The contribution of electrochemical reactions to sulfur transfer from electrode to slag in electroslag remelting
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I. INTRODUCTION
IT is
well known that in e l e c t r o s l a g r e m e l t i n g (ESR) e l e c t r o c h e m i c a l r e a c t i o n s can play an i m p o r t a n t r o l e in r e f i n i n g s u c h as the r e m o v a l of s u l f u r and oxygen when using DC power 1 and, to a l i m i t e d extent, when using AC due to the r e c t i f i c a t i o n that o c c u r s with a " l i v e " mold. 2-4 E v e n without a p p r e c i a b l e r e c t i f i c a t i o n d u r i n g AC r e f i n i n g , it has b e e n suggested that net e l e c t r o c h e m i c a l r e a c t i o n s at the e l e c t r o d e and ingot s u r faces c o n t r i b u t e to r e f i n i n g . 5'6 The d i r e c t i o n and extent of these e l e c t r o c h e m i c a l r e a c t i o n s , which may o c c u r at both the e l e c t r o d e / s l a g and liquid m e t a l p o o l / s l a g i n t e r f a c e s , depend on a n u m b e r of f a c t o r s , which have not b e e n studied sufficiently, including the type of r e f i n i n g c u r r e n t , i.e. AC or DC with d i r e c t (electrode negative) or r e v e r s e (electrode positive) p o l a r i t y . In the case of d e s u l f u r i z a t i o n , H o l z g r u b e r et al7 have o b s e r v e d that good s u l f u r r e m o v a l t a k e s place with DC e l e c t r o d e p o s i t i v e ( r e v e r s e p o l a r i t y ) in ingot s i z e s up to about 15 c m d i a m , no a p p r e c i a b l e d e s u l f u r i z a t i o n being o b s e r v e d with DC e l e c t r o d e n e g a t i v e (direct p o l a r i t y ) . F o r l a r g e r ingots, 20 to 45 c m d i a m , b e t t e r d e s u l f u r i z a t i o n is c l a i m e d for DC d i r e c t p o l a r i t y o p e r a tion.7-9 The o b s e r v a t i o n s by H o l z g r u b e r el al for s m a l l e r d i a m e t e r ingots have been c o n f i r m e d by Swinden 1~ and M e l l b e r g and S a n d b e r g . l l Swinden~O c o n c l u d e s that the m a j o r r e a c t i o n site for t h e s e ingots is the liquid m e t a l p o o l / s l a g i n t e r f a c e . M e l l b e r g and Sandberg n found, in e x p e r i m e n t s on 10 c m d i a m ingots, that a p p r e c i a b l e d e s u l f u r i z a t i o n o c c u r r e d at the liquid m e t a l p o o l / s l a g interface with D C electrode positive and a slight sulfurization at the s a m e interface with D C electrode negative. O n the other hand, when refining a 13 c m diam electrode into a 17 c m diam mold, G r u n b a u m and G u s t a f s s o n ~2 obtained f a v o r a b l e s u l f u r e l i m i n a t i o n with N, Q. MINH and T. B. KING are Visiting Scientist and Professor, respectively, Department of Materials Science and Engineering, Massachusetts Institute of Technology, Cambridge, MA 02139. Manuscript submitted April 2, 1979. METALLURGICALTRANSACTIONSB
DC e l e c t r o d e n e g a t i v e , in a g r e e m e n t with other r e s u l t s for l a r g e r size i n g o t s , v-9 Many a u t h o r s %n,~3 a s c r i b e the d i f f e r e n c e in d e s u l f u r i z a t i o n b e h a v i o r with l a r g e an
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