Thermodynamics of austenitic Fe-C alloys
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N e w d a t a on the a c t i v i t y of c a r b o n i n a u s t e n i t e h a v e b e e n o b t a i n e d b y C O s / C O e q u i l i b r a t i o n o f F e - C a l l o y s in the t e m p e r a t u r e r a n g e 900 ~ t o 1400~ E q u a t i o n s f o r the t h e r m o d y n a m i c p r o p e r t i e s of c a r b o n a n d i r o n in a u s t e n i t e a r e o b t a i n e d f r o m the d a t a c o m b i n e d with s e l e c t e d d a t a f r o m the l i t e r a t u r e . A s l i g h t l y m o d i f i e d p h a s e d i a g r a m i s p r e s e n t e d . T h e s t a b i l i t y of c e m e n t i t e i s a l s o d e t e r m i n e d f r o m d a t a p u b l i s h e d e a r l i e r a n d t h e r e s u l t s of t h e p r e s e n t s t u d y . P a r a m e t e r s of t h e v a r i o u s m o d e l s of t h e b e h a v i o r of c a r b o n in a u s t e n i t e t a k e n f r o m t h e l i t e r a t u r e a r e a l s o c a l c u l a t e d f r o m the d a t a .
S Tof the activity U ofDcarbon inYaustenite by CO2/CO equilibration, as reported in the previous paper for 1150~ ' has been extended to other temperatures. The experimental method was described e a r l i e r ' and the current results are shown in Table I. The equilibrium investigated was p2 CO2(g) + COn y F e ) = 2 C O ( g ) ;
K'(z) =
YC ZC - 1 -- YC
CO
PC02 "Zc
K'(z) = log K + 0.4340CY C
900~ [1]
[2]
w h e r e 0 c -= (~ in K' (e)/SyC)PT. V a l u e s of t h e two c o n s t a n t s a t e a c h e x p e r i m e n t a l t e m p e r a t u r e a r e g i v e n in T a b l e II. S l o p e s of t h e l i n e s a r e s u b j e c t t o s o m e u n c e r t a i n t y b u t it i s q u i t e o b v i o u s t h a t the s l o p e d e c r e a s e s w i t h i n c r e a s i n g t e m p e r a t u r e . V a l u e s of l o g K a t YC = 0 a r e a s s i g n e d an u n c e r t a i n t y e q u a l to a f o r e a c h g r o u p of d a t a . It i s a p p a r e n t t h a t t h e s e v a l u e s a r e i n d e p e n d e n t of t h e m o l a r c o n c e n t r a t i o n v a r i a b l e u s e d in e x p r e s s i n g K ' . T h u s , if one w i s h e s to e x p r e s s t h e r e s u l t s i n t e r m s of x C o r YC t h e s a m e v a l u e of l o g K i s to b e u s e d . F o r l o g K ' t h e appropriate activity coefficient must be employed. At h i g h e r t e m p e r a t u r e s t h e r e s u l t s b e c o m e l e s s c e r t a i n b e c a u s e of t h e v e r y s m a l l f r a c t i o n of CO2 in t h e e q u i l i b r i u m g a s a n d t h e e n h a n c e d p o s s i b i l i t y of i t s f o r m a t i o n b y d e c o m p o s i t i o n o f CO. In the e x p e r i m e n t s a t 1150~ it w a s f o u n d t h a t t h e g a s c o m p o s i t i o n a t t h e p o s i t i o n of t h e i r o n s p e c i m e n c o r r e s p o n d e d c l o s e l y to t h a t of the e n t r a n t g a s . T h e s a m e c h e c k w a s not m a d e at the h i g h e r t e m p e r a t u r e but outlet gas c o m p o s i t i o n s w e r e r e c o r d e d . T h e s e s h o w e d e v i d e n c e of c o n s i d e r a b l e d e c o m p o s i t
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