Ferrite and bainite in alloy steels
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decade before World War II was one of the m o s t f r u i t f u l in the d e v e l o p m e n t of our u n d e r s t a n d i n g of the heat t r e a t m e n t of s t e e l s . T h i s was in no s m a l l m e a s u r e due to the efforts of a group of A m e r i c a n m e t a l l u r g i s t s both in i n d u s t r y and u n i v e r s i t i e s , who laid the foundations of the m e t a l l u r g y of heat t r e a t m e n t . E d g a r Bain, whose eightieth b i r t h d a y we a r e c e l e b r a t i n g with this s e m i n a r , was an o u t s t a n d i n g figure in this p e r i o d , who by his c l a s s i c a l work on i s o t h e r m a l t r a n s f o r m a tion of s t e e l s p r o v i d e d the f r a m e w o r k on which the p r e s e n t day technology of heat t r e a t m e n t is based. Bain s u c c e s s f u l l y b r i d g e d the gap b e t w e e n a c a d e m i c e n q u i r y and the a p p l i c a t i o n of s c i e n t i f i c p r i n c i p l e s to the d e v e l o p m e n t of s t e e l s in a way which has e a r n e d the e n d u r i n g r e s p e c t of s u c c e e d i n g g e n e r a t i o n s of m e t a l l u r g i s t s in m a n y c o u n t r i e s . With other p i o n e e r s such as M a r t e n s , R o b e r t s - A u s t e n , and Widmanstfitten, his n a m e will always be e n s h r i n e d in the n o m e n c l a t u r e of the subject. The p r e s e n t p a p e r d i s c u s s e s s o m e a s p e c t s of f e r r i t i c and b a i n i t i c m i c r o s t r u c t u r e s in an effort to r e late these to the m e c h a n i c a l p r o p e r t i e s of alloy s t e e l s . We have chosen to deal with both f e r r i t e and b a i n i t e t o g e t h e r b e c a u s e of the difficulty in defining the dem a r c a t i o n b e t w e e n the two c a t e g o r i e s of s t r u c t u r e in p r a c t i c e . The s u b j e c t is a m i c r o c o s m of p h y s i c a l m e t a l l u r g y , in so f a r as it c o v e r s the whole s p e c t r u m of t r a n s f o r m a t i o n and p r e c i p i t a t i o n r e a c t i o n s , i n c l u d ing both d i f f u s i o n - c o n t r o l l e d and s h e a r t r a n s f o r m a tions, n u c l e a t i o n of p r e c i p i t a t e s on i n t e r f a c e s , d i s l o c a tions and g r a i n b o u n d a r i e s , and the c o o p e r a t i v e growth of both p r e c i p i t a t e and m a j o r phase.
and c a r b i d e s , but the c a r b i d e and f e r r i t e c o m p o s i t i o n s on the one hand, and t h e i r m i c r o s t r u c t u r e on the other, change both with the t r a n s f o r m a t i o n and the c o m p o s i tion of the steel. We shall now d i s c u s s each r e a c t i o n p r o d u c t in turn, c o m m e n c i n g at the h i g h e r t r a n s f o r m a tion t e m p e r a t u r e s . 1) P o l y h e d r a l F e r r i t e In p l a i n c a r b o n s t e e l s at high 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 s (750 ~ to 850~ f e r r i t e n u c l e a t e s at the a u s tenite g r a i n b o u n d a r i e s in s e v e r a l m a s s i v e m o
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