e r r i t e formation u n t i l the stresses are relaxed and the carbon d i f fused, away at the higher temperature. P a r t i a l bainite reaction at a higher temperature retards subsequent transformation at a lower temperature, and vice versa. . The effect i s more noticeable i n higher-carbon steel? ). 0  This effect i s . s a t i s f a c t o r i l y explained by the be*-.  haviour of the carbon? ). 0  At higher temperatures the austenite  adjacent to a bainite plate i s enriched with carbon and so retards the bainite formation at ,a lower temperature, where such enrichment i s not so great.  Transformation at a lower  temperature leaves less carbon nearby and so accelerates the transformation at a higher temperature.  The differences i n  residual stresses i n the temperature range for which data i s available, are of less importance than the carbon effect,,as  is  evidenced by the larger v a r i a t i o n with steel of higher carbon content. Holding i n the carbide range apparently retards s l i g h t l y the subsequent formation of b a i n i t e ? * ? ) . 0  1  This has  been explained as carbide nucleating at severe discontinuities and so rendering them unavailable for bainite nucleation? ). 0  While t h i s might be a contributing effect i t must be noted also that the precipitaion of carbides w i l l harden the matrix,as ini age hardening, and so make the shear reaction more d i f f i c u l t . A relationship has been suggested between the strength of the austenite and i t s s t a b i l i t y i n the lower transformation range?2).  30  Some data has been advanced30) on the e f f e c t of p a r t i a l transformation on subsequent cementite p r e c i p i t a t i o n . The data, given i n d i c a t e that h o l d i n g i n the b a i n i t e range f o r moderate f r a c t i o n s of the time necessary f o r 1% b a i n i t e to form r e t a r d s the subsequent carbide p r e c i p i t a t i o n , w h i l e h o l d i n g f o r shorter times a c c e l e r a t e s the p r e c i p i t a t i o n .  The l a t t e r e f f e c t  appears more predominant w i t h h o l d i n g at a lower temperature i n the b a i n i t e range.  The r e t a r d i n g e f f e c t of moderate q u a n t i t i e s  of b a i n i t e has been explained3°) by the u t i l i z a t i o n of nuclea t i o n s i t e s by the b a i n i t e , reducing the number a v a i l a b l e f o r cementite n u c l e a t i o n .  To e x p l a i n why cementite was not nucle-  ated on the cementite p a r t i c l e s p r e c i p i t a t e d i n conjunction with the*; b a i n i t e , the p o s s i b i l i t y of d i f f e r i n g o r i e n t a t i o n s of baini t l c and proeutectoid cementite was advanced.  Since there i s no  experimental evidence concerning the o r i e n t a t i o n of the  cementite  t h i s must he considered as a p o s s i b i l i t y because of the d i f f e r e n t modes of formation.  Accepting the d e p l e t i o n o f n u c l e a t i o n s i t e s  by b a i n i t e , the e f f e c t o f the carbides already present may  be  considered i n the l i g h t of the mechanism of growth here advanced. When the b a i n i t e i s formed, the carbon content i s increased i n the adjacent r e g i o n .  Carbides are then p r e c i p i t a t e d at the  i n t e r f a c e , some of which at higher temperatures may  redissolve.  The remaining carbides w i l l grow and cause a carbon d e p l e t i o n of the surrounding a u s t e n i t e , a l l o w i n g the growth of the b a i n i t e near them.  I n t h i s manner the cementite, o r i g i n a l l y on the  a u s t e n i t e - f e r r i t e i n t e r f a c e , w i l l become surrounded by f e r r i t e . This process w i l l occur more r a p i d l y at higher temperatures.  31  The cementite p a r t i c l e s surrounded by f e r r i t e may  be thought  of as being made i n a c t i v e as n u c l e a t i o n s i t e s by the enveloping f e r r i t e .  This provides an explanation of the a c c e l e r a -  t i o n of subsequent cementite by short periods of h o l d i n g i n the b a i n i t e r e g i o n , f o r , although the b a i n i t e w i l l remove some s i t e s of p o s s i b l e n u c l e a t i o n , i t w i l l provide many others i f the time of r e a c t i o n i s not l o n g enough to permit envelopment of the p r e c i p i t a t e d carbides.  This e f f e c t i s expected to be more n o t i -  ceable at lower temperatures where the enveloping growth i s much slower...  I n t h i s regard i t must be noted t h a t the number of  carbides p r e c i p i t a t e d i n the i n i t i a l stages of b a i n i t e formation i s much greater than the number subsequently growing edges of the b a i n i t e .  p r e c i p i t a t e d at the  The a c c e l e r a t i n g e f f e c t of the  quenching s t r e s s e s w i l l allow the e f f e c t of I n i t i a l b a i n i t e forma t i o n by shear to predominate over the e f f e c t of the growing edges of b a i n i t e , since the amount of carbon entrapped (and hence the number of carbides p r e c i p i t a t e d at the interface') w i l l  be  greater when the s t r e s s e s promoting the shear are l a r g e r .  As  the i n t e r n a l s t r e s s becomes: l e s s c o n t r o l l e d by the quench and more by the b a i n i t e formation the f l u c t u a t i o n s i n carbon may  be  expected to a s s i s t i n the r e a c t i o n , that i s , the b a i n i t e w i l l form i n a region when the cafbon concentration i n that r e g i o n f l u c t u a t e s towards lower values.  The formation o f b a i n i t e  under these c o n d i t i o n s w i l l tend l e s s to cause adjaeent  regions  of increased carbon concentration conduiiive to r a p i d oarbide precipitation.  22  17 - SUMMARY  The i n i t i a t i o n and course of the i s o t h e r m a l decomp o s i t i o n of a u s t e n i t e i n the b a i n i t e r e g i o n has been i n v e s t i gated, and a theory proposed to account f o r the observed phenomena associated, w i t h t h i s transformation.. I n the b a i n i t e r e g i o n decomposition may take place by two mechanisms, (a) p r e c i p i t a t i o n and growth by d i f f u s i o n , and (b) phase change by m a r t e n s i t i c shear.  Since the shear  r e a c t i o n sets up r e s i d u a l s t r e s s e s i n the surrounding austen i t e and entraps carbon atoms, the f r e e energy change of a r e g i o n of a u s t e n i t e transforming t o b a i n i t e by shear i s o f the form dG = dGjp f CdGQ e  where  CTdS t dU  ^-^je * * energy change of the i r o n i n going from the f . c c . s t r u c t u r e t o the b.c.c. dG i s the d i f f e r e n c e i n f r e e energy between carbon i n the f . c . c . l a t t i c e and carbon i n the b.c.c. l a t t i c e . C i s the carbon c o n c e n t r a t i o n T i s the temperature dS i s the change i n entropy of the carbon dU i s the s t r a i n energy associated w i t h the transformation s  n e  f r e e  c  The s t r a i n energy dU i s such as to i n h i b i t f u r t h e r transformation by shear along planes of s i m i l a r o r i e n t a t i o n i n the same aust e n i t e g r a i n , and to a s s i s t transformation' by shear along planes of c e r t a i n complementary o r i e n t a t i o n s .  Since the a c t i v a t i o n  energy o f m a r t e n s i t e - l i k e r e a c t i o n s i s n e g l i g i b l e , the extension _o£ A growing edge of a b a i n i t e p l a t e by shear i s assumed to occur  33  whenever the free energy of the adjacent austenite i n the d i r ection of growth may be reduced by the reaction,  At any temp-  erature, dGj, , dGg, and dS are independent of time, but C and Q  dU are functions of the time and the progress of the transformation.  dU I n i t i a l l y varies widely throughout the specimen  because of quenching, i s relaxed at a rate dependent upon the temperature, and i s increased by the shear transformation.  The  progress of the shear reaction i s thereby dependent upon the rate of stress relaxation.  The carbon concentraion w i l l f l u c -  tuate with time by chance diffusion and by variations i n i n t e r nal stress conditions, w i l l be increased by diffusion of carbon entrapped i n bainite and decreased by p r e c i p i t a t i o n of carbon as carbide.  /  l a t t i c e coherency between bainite and the parent austenite i s improbable except near the advancing edge of a bainite plate. After the formation by shear of a region of b a i n i t e , carbon w i l l diffuse out of the bainite into the surrounding austenite, l o c a l l y enriching the austenite and p r e c i p i t a t i n g highly-dispersed carbides at the disordered austenite-bainite interface.  The carbon-enrichment w i l l be greater at higher  temperatures. The decomposition of austenite i n addition takes place through the growth of f e r r i t e regions by diffusion of individual iron atoms from austenite to f e r r i t e .  Such a reaction has an  associated activation energy, and i t s rate of reaction i s r e s t r i c t e d by the rate of diffusion of carbon away from the austenite-  34  f e r r i t e interface.  The rate of transformation of austenite by  t h i s means i s proportional to the area of  austenite-ferrite  interface, and hence i s highly dependent upon the amount of previously-formed sheared product. Bainite i s nucleated on the grain boundaries. quenching stresses promote rapid i n i t i a l nucleation.  The  The shear  reaction sets up i n h i b i t i n g residual stress conditions which r e s t r i c t the growth rate and number of operative n u c l e i . The decomposition «Sf austenite i n the bainite region begins as a shear transformation.  The lamellae formed by shear  grow edgewise by continued shear u n t i l obstructed, as by grain boundaries, and sidewise by a diffusive process which causes them to agglomerate.  The rate of the diffusive process decreases  more rapidly with temperature than does the rate of the shear process..  At higher temperatures the decomposition of austentjebe  i s i n i t i a l l y by shear, but goes to completion by the diffusive reaction.  At lower temperatures the decomposition progresses  by the continued formation of new lamellae throughout the react i o n period.  The internal stresses set up by the shear trans-  formation i n l i b i t the growth of s i m i l a r l y orieh^ited lamellae, which as the temperature i s lowered and hence relaxation rate i s decreased, reduces the tendency of p a r a l l e l plates of bainite to occur i n groups, as i s to be expected from the effect of internal stress on nucleation. A decrease i n grain size accelerates the l a t e r stages of the decompesition of austenite i n the upper bainite region, but has a negligible effect i n the lower regions because the  35  r a t e of growth by shear i s determined by the i n t e r n a l s t r e s s , not by the number of n u c l e i .  ,The l a r g e r area of g r a i n bound-  ary provides a l a r g e r a u s t e n i t e - f e r r i t e i n t e r f a c e area, a c c e l e r a t i n g the decomposition by. the d i f f u s i v e process, which i s predominant  i n the l a t e r stages of decomposition i n the upper  b a i n i t e range. The proposed theory has been a p p l i e d to account f o r the e f f e c t s i n the e a r l y stages of transformation observed w i t h anisothermal transformation procedures.  ?6  V - ACKNOWLEDGEMENTS  The author i s grateful to Mr. F. A. Forward, Head of the Department of Mining and Metallurgy for his consideration and interest, and to Associate Professor W. M. Armstrong for his c r i t i c i s m and generous encouragement during the past year. The author i s indebted to the National Research Council for a research grant during the summer of 1 9 4 8 , and for a bursary during the winter of  1948-49.  37  Yl - BIBLIOGRAPHY  S c o t t , D. A., Armstrong, ¥• 1 , and Forward, F. A.  DeSy, A. L., and Haemers  Smith, G. V., and Mehl, R. F.  A l l e n , N. P., P f e i l , L. B., and G r i f f i t h s , W. T.  Hultgren, A x e l  Greninger, A. B., and Troiano, A. R.  Avrami, M.  "The Influence o f N i c k e l and Molybdenum on Isothermal Transformation o f A u s t e n i t e i n Pure I r o n - N i c k e l and I r o n N i c k e l Molybdenum A l l o y s Containing 0.55% Carbon", A. S. M. Trans., Preprint No. 5 (1948). " E l e c t r o l y t i c Rapid Method o f Etch-Polishing Metallographic Specimens", T r a n s l a t e d from S t a h l und.Eisen, 6 l , 185-. 187 (1941) by Henry Brutcher, T r a n s l a t i o n No. IO98. " L a t t i c e R e l a t i o n s h i p s i n the Decomposition o f A u s t e n i t e t o P e a r l i t e , B a i n i t e and Martens i t e " , A. I . Iff. E. Trans., 150. 211-226 (1942).  "The Determination o f the Transformation C h a r a c t e r i s t i c s o f A l l o y S t e e l s " , Second Report of the A l l o y S t e e l s Research Committee I r o n & S t e e l I n s t i t u t e S p e c i a l Report 24, 569390, (1939). "Isothermal Transformation o f A u s t e n i t e " , A. S. M. Trans., 22., 915-1005, (1947). "Crystallography o f A u s t e n i t e Decomposition", A. I . M. E. Trans. 140. 507-336 (1940). " K i n e t i c s o f Phase Change I I " , J . o f Chem. Phys. 8, (1940).  38  8.  K l e i r , E . P . , and Lyman, T. .  9.  Zener, C.  10.  11.  Lyman, T., and T r o i a n o , A. R.  Benard, J .  "The B a i n i t e R e a c t i o n i n Hypoe u t e c t o i d S t e e l s " , A. I . M. E . Trans. 158. 394-422 (1944). " K i n e t i c s o f t h e Decomposition . o f A u s t e n i t e " , A. I . M. E.,T.P. (1925), Met. Tech., Jan. 1946. "Isothermal T r a n s f o r m a t i o n o f , A u s t e n i t e i n One P e r Cent Carbon, High-Chromium S t e e l s " , A. I . M. E . T r a n s . 162, 187196 (1945). "La S t r u c t u r e C r y s t a l l i n e F a c t e u r Des R e a c t i o n s Dans L * E t a t S o l i d e " , B u l l . Soc. Ghem. de France, Sept.-Oct. 1946, 511-  321.  12.  F o r s t e r , IF., und Scheil, E.  13.  Nabarro, F. R. N.  "The S t r a i n s Produced by P r e c i p i t a t i o n i n A l l o y s " , Proc. Roy. Soc. London, A 175. 519538 (1940).  14.  Orowan, E .  " C l a s s i f i c a t i o n and Nomenclature o f I n t e r n a l S t r e s s e s " , Symp. on I n t e r n a l S t r e s s e s i n M e t a l s and A l l o y s , I n s t i t u t e o f M e t a l s Monograph & Report S e r i e s No. 5, . I n s t , o f M e t a l s , 47-60 (1948).  13.  Boas, W.  "An I n t r o d u c t i o n t o the P h y s i c s o f M e t a l s and A l l o y s " , (1947) - John W i l e y & Sons, I n c . , New York.  16.  Snook, J". L .  P h y s i c a 8, 711 (1941).  17.  W e l l s , C. and Mehl, R.  J . App. Phys. 19_, 217, (1948).  "Untersuchung des z e i t l i c h e n A h l a u f e s von Umklappvorgfingen i n M e t a l l e n " , Z. Metallkunde ^2.-6, 165-173, (1940).  39  18.  Nabarro, F. R. N.  "Diffusion and P r e c i p i t a t i o n i n A l l o y s " , Symposium on Internal Stresses i n Metals and A l l o y s , Institute of Metals Monograph & Report Series No. 5, Inst, of Metals 227-250, (19*8).  19.  Sirota, N. N.  "Effect of Transformation Temperature of Supercooled Austenite on Composition of Separated Carbides", Comptes Rendus (Doklady) de l Acad«3iie des Sciences de l'U. R. S. S. 22.-3, 111-114 (1943). !  20.  Zener, C.  21.  Konobeevsky, S. T., Rovenski, G. M., and Iveronbva, Y. I.  " E l a s t i c i t y and A n e l a s t i c i t y of Metals" (1948), - Univ. of Chicago Press, Chicago.  "The Relaxation of E l a s t i c S t r a i n by »Uphill' D i f f u s i o n i n S o l i d Solution",-J. S c i . Instr. 24, 13,  22.  Hollomon, J . H., J a f f a , L. D., and Norton, M. R.  (1947J.  "Anisothermal Decomposition of Austenite", Met. Tech., Aug. 1946,  T.  P.  2008.  23.  Cot t r a i l , A. H.  "Tensile Properties of Unstable Austenite and I t s Low-Temperature Decomposition Products", J . Iron & Steel Inst. 151-1 1945.  24.  Avrami, M.  "Kinetics of Phase Change I", J . of Chem. Phys. 2, (1939).  25.  Howard, and Cohen, M.  "Austenite Transformation Above and Within the Martensite Range", Met. Tech., Sept. 1947.  26.  Nabarro, F. R. N.  "La'szlgf's Papers on Tessellated Stresses: a Review", Symp. on Internal Stresses i n Metals and A l l o y s , Inst, of Metals Mdnogr. and Report Series No. 5, Inst, of Metals, 61-72 (1948).  4 0  27.  Thompson, F. C , Stanton, L . R.  28.  Davenport, S. S., Grange, R. A., and H a f s t e n , R. J .  "Some O b s e r v a t i o n s on the , Austempering and I s o t h e r m a l Transformation of S t e e l s , w i t h S p e c i a l Reference t o the P r o d u c t i o n o f M a r t e n s i t e " , J . I r o n & S t e e l I n s t . 151-1. 122 (1945).  "Influence of Austenite Grain S i z e upon Isothermal T r a n s f o r m a t i o n Behaviour o f SAE 4140 S t e e l " , A. I . M. 13. Trans. 145. 301-314 (1941). "Uber d i e Umwandlung des Austenits i n Martensit i n EisenNickellegierungen unter B e l a s t u n g " , Z. Anorgan & A l l g e m . Chemi. 2 1 , 2 0 7 , (1922).  2:9.  Schiel,  20.  Jaffe, L.  21.  Lange, H., and Mathieu, K.  22.  and  S.  "Anisothermal Fosrmation o f B a i n i t e and P r o e u t e c t o i d Constituents i n Steels", Met. Tech. Dec. 1 9 4 7 , T. P. 2290.  D.  Thompson, F.  C.  "Uber den A b l a u f .der Austenitumwandlung im u n t e r k u h l t e n Zustand b e i E i s e n - K i c k e l Kbhlenstoff-Legierungen", M i t t . K-Wilh.-Inst. E i s e n f o r s c h g . 20, 125-12* (1928). " I n t e r n a l S t r e s s e s A r i s i n g from Transformations i n M e t a l s and A l l o y s " , Symposium on I n t e r n a l S t r e s s e s i n Metals and A l l o y s , I n s t i t u t e o f M e t a l s Monograph and Report S e r i e s No. 5 , I n s t , o f M e t a l s 227-222 (1948).  

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