The bainite transformation in a silicon steel
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bainite transformation r e m a i n s the l e a s t u n d e r s t o o d of a l l the d e c o m p o s i t i o n r e a c t i o n s of the h i g h t e m p e r a t u r e a u s t e n i t i c p h a s e in s t e e l s . T h e c o m p l e x i t i e s of i t s f o r m a t i o n m e c h a n i s m a n d k i n e t i c s , a n d t h e a p p a r e n t d i v e r s i t y in i t s m i c r o s t r u c t u r a l a p p e a r a n c e , e v e n c r e a t e d i s a g r e e m e n t in i d e n t i f y i n g i t s c o r r e c t d e f i n i t i o n . 1,2 H o w e v e r , the w e l l k n o w n d i f f e r e n c e i n c a r b i d e distribution between bainite f o r m e d at high a n d low temperatures, v i z . , intralath and interlath, r e s p e c t i v e l y , a p p e a r s t o e x i s t i n a m a j o r i t y of s t e e l s a n d m a k e s t h e c l a s s i c a l n o m e n c l a t u r e of u p p e r a n d l o w e r b a i n i t e u s e f u l , b o t h in d e s c r i b i n g t h e m i c r o s t r u c t u r a l a p p e a r a n c e a n d i n c l a s s i f y i n g the o v e r a l l r e a c t i o n m e c h a n i s m . 3"° I n t h i s r e s p e c t , t h e s e t e r m s a r e u s e d in the present w o r k . S i n c e b a i n i t e f o r m s in t h e t e m p e r a t u r e r a n g e d i v i d i n g the r e c o n s t r u c t i v e f e r r i t e / p e a r l i t e r e a c t i o n s a n d t h e displacive martensitic reaction, t h e r e has been a n a t u r a l d e s i r e t o r e l a t e b a i n i t e t o o n e o r o t h e r of t h e s e phases, w h i c h has led t o much controversial d e b a t e c o n c e r n i n g t h e m e c h a n i s m of i t s f o r m a t i o n . 1,1°-14 O n e s c h o o l of t h o u g h t c o n s i d e r s t h a t t h e f e r r i t i c c o m p o n e n t of b a i n i t e d e v e l o p s o v e r t h e w h o l e b a i n i t i c t e m p e r a t u r e r a n g e by a d i f f u s i o n a l l e d g e m e c h a n i s m a n a l o gous to t h e p r o p o s a l s made t o a c c o u n t f o r t h e f o r m a t i o n of W i d m a n s t a t t e n p r o - e u t e c t o i d f e r r i t e . 1 T h e c a r b o n c o n t e n t o f t h i s f e r r i t e i s c o n s i d e r e d t o be b e t w e e n t h e ~/oz + F e 3 C a n d t h e e x t r a p o l a t e d ~ / ~ + 7 p h a s e b o u n d a r i e s , a n d t h e b a i n i t i c c a r b i d e s a r e c o n s i d e r e d to f o r m p r i m a r i l y on t h e a u s t e n i t i c side of t h e a u s t e n i f e / f e r rite interface.1 R e c e n t detailed electron diffraction s t u d i e s of t h e c a r b i d e p r e c i p i t a t i o n r e a c t i o n s h a v e b e e n i n t e r p r e t e d to g i v e s u p p o r t t o t h i s h y p o t h e s i s , is A s o l u t e d r a g m o d e l i s i n v o k e d t o e x p l a i n the b a y i n the T T T c u r v e a t t h e B s t e m p e r a t u r e . 1~ T h e i n c o m p l e t e H. K. D. H. BHADESHIA and D. V. EDMONDS are Research Student and Royal Society Warren Research Fellow, respectively, Department of Metallurgy and Materials Science, University of Cambrid
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