On the precipitation of epsilon-carbide in lower bainite
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The t r a n s f o r m a t i o n m e c h a n i s m s of b a i n i t e f r o m m e t a s t a b l e a u s t e n i t e has b e e n a topic of d e b a t e for m a n y y e a r s . The p r o b l e m is c o m p l i c a t e d b e c a u s e the b a i n i t e t r a n s f o r m a t i o n e x h i b i t s f e a t u r e s c o m m o n to both diffusion c o n t r o l l e d and m a r t e n s i t i c t r a n s f o r m a t i o n s J It i s known that t h e r e a r e two m a j o r f o r m s of b a i n i t e ; n a m e l y , u p p e r and l o w e r b a i n i t e . R e c e n t t r a n s m i s s i o n e l e c t r o n m i c r o s c o p e s t u d i e s a's have shown that t h e r e i s a d i s t i n c t m o r p h o l o g i c a l d i f f e r e n c e b e t w e e n t h e s e two f o r m s of b a i n i t e . In u p p e r b a i n i t e , c e m e n t i t e p r e c i p i t a t e s b e t w e e n f e r r i t e l a t h s , while in lower bainite, cementite precipitates unidirectionally within the b a i n i t i c f e r r i t e l a t h at an a n g l e of about 55 to 65 d e g to the long d i r e c t i o n of the lath. The p r e c i p i t a t i o n of ~ c a r b i d e in l o w e r b a i n i t e h a s a l s o been o b s e r v e d , 4 although the p r e c i p i t a t i o n p r o c e s s i s not c l e a r l y u n d e r s t o o d . In a r e c e n t b a i n i t e d e b a t e , ~ H e h e m a n n p r o p o s e d the e x i s t e n c e of the m e t a s t a b l e e u t e c told r e a c t i o n ? ~ a + e c a r b i d e a t 350~ He a r g u e d that below 350~ c c a r b i d e p r e c i p i t a t e s v e r y r a p i d l y from supersaturated ferrite, since a-E carbide is s t a b l e r e l a t i v e to ~ - 7 . This t e r m i n a t e s the p a r t i t i o n of c a r b o n to a u s t e n i t e and r e s u l t s in a l o w e r b a i n i t e s t r u c t u r e . In n o n s i l i c o n s t e e l s , c e m e n t i t e s w i l l r a p i d l y r e p l a c e ~ c a r b i d e s a n a l o g o u s to t e m p e r i n g of m a r t e n s i t e . Above 350~ the ~ - ~ c a r b i d e a g g r e g a t e is m e t a s t a b l e r e l a t i v e to an ~ - 7 a g g r e g a t e . The u p p e r b a i n i t e s f o r m e d in n o n s i l i c o n s t e e l s would contain c e m e n t i t e s which could p r e c i p i t a t e f r o m both f e r r i t e and a u s t e n i t e . In s i l i c o n s t e e l s , h o w e v e r , the p r e c i p i t a t i o n of c e m e n t i t e is r e t a r d e d due to the p r e s e n c e of s i l i c o n , the r e s u l t i n g u p p e r b a i n i t e s would b e f r e e of c a r b i d e s . On the o t h e r hand, K i n s m a n and A a r o n s o n 5 b a s e d on the m e a s u r e m e n t s of b a i n i t e growth k i n e t i c s and of the c a r b o n c o n t e n t of 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 , c o n c l u d e d that b a i n i t i c c a r b i d e is m o r e l i k e l y to p r e cipitate from austenite: ferrite boundaries. One p o s s i b l e a p p r o a c h to the a n s w e r of the q u e s t i o n a s to the s o u r c e of b a i n i t i c c a r b i
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