Nitrogenation and decarburization of stainless steel
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S T A I N L E S S
s t e e l s m a d e by the AOD p r o c e s s contain l e s s n i t r o g e n than t h o s e made by conventional a r c f u r n a c e p r a c t i c e , and s o m e t i m e s n i t r o g e n must be added to the s t e e l to i m p r o v e m e c h a n i c a l p r o p e r t i e s . Making n i t r o g e n additions by adding n i t r o g e n - b e a r i n g f e r r o a l l o y s is c o s t l y and the c o n t r o l is poor. N i t r o g e n can be added to the s t e e l by using gaseous n i t r o g e n in p l a c e of argon in a c o n t r o l l e d m a n n e r . ~ N i t r o g e n or air m a y a l s o be used in the e a r l y s t a g e s of the p r o c e s s in p l a c e of argon to r e d u c e c o s t s . To a c c o m p l i s h t h e s e goals in the b e s t m a n n e r a knowledge of the r a t e of abs o r p t i o n of n i t r o g e n when it is bubbled through liquid s t e e l is helpful. The r a t e of n i t r o g e n a b s o r p t i o n in liquid i r o n a l l o y s has been e x t e n s i v e l y studied, howe v e r t h e r e is l i t t l e known about the r a t e when the gas is being bubbled through the m e t a l . In the p r e s e n t work the r a t e of n i t r o g e n a t i o n of i r o n and s t a i n l e s s s t e e l by bubbling n i t r o g e n has been i n v e s t i g a t e d , and the r e s u l t s are described herein. In AOD s t a i n l e s s s t e e l m a k i n g , a r g o n - o x y g e n gas m i x t u r e s a r e used and c a r b o n is o x i d i z e d in p r e f e r e n c e to c h r o m i u m b e c a u s e the r e s u l t i n g CO is diluted with a r gon. The o v e r a l l b a s i c c h e m i c a l r e a c t i o n d e s c r i b i n g the p r o c e s s can be r e p r e s e n t e d by Cr=Os(s) + 3 C ( i n Fe) - - 3CO(g) + 2Cr (in Fe).
p r e s e n t work is not intended to model an AOD o p e r a tion. The v e s s e l s i z e , blowing r a t e s , t u y e r e s i z e , e t c a r e v a s t l y d i f f e r e n t f r o m a c o m m e r c i a l AOD. The e x p e r i m e n t s w e r e d e s i g n e d to obtain i n f o r m a t i o n conc e r n i n g the r e a c t i o n s e q u e n c e and m e c h a n i s m s for the oxidation of c a r b o n and c h r o m i u m . EXPERIMENTAL Most of the experiments were conducted in an induction furnace with 4200 g of metal contained in a i0 cm ID ZrO 2 crucible. The starting material was Type 304 stainless steel (18.4 pct Cr and 8.1 pet Ni) or electrolytic iron. The temperature was measured with a l:~Pf/10 pct Rh thermocouple and maintained at about 1610~ The metal was melted under a protective argon atmosphere. The gas injector was a molybdenum cermet (80 pct Mo and 20 pct Al2Oa) closed-end tube with a 0.2 cm-diam hole cut into the side near the bottom such that the gas was injected horizontally. The metal height above the orifice was 7.5 cm, 09
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T h e r e a r e s o m e r e s e r v a t i o n s c o n c e r n i n g this r e a c t i o n ; for e x a m p l e , the oxide of c h r o m i u m may be Cr304 or FeCr204. The e q u i l i b r i u m c u r v e s for React
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