Spheroidization of binary Fe-C alloys over a range of temperatures
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Temperatures K. M. VEDULA
AND
R. W. HECKEL
The s p h e r o i d i z a t i o n of c e m e n t i t e in b i n a r y F e - C a l l o y s , 0.24, 0.42, and 0.79 wt pct C , was i n v e s t i g a t e d o v e r a range of t e m p e r a t u r e s , 594 ~ 649 ~ and 704~ f o r t i m e s up to about 106 s e c . Quantitative m e t a l l o g r a p h y t e c h n i q u e s w e r e u s e d to obtain the following m i c r o s t r u c t u r a l d a t a on the c e m e n t i t e p a r t i c l e s : s h a p e , s i z e d i s t r i b u t i o n , m e a n s i z e , n u m b e r of p a r t i c l e s p e r unit v o l u m e , and growth (and s h r i n k a g e ) r a t e s of v a r i o u s s i z e s in the s i z e d i s t r i b u t i o n . The v a r i a tions of t h e s e m i c r o s t r u c t u r a l p a r a m e t e r s w e r e a n a l y z e d in t e r m s of e x i s t i n g m o d e l s for the s p h e r o i d i z a t i o n p r o c e s s . The L i f s h i t z - W a g n e r a n a l y s i s i s shown to have l i m i t e d a p p l i c a b i l i t y to the s p h e r o i d i z a t i o n of c e m e n t i t e in b i n a r y s t e e l s , since the r e q u i r e d s t e a d y - s t a t e s i z e d i s t r i b u t i o n i s not a t t a i n e d in t i m e s l e s s than about 106 s e c . An a n a l y s i s s i m i l a r to that of L i f s h i t z and W a g n e r , but r e q u i r i n g no s p e c i f i c a t i o n of the shape of the s i z e d i s t r i b u t i o n , i s shown to a p p l y and i n d i c a t e s that the o b s e r v e d s p h e r o i d i z a t i o n was d i f f u s i o n - c o n t r o l l e d . The effective diffusion c o e f f i c i e n t was between the v a l u e s f o r the diffusion of c a r b o n and i r o n in f e r r i t e and a p p r o x i m a t e d the coupled diffusion c o e f f i c i e n t s d e v e l o p e d by O r i a n i and Li, B l a k e l y , and Feingold.
T H E s p h e r o i d i z a t i o n of c e m e n t i t e in the F e - C s y s tem h a s b e e n c o n s i d e r e d p r e v i o u s l y in s e v e r a l t h e o r e t i c a l and e x p e r i m e n t a l i n v e s t i g a t i o n s . The L i f s h i t z W a g n e r I-4 t h e o r y p r e d i c t s that, when the s t e a d y - s t a t e s i z e d i s t r i b u t i o n of c e m e n t i t e p a r t i c l e s is a t t a i n e d , the k i n e t i c s m a y be d e s c r i b e d by Eq. [1] o r [2]: i) f o r d i f f u s i o n - c o n t r o l l e d growth: ~ 3 _ ~,o z
8(rDC =
oV2m(t-
t'o)
9uRT
[1]
ii) for i n t e r f a c e - c o n t r o l l e d growth: ~ 2 _ ~,o2 =
6 4 a k C oY2m(t - t'o) 81~,RT
[2]
whe re /~ i s the m e a n p a r t i c I e r a d i u s at any t i m e t, / ~ i s the m e a n p a r t i c l e r a d i u s at any t i m e , t~, when s t e a d y - s t a t e growth b e g i n s , i s the s u r f a c e f r e e e n e r g y of the i n t e r f a c e , a s sumed isotropic,
k i s the r e a c t i o n c o n s t a n t , and v i s the s t o i c h i o m e t r i c f a c t o r (weight f r a c t i o n of solute in p a r t i c l e s ) . Bannyh, Modin, and Modin s w e r e a m
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