The formation of austenite in a low-alloy steel
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D. V. EDMONDS
A u s t e n i t e f o r m a t i o n f r o m different m i c r o s t r u c t u r a l c o n d i t i o n s has been s t u d i e d in an F e - I V - 0 . 2 C s t e e l . The s t a r t i n g m i c r o s t r u c t u r e s of f e r r i t e , b a i n i t e , and m a r t e n s i t e , and the morphology of a u s t e n i t e f o r m a t i o n have b e e n e x a m i n e d in d e tail by light m i c r o s c o p y 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 . R e t a i n e d a u s t e nite in quenched m a r t e n s i t i c a r e a s has b e e n used to e s t a b l i s h the c r y s t a l l o g r a p h i c r e l a t i o n s h i p s d u r i n g a u s t e n i t e n u c l e a t i o n , and to i n d i c a t e the b e h a v i o r of the i n i t i a l v a n a d i u m c a r b i d e d i s p e r s i o n p r e s e n t in the f e r r i t e s t a r t i n g m i c r o s t r u c t u r e . L i m i t e d m e a s u r e m e n t s w e r e also made of the k i n e t i c s of a u s t e n i t i z a t i o n . 1. INTRODUCTION F O R M A T I O N of the high t e m p e r a t u r e a u s t e n i t i c phase is an inevitable o c c u r r e n c e d u r i n g the heat t r e a t m e n t of the v a s t m a j o r i t y of c o m m e r c i a l s t e e l s . D e s p i t e this c o n s i d e r a t i o n , t h e r e has been little work c a r r i e d out on the f o r m a t i o n of a u s t e n i t e as c o m p a r e d with the v a s t a m o u n t of r e s e a r c h on its d e c o m p o s i t i o n . A u s t e n i t i z i n g is n e c e s s a r y , often m o r e than once, b e fore t r a n s f o r n m t i o n to the f i n a l f e r r i t i c , p e a r l i t i c , b a i n i t i c , or n m r t e n s i t i c s t r u c t u r e s . Although d e s t r o y e d by r e a c t i o n to these p h a s e s , the i n i t i a l a u s t e n i t i c c o n dition, p a r t i c u l a r l y the g r a i n size, is i m p o r t a n t to the d e v e l o p m e n t of the f i n a l m i c r o s t r u c t u r e and its m e c h a n i c a l p r o p e r t i e s . T h i s has b e e n r e c o g n i z e d for s o m e t i m e , and p a r t i c u l a r l y by i n v e s t i g a t o r s c o n c e r n e d with a b n o r m a l g r a i n growth ~,2 and the r e f i n e m e n t of f i n a l g r a i n sizes.S The r e c e n t d e v e l o p m e n t of dual phase f e r r i t e / m a r t e n s i t e s t r u c t u r e s by p a r t i a l r e a u s t e n i t i zation, which has led to i m p r o v e m e n t s in the f o r m a b i l i t y of low c a r b o n s t e e l sheet, 4,s has a l s o r e v i v e d i n t e r e s t in the heating p a r t of the heat t r e a t m e n t c y c l e . E a r l y work on a u s t e n i t i z a t i o n p r i o r to 1940 was s u m m a r i z e d in a p a p e r by R o b e r t s and Mehl ~ which a l s o r e p o r t e d a study of a u s t e n i t e f o r m a t i o n f r o m f e r r i t e / p e a r l i t e and f e r r i t e / s p h e r o i d i t e a g g r e g a t e s e s t a b l i s h i n g the n u c l e a t i o n and growth c h a r a c t e r of the t r a n s f o r
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