The rate of carburization of iron in CO-H 2 atmospheres: Part I. Effect of temperature and CO and H 2 -pressures
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s e r i o u s p r o b l e m in the d i r e c t r e d u c t i o n of i r o n o r e in g a s m i x t u r e s c o n t a i n i n g CO and Ha is that of s o o t f o r m a t i o n c a t a l y z e d b y r e d u c e d i r o n o x i d e s . In o r d e r to o b t a i n an u n d e r s t a n d i n g of the c h e m i c a l r e a c t i o n s involved, the r a t e of c a r b u r i z a t i o n of i r o n s t r i p s in COHe and CO-Ha a t m o s p h e r e s a t 915 and 1000~ have b e e n investigated.
r e m a i n e d r e l a t i v e l y c o n s t a n t up to about 0.4 wt p c t C. It is a p p a r e n t f r o m Fig. 3 that f o r CO-H2 m i x t u r e s c a r b o n diffusion in the m e t a l m a y a f f e c t the r a t e only at l a t e r s t a g e s of c a r b u r i z a t i o n . 1"he r a t e c o n t r o l l e d b y diffusion of c a r b o n in i r o n was c a l c u l a t e d a s s u m i n g a f a s t c h e m i c a l r e a c t i o n on the s u r f a c e and using the diffusion d a t a of Smith. 1 However, it is p o s s i b l e to e x a m i n e the i n i t i a l r a t e s s i n c e diffusion will not affect
EXPERIMENTAL The s a m p l e s w e r e m a c h i n e d into s t r i p s 2.5 • 1.5 x 0 . 0 5 c m f r o m high p u r i t y i r o n (99.99 pct). The r a t e of c a r b u r t z a t i o n was' d e t e r m i n e d b y m e a s u r i n g the weight gain with a s e m i m t c r o r e c o r d i n g b a l a n c e which can m e a s u r e within • 0.02 m g o r about 0.01 wt p c t C f o r the s a m p l e s used. This technique of d e t e r m i n i n g c a r b o n c o n t e n t s h a s p r o v e d to be s a t i s f a c t o r y f o r the present purpose. ~"he g a s m i x t u r e s w e r e p r e p a r e d u s i n g c o n v e n t i o n a l flow m e t e r s f r o m p u r i f i e d CO, He, and Ha; c a r e was taken to r e m o v e any w a t e r v a p o r o r COa p r e s e n t in the g a s e s . In s e v e r a l e x p e r i m e n t s the s a m p l e s w e r e p r e * r e a t e d at the r e a c t i o n t e m p e r a t u r e in h y d r o g e n for s e v e r a l h o u r s p r i o r to c a r b u r t z a t i o n in o r d e r to i n s u r e t h a t t h e r e was no oxygen p r e s e n t . The r e s u l t s o b t a i n e d u s i n g t h i s p r o c e d u r e w e r e in a g r e e m e n t with t h o s e o b t a i n e d without p r e t r e a t m e n t in h y d r o g e n .
301100~ / //50% CO .... DIFFUSIOCONTROL N 25I_- _/_GR_AP_HIT_E SATU R ~ ....
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~
0
100
8.5%C0
I
I
I
200 300 400 500 TIME, rain Fig. 1 - - C a r b u r i z a t i o n of i r o n s t r i p s in C O - H e m i x t u r e s (PT = 0.965 atm) at 1273 K ( a r e a = 7.5 s q c m , t h i c k n e s s 0.05 c m ) . 50 ....
RESULTS 2"he r a t e of c a r b u r t z a t t o n of I r o n in CO-He m i x t u r e s r a n g i n g f r o m 100 to 5 p c t CO was d e t e r m i n e d at 1000 and 915~ The r e s u l t s f o r t y p i c a l e x p e r i m e n t s a r e shown in F i g s . 1 and 2. The r a t e was a l s o d e t e r m i n e d f o r C O - H a m i x t u r e s a t the s a m e t e m p e r a t u r
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