The inverse bainite reaction in hypereutectoid Fe-C alloys
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Optical microscopy studies have shown that bainite t h u s d e f i n e d (but p r e s e r v i n g the o r i g i n a l r e s t r i c t i o n that the f e r r i t i c c o m p o n e n t m u s t a p p e a r in a W i d m a n statten morphology) forms throughout a considerable t e m p e r a t u r e - c o m p o s i t i o n r e g i o n a b o v e the To t e m p e r a t u r e , at which a u s t e n i t e a n d s t r e s s - f r e e f e r r i t e of the s a m e c o m p o s i t i o n a r e in m e t a s t a b l e e q u i l i b r i u m , 2 in h y p e r e u t e c t o i d F e - C a l l o y s , b3 F i g . 1.* *The major uncertainty in the calculationof To presently appears to be the numerical value of A-/ffC,the partml molarheat of solutmn of carbon in ferrlte The two versmnsof the To-carbon content curveplotted m Fig. 1 were computed using the highest and the lowestvalue of this quantity now available?,a'7 The equation for T0 employed1,3 is based upon the statistical thermodynamicanalysis of Lacher8 and Fowler and Guggenheim? However,severalother formulations have been shown to yield qmte sirmlarresults? Within t h i s r e g i o n , it is t h e r m o d y n a m i c a l l y i m p o s s i b l e f o r the 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 to i n h e r i t t h e f u l l c a r b o n c o n t e n t of the ( c o m p o s i t i o n a l l y u n d i s t u r b e d ) a u s t e n i t e m a t r i x . S i n c e the a p p e a r a n c e of t h i s b a i n i t e , a s s e e n in the o p t i c a l m i c r o s c o p e , a p p e a r s to b e e s s e n t i a l l y the s a m e a s t h a t of the b a i n i t e f o r m e d in h y p o e u t e c t o i d F e - C a l l o y s , the c o n c l u s i o n h a s b e e n d r a w n t h a t the i n i t i a l c a r b o n c o n t e n t of the f e r r i t i c c o m p o n e n t of h y p o e u t e c t o i d b a i n i t e i s u n l i k e l y to b e the s a m e a s t h a t of i t s a u s t e n i t e m a t r i x e v e n though t h i s i s t h e r m o d y n a m i c a l l y p e r m i s s i b l e a t m o s t t e m p e r a t u r e s . 1,s The p r e s e n t s t u d y w a s u n d e r t a k e n p r i m a r i l y to r e e x a m i n e the b a i n i t e s t r u c t u r e s f o r m e d a b o v e To in h y p e r e u t e c t o i d F e - C a l l o y s with r e p l i c a tion and 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 y . T h i s h a s b e e n done in o r d e r to a s c e r t a i n w h e t h e r or not the f o r m a t i o n of " i n v e r s e b a i n i t e " , 1 0 a t r a n s f o r m a t i o n p r o d u c t found in h y p e r e u t e c t o i d a l l o y s but not u s u a l l y r e s o l v a b l e with o p t i c a l m i c r o s c o p y , r e q u i r e s any a l t e r a t i o n in t h e s e d e d u c t i o n s . T h e b a s i c unit of i n v e r s e bainite is a proeutectoid cementite "spine" surr o u n d e d b y a s h e a t h of f e r r i t e . 1~ S i n c e c e m e n t i t e is the n u c l e a t i n g p h a s e of i n v e r s e b a i n i t e , ~~ if the b a i n i t e f o r m e d a b o v e the To t e m p e r a
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