The transformation to austenite in a fine grained tool steel
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S. F . D I R N F E L D , B. M. KOREVAAR, and F . V A N ' t S P I J K E R The t r a n s f o r m a t i o n to a u s t e n i t e in a fine g r a i n e d tool s t e e l has b e e n i n v e s t i g a t e d q u a n t i t a t i v e l y until the d i s a p p e a r a n c e of the f e r r i t e . The i n i t i a l s t r u c t u r e of the s t e e l c o n s i s t e d of f e r r i t e and g l o b u l a r c a r b i d e s . The n u c l e a t i o n s t a r t s at c a r b i d e s which l i e at the f e r r i t e g r a i n b o u n d a r i e s . T h e k i n e t i c s a r e in good a g r e e m e n t with C a h n ' s t h e o r y of g r a i n b o u n d a r y n u c l e a t e d t r a n s f o r m a t i o n . A c o n s t a n t g r o w t h r a t e was found up to 30 pct t r a n s f o r m a t i o n . Site s a t u r a t i o n o c c u r s e a r l y in the r e a c t i o n ; this was c o n f i r m e d by m e t a l l o g r a p h i c e x a m i n a tion. The r a t e law i s c o n t r o l l e d by g r o w t h and i s i n d e p e n d e n t of the n u c l e a t i o n r a t e . The m e c h a n i s m which i s c o n t r o l l i n g t h i s g r o w t h i s the a d v a n c i n g f e r r i t e - a u s t e n i t e i n t e r f a c e r e a c t i o n . The a l l o y i n g e l e m e n t s i n f l u e n c e the a t o m i c m o b i l i t i e s , i n c r e a s i n g the n e c e s s a r y a c t i v a t i o n e n e r g y to about 110 k c a l / m o l .
PREWOUS
i n v e s t i g a t i o n s of the k i n e t i c s of the t r a n s f o r m a t i o n of f e r r i t e - c a r b i d e a g g r e g a t e s to a u s t e n i t e in the p r e s e n c e of a l l o y i n g e l e m e n t s , a r e not e n t i r e l y s a t i s f a c t o r y with r e g a r d to the r a t e c o n t r o l l i n g m e c h a n i s m of this p r o c e s s . In the e a r l y w o r k of R o b e r t s and M e h l * the g r e a t i n f l u e n c e of the i n i t i a l s t r u c t u r e on the s p e e d of f o r m a t i o n of the a u s t e n i t e was d e m o n s t r a t e d . S e v e r a l a u t h o r s 2-s p r e s e n t t h e i r r e s u l t s of the a u s t e n i t i z i n g p r o c e s s in T . T . T . - d i a g r a m s . F r o m t h e s e it i s e v i d e n t that the d i s s o l u t i o n of the r e s i d u a l c a r b i d e s a f t e r the c o m p l e t e t r a n s f o r m a t i o n of the f e r r i t e i s a v e r y slow p r o c e s s . In o r d e r to d e t e r m i n e which m e c h a n i s m i s r e s p o n s i b l e f o r the f e r r i t e to a u s t e n i t e t r a n s f o r m a t i o n in a l l o y s t e e l s , the t i m e p o w e r n in the s i g m o i d a l r e l a t i o n s h i p ( f = 1 - exp(-Ktn)) and the r e a c t i o n constant K are usually considered. Many a u t h o r s 4-5 have d e t e r m i n e d an a c t i v a t i o n ene r g y f r o m the e x p e r i m e n t a l k i n e t i c d a t a , u s i n g the s l o p e of ln(1/t) vs 1 / T p l o t s , although the r e a c t i o n e x p o n e n t s n v a r y with the 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 (non i s o k i n e t
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