Diffusional growth limitation and hardenability
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E A R L Y in his or h e r p r o f e s s i o n a l education, the m e t a l l u r g i s t is i n s t r u c t e d that the " h a r d e n a b i l i t y of s t e e l is the p r o p e r t y that d e t e r m i n e s the depth and d i s t r i b u tion of h a r d n e s s induced by quenching. ''~ U n f o r t u n a t e l y , this definition r e v e a l s nothing f u n d a m e n t a l about the a s s o c i a t e d p h e n o m e n a . T h e r e f o r e , let us c o m m e n c e with a s t a t e m e n t of what a. h a r d e n a b i l i t y m e a s u r e m e n t det e r m i n e s and identify those f u n d a m e n t a l p r o p e r t i e s which u n d e r l i e the h a r d e n a b i l i t y p r o p e r t y . P r i m a r i l y , the depth and d i s t r i b u t i o n of h a r d n e s s is d e t e r m i n e d by the depth and d i s t r i b u t i o n of m a r t e n s i t e . The a b i l i t y to f o r m m a r t e n s i t e is dependent, in t u r n , on the a b i l i t y to s u p p r e s s ( p a r t i a l l y or completely) the f o r m a t i o n of other a u s t e n i t e d e c o m p o s i t i o n p r o d u c t s such as p r o e u tectoid f e r r t t e and c e m e n t i t e , p e a r l i t e , and b a i n i t e . Although the r a t e s of f o r m a t i o n of t h e s e p r o d u c t s a r e i n fluenced s i g n i f i c a n t l y b y cooling r a t e and a u s t e n i t i c g r a i n s i z e , the p r e s e n t p a p e r is c o n c e r n e d only with the influence of a u s t e n i t i c c o m p o s i t i o n ( a s s u m e d to be i n i t i a l l y uniform). F o r a given s t e e l , a useful index of the a b i l i t y to s u p press diffusion-controlled austenite decomposition rea c t i o n s * is the location of the nose (or noses) on the
n e t i c s for such s y s t e m s is expected to be a c o m p l e x and u n r e w a r d i n g task. In the p r e s e n t p a p e r , we t h e r e f o r e c o n s i d e r a s i m p l e a r c h e t y p a l s y s t e m , F e - C - X , where X is a s u b s t i t u t i o n a l alloying e l e m e n t such as Cr, Mn, Mo, Ni, Co, Si, and so forth, and i n q u i r e into the r o l e of X in i n f l u e n c i n g growth k i n e t i c s . Our objective is to s u m m a r i z e , and to s o m e extent s y n t h e s i z e , the v a r ious a t t e m p t s which have b e e n made to e l u c i d a t e this role. We a r e c o n c e r n e d with the d e c o m p o s i t i o n of a u s t e n ite by diffusional p r o c e s s e s and t h e r e f o r e c o m m e n c e with a b r i e f d e s c r i p t i o n of diffusion in s y s t e m s of the f o r m F e - C - X . The i n f l u e n c e s of X on the growth k i n e t ics of p r o e u t e c t o i d f e r r i t e a r e then e x a m i n e d in d e t a i l . A s i m i l a r e x a m i n a t i o n of the p e a r l i t e r e a c t i o n is s u b s e q u e n t l y u n d e r t a k e n . It t r a n s p i r e s that m a n y fundam e n t a l c o n s i d e r a t i o n s a r e c o m m o n to both r e a c t i o n s . Although the b a i n i t e r e a c t i o n is of c o n s i d e r a b l
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