Influence of al, co, and si upon the kinetics of the proeutectoid ferrite reaction
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AND
H. I. AARONSON
The e f f e c t s of c a . 3 at. pct of A1, Si, or Co upon the k i n e t i c s of g r a i n boundary f e r r i t e a l l o t r i o m o r p h f o r m a t i o n (and thus upon h a r d e n a b i l i t y ) r e l a t i v e to t h o se in F e - C a l l o y s of c o m p a r a b l e c a r b o n content w e r e e v a l u a t e d . All t h r e e a l l o y i n g e l e m e n t s d i s p l a c e the T T T c u r v e for the i n i t i a t i o n of t r a n s f o r m a t i o n to s h o r t e r t i m e s at the higher r e a c t i o n t e m p e r a t u r e s . Both a l u m i n u m and s i l i c o n i n c r e a s e the p a r a b o l i c r a t e constant for a l l o t r i o m o r p h thickening, ~, r e l a t i v e to that in t h e i r c o u n t e r p a r t F e - C a l l o y s ; the i n f l u en ce of cobalt upon a , if any, is a p p r e c i a b l y l e s s . In the F e - C - A 1 and F e - C - S i a l l o y s , thickening p r o c e e d s n o t i c e a b l y l e s s r a p i d l y than v o l u m e diffusion c o n t r o l (as a s s e s s e d by A t k i n s o n ' s a n a l y s i s of the g r o w t h of an oblate e l l i p s o i d ) a l l o w s . In the F e - C - C o and F e - C a l l o y s , the a v e r a g e c a l c u l a t e d and e x p e r i m e n t a l a ' s a r e in b e t t e r a g r e e m e n t but, e v i d e n t l y a s a r e sult of the p r e s e n c e of d i s l o c a t i o n f a c e t s at a b r o a d face of a l l o t r i o m o r p h s , s o m e a l l o t r i o m o r p h s a c t u a l l y t h i c k e n e d m o r e r a p i d l y than c a l c u l a t e d . The s u b s t a n t i a l s c a t t e r in a in all a l l o y s w a s a l s o ~ t t r i b u t e d to t h e s e f a c e t s . I n d i r e c t d e t e r m i n a t i o n s i n d i c a t e d that all t h r e e e l e m e n t s i n c r e a s e d the r a t e of n u c l e a t i o n of f e r r i t e a l l o t r i o m o r p h s .
C O B A L T has long been c o n s i d e r e d to e x e r t a unique and a n o m a l o u s effect upon the k i n e t i c s of the d e c o m p o sition of a u s t e n i t e . E a r l y w o r k d e m o n s t r a t e d that cobalt d e c r e a s e s the h a r d e n a b i l i t y of s t e e l . ~-3 D a v e n p o r t 4 o b s e r v e d that cobalt d i s p l a c e s the T T T c u r v e s for both the beginning and the ending of t r a n s f o r m a t i o n in 0.95 pct C s t e e l s to s h o r t e r t i m e s . U s i n g the s a m e s t e e l s , Hawkes and Mehl 5 found that cobalt i n c r e a s e s both the r a t e of n u c l e a t i o n and the r a t e of g r o w t h of p e a r l i t e . Z e n e r 6 f o r e c a s t the l a t t e r r e s u l t s by p r e d i c t i n g that c o balt should i n c r e a s e the f r e e e n e r g y change a s s o c i a t e d with a u s t e n i t e d e c o m p o s i t i o n . T h i s p r e d i c t i o n was conf i r m e d c a l o r i m e t r i c a l l y for the p e a r l i t e - t o - a u s t e n i t e t r a n s f o r m a t i o n by Hagel, Pound, and Mehl. 7 Although m e a s u r e m e n t s of n u c l e a t i o n and growth
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