The kinetics of carbon clustering in martensite
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DeCRISTOFARO,
ROY
KAPLOW,
AND
WALTER
S. OWEN
The r a t e of c l u s t e r i n g of c a r b o n a t o m s into r e g i o n s of o r d e r e d Fe4C has b e e n studied by M S s s b a u e r s p e c t r o s c o p y . The a c t i v a t i o n e n e r g y a s s o c i a t e d with this p r o c e s s is 89,500 12,000 J p e r mole (21.4 k c a l p e r mole). 1. INTRODUCTION
THE
s e g r e g a t i o n and r e d i s t r i b u t i o n of c a r b o n a t o m s in m a r t e n s i t i c s t e e l s p r i o r to p u r p o s e f u l t r e a t m e n t at e l e v a t e d t e m p e r a t u r e is a well r e c o g n i z e d p h e n o m e non. 1 " A u t o t e m p e r i n g " is an e x a m p l e of such a p r o c e s s o c c u r r i n g in l o w - c a r b o n l o w - a l l o y s t e e l s with high M s t e m p e r a t u r e s . D u r i n g the quench, c a r bon a t o m s i n the f r e s h l y f o r m e d m a r t e n s i t e m i g r a t e to s i t e s at lattice defects, such as d i s l o c a t i o n s , s i n c e the i n t e r s t i t i a l atom m o b i l i t y is high and the s t r e s s fields a s s o c i a t e d with the defects p r o v i d e e n v i r o n m e n t s of r e l a t i v e l y low e n e r g y . T h i s type of p r o c e s s also m a y o c c u r in s t e e l s with s u b a m b i e n t M s t e m p e r a t u r e s when the e x i s t i n g m a r t e n s i t i c phase is allowed to age at r o o m t e m p e r a t u r e , s i n c e the jump f r e q u e n c y of c a r b o n is s t i l l r a p i d enough to p e r m i t local r e a r r a n g e ment. 2 M e a s u r e m e n t s of the r e s i s t i v i t y of m a r t e n s ire as a f u n c t i o n of c a r b o n content s u g g e s t that the p r e f e r r e d defect s i t e s b e c o m e s a t u r a t e d with c a r b o n a t o m s when the c a r b o n content e x c e e d s 0.2 wt pct. The s u b s t r u c t u r e of h i g h - c a r b o n m a r t e n s i t e , which f o r m s in s t e e l s with m o r e than about 0.4 wt pct c a r bon, c o n s i s t s of fine twins and a d i s l o c a t i o n d e n s i t y s i g n i f i c a n t l y s m a l l e r than that of the l o w - c a r b o n , lath m a r t e n s i t e . E s t i m a t e s of the d e n s i t y of d e f e c t - r e lated s i t e s in h i g h - c a r b o n m a r t e n s i t e indicate that c o n s i d e r a b l y l e s s than 0.2 wt pct c a r b o n should be sufficient to s a t u r a t e t h e m . However, the aging b e havior of the r e s i s t i v i t y of h i g h - c a r b o n s t e e l s in the v i c i n i t y of r o o m t e m p e r a t u r e r e v e a l s that a d d i t i o n a l changes o c c u r . A c t i v a t i o n e n e r g i e s d e r i v e d f r o m those m e a s u r e m e n t s v a r y b e t w e e n 75,300 J (18 kcal) and 100,400 J (24 kcal) p e r mole. 3'4 The value b e t w e e n t h e s e e x t r e m e s - 8 7 , 8 5 0 J p e r m o l e - i s c o n s i s t e n t with the p r e d i c t e d a c t i v a t i o n e n e r g y for the diffusion of c a r b o n in m a r t e n s i t e of the r e l e v a n t c a r b o n c o n
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