A study of the peculiarities of austenite during the formation of bainite
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L e H O U I L L I E R , G. B t ~ G I N , A N D A . D U B l ~
T h e f o r m a t i o n of b a i n i t e i n s t e e l i s g e n e r a l l y a c c o m p a n i e d by a n e n r i c h m e n t i n c a r b o n o f t h e a d j a c e n t a u s t e n i t e w h i c h c a n b e c o m e r e m a r k a b l y s t a b l e a s e v i d e n c e d by i t s v e r y s l o w t r a n s f o r m a t i o n r a t e a n d i t s v e r y l o w Ms p o i n t . T h i s p a p e r p r e s e n t s t h e r e s u l t s of a s t u d y o f t h i s r e s i d u a l a u s t e n i t e in a n S A E - 9 2 6 2 s t e e l . B o t h t h e c a r b o n c o n t e n t a n d t h e a m o u n t o f r e t a i n e d a u s t e n i t e h a v e b e e n d e t e r m i n e d a s a f u n c t i o n of 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 . It h a s b e e n s h o w n t h a t t h e c a r b o n c o n t e n t of the e n r i c h e d a u s t e n i t e p a s s e s t h r o u g h a m a x i m u m o f 1.7 p c t a t a r e a c t i o n t e m p e r a t u r e o f 4 0 0 ° C . H o w e v e r , t h i s r e m a r k a b l y h i g h c a r b o n c o n t e n t f a l l s s h o r t of t h e o n e p r e d i c t e d by the K i n s m a n - A a r o n s o n e x t r a p o l a t i o n o f t h e A3 c u r v e t h u s i n d i c a t i n g that t h e b a i n i t i c t r a n s f o r m a t i o n c a n n o t be c o n s i d e r e d s i m p l y a s a n e x t e n s i o n o f t h e p r o e u t e c t o i d t r a n s f o r m a t i o n . I n v i e w of t h e i n a d e q u a c y of t h e s t a n d a r d t h e r m o d y n a m i c s t h e o r y o f t h e B s t e m p e r a t u r e , a k i n e t i c p o i n t o f v i e w is p r o p o s e d f o r t h e d e f i n i t i o n o f t h i s temperature. T i T is w e l l k n o w n that the f o r m a t i o n of b a i n i t e is a c c o m p a n i e d b y a m a r k e d e n r i c h e m e n t in c a r b o n of the a d j a c e n t a u s t e n i t e . T h e e n r i c h e d a u s t e n i t e is r e m a r k a b l y s t a b l e , a s e v i d e n c e d b y its s l o w t r a n s f o r m a t i o n rate at h i g h t e m p e r a t u r e s a n d its v e r y l o w M s p o i n t ,z-3 A l t h o u g h this p h e n o m e n o n h a s b e e n o b s e r v e d in m a n y m e d i u m a l l o y s t e e l s , it h a s b e e n p a r t i c u l a r y s t u d i e d in s i l i c o n s t e e l s of the c a t e g o r y S A E - 9 2 X X b y H e h e m a n n a n d his c o w o r k e r s~ in the U . S. a n d b y D e l i r y3 a n d P o m e ys in F r a n c e . D e l i r y h a s s h o w n that the c a r b o n c o n t e n t of the e n r i c h e d a u s t e n i t e is a f u n c t i o n of the 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 a n d m a y r e a c h v a l u e s a s h i g h a s 1.8 pct, w h i c h is n e a r l y t w i c e the n o m i n a l c o n c e n t r a t i o n of this e l e m e n t in a s t e e l of the c o m p o s i t i o n S A E - 9 2 1 0 0 . A l t h o u g h m a n y significant o b s e r v a t i o n s have t h u s b e e n g a t h e r e d f r o m t h e s e s t u d i e s , no simultaneous m e a s u r e m e n
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