Schematic transformation diagrams for steel
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I. INTRODUCTION T H E i n t e r p r e t a t i o n of the s t r u c t u r e s of s t e e l s cont a i n i n g diffusional t r a n s f o r m a t i o n p r o d u c t s is f r e quently made in t e r m s of the a p p r o p r i a t e i s o t h e r m a l t r a n s f o r m a t i o n (IT) d i a g r a m a l t e r n a t i v e l y known as the t i m e - t e m p e r a t u r e - t r a n s f o r m a t i o n ( T T T ) d i a g r a m , or if it is a v a i l a b l e , the continuous cooling (CC) diag r a m . It is well known that the conditions for isot h e r m a l t r a n s f o r m a t i o n were defined by D a v e n p o r t and Bain ~ who d e t e r m i n e d the f i r s t " S - c u r v e " and that l a t e r , Cohen 2 modified the curve by adding to it the concept of the M s and M f t e m p e r a t u r e s . Subsequently; I T d i a g r a m s showing the t i m e r e q u i r e d for diffusional t r a n s f o r m a t i o n to c o m m e n c e and to f i n i s h as a function of t e m p e r a t u r e have been obtained for a wide r a n g e of s t e e l s (e.g., Ref. 3). In g e n e r a l , these diag r a m s do not identify the t r a n s f o r m a t i o n p r o d u c t s a l though it is i m p l i c i t that u n d e r a p p r o p r i a t e conditions p e a r l i t e , * upper and lower bainite* and m a r t e n s i t e *
lite and b a i n i t e may f o r m d u r i n g cooling and t h e r e is s o m e s u p p o r t 8'9,1z,1~ for the p o s s i b i l i t y that both p r o ducts may o c c u r in s t e e l s t r a n s f o r m e d at t e m p e r a t u r e s n e a r the " k n e e " or " n o s e " of the I T d i a g r a m . C l e a r l y t h e r e is much confusion c o n c e r n i n g the conditions u n d e r which p e a r l i t e and b a i n i t e may f o r m , and in p a r t i c u l a r it does not s e e m p o s s i b l e to r e c o n c i l e the f r e q u e n t l y o b s e r v e d p r e s e n c e of b a i n i t e a f t e r c o n t i n u ous cooling, n o r the c o e x i s t e n c e of p e a r l i t e and b a i n i t e a f t e r i s o t h e r m a l t r a n s f o r m a t i o n , with e i t h e r the CC or I T d i a g r a m as p r e s e n t l y c o n s t r u c t e d for p l a i n c a r b o n s t e e l s . T h i s p a p e r s e e k s to explain the a n o m a l y by e s t a b l i s h i n g that the u s u a l f o r m of the I T d i a g r a m is in fact an " e n v e l o p e c u r v e " , and to avoid m i s r e p r e s e n t ing the t r a n s f o r m a t i o n b e h a v i o r of a u s t e n i t e , s e p a r a t e C - c u r v e s a r e r e q u i r e d for each of s e v e r a l diffusional p r o d u c t s . The a p p r o a c h will be used to develop s c h e m a t i c v e r s i o n s of each kind of t r a n s f o r m a t i o n d i a g r a m c o n s i s t e n t with the e x p e r i m e n t a l i n f o r m a t i o n for the case of a p l a i n c a r b o n eutectoid s t e e l .
*See Ref.4 for discussionof the valueof this terminology. will be f o r m e d . D e s p i t e wide a v a i l a b i l i t y , I T d i a g r a m s a r e not dir e c t l y r e l e v
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