Strain tempering of bainite
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In the p r e s e n t i n v e s t i g a t i o n the s t r a i n t e m p e r i n g of b a i n i t e has b e e n c a r r i e d out with (EN 24) a m e d i u m c a r b o n low alloy s t e e l . The s p e c i m e n s were a u s t e n i t i z e d at 850~ and i s o t h e r m a l l y t r a n s f o r m e d at 300 ~ 360 ~ and 400~ to produce b a i n i t e , s t r a i n e d and t e m p e r e d in the range 100~ to 400~ The b a i n i t e f o r m e d at 360~ has b e e n s p e c i f i c a l l y e x a m i n e d by t h e r m o m a g n e t i c a n a l y s i s to c o n f i r m q u a n t i t a t i v e l y the s o l u t i o n of E c a r b i d e . T h e r m o m a g n e t i c a n a l y s i s has shown that the c a r b i d e phase in b a i n i t e f o r m e d even at 360~ is a m i x t u r e of c c a r b i d e and c e m e n t i t e . It has been found that with l o w e r i n g of b a i n i t e t r a n s f o r m a t i o n t e m p e r a t u r e , the s t r a i n t e m p e r i n g t r e a t m e n t r e s u l t s in h i g h e r s t r e n g t h c o n s i s t e n t with good ductility. The p r e s ent i n v e s t i g a t i o n f a v o r s the c a r b o n d i s l o c a t i o n t r a p p i n g model for the m e c h a n i s m of s t r a i n t e m p e r i n g of b a i n i t e , s i m i l a r to that p r o p o s e d for the s t r a i n t e m p e r i n g o f m a r t e n s i t e .
W H I L E the s t r a i n t e m p e r i n g of m a r t e n s i t e has b e e n studied e x t e n s i v e l y , the s t r a i n t e m p e r i n g of b a i n i t e has not b e e n studied in d e t a i l . S e v e r a l a u t h o r s 1-7 have i n v e s t i g a t e d s t r a i n t e m p e r ing of quenched and t e m p e r e d m a r t e n s i t e to i) d e t e r mine the m e c h a n i c a l p r o p e r t i e s r e s u l t i n g f r o m the t r e a t m e n t and ii) e s t a b l i s h the s t r e n g t h e n i n g m e c h a n i s m s involved. In g e n e r a l , it is e n v i s a g e d that when m a r t e n s i t e i s t e m p e r e d at a low t e m p e r a t u r e , so a s to produce E c a r b i d e and s u b s e q u e n t l y s t r a i n e d and t e m p e r e d , the e c a r b i d e goes into s o l u t i o n in the p l a s t i c a l l y d e f o r m e d r e g i o n s b e c a u s e c a r b o n a t o m s diffuse to d i s l o c a t i o n s , where they a r e in a m o r e stable state. With the f o r m a t i o n of such c a r b o n a t m o s p h e r e s , the d i s l o c a t i o n s a r e locked and b e c o m e i m m o b i l e , thus r a i s i n g the s t r e n g t h of the s t e e l . However, if the s t e e l is t e m p e r e d at a high t e m p e r a t u r e , the c a r b o n a t o m s f r o m the a t m o s p h e r e s diffuse out to f o r m c e m e n t i t e , a s it is a m o r e stable phase than the a t m o s p h e r e and thus the d i s l o c a t i o n s are left unlocked, l e a d i n g to softening of the s t e e l . T h e r e has b e e n v e r y little l i t e r a t u r e c o n c e r n i n g the s t r a i n t e m p e r i n g of b a i n i t e . A r e c e n t c o m p r e h e n
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