Effect of microstructural banding in steel
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IVI!CROSTRUCTURAL b a n d i n g in w r o u g h t s t e e l is a m a n i f e s t a t i o n of the s e g r e g a t i o n of one o r m o r e e l e m e n t s . The s e g r e g a t i o n a r i s e s d u r i n g s o l i d i f i c a t i o n as a r e s u l t of s u b s t a n t i a l t e m p e r a t u r e d i f f e r e n c e between the l i q u i d u s and s o l i d u s . Such d e n d r i t i c s e g r e g a t i o n is hence unavoidable in c a s t s t e e l and, though u s u a l l y r e duced somewhat d u r i n g hot w o r k i n g , p e r s i s t s in the w r o u g h t p r o d u c t . The a p p a r e n t s e v e r i t y of m i c r o s t r u c t u r a l banding in wrought s t e e l v a r i e s with the f i n a l h e a t t r e a t m e n t . 1'2 While the o r i g i n and m i c r o s t r u c t u r a l a s pects of b a n d i n g a r e well d o c u m e n t e d in the l i t e r a t u r e , t h e r e a r e few data which r e v e a l the effect of banding on m e c h a n i c a l p r o p e r t i e s . To obtain such data, it is n e c e s s a r y that a p a r t i c u l a r s t e e l be p r o c e s s e d to the s a m e m i c r o s t r u c t u r e and g r a i n size in both the banded mad unbanded condition. H o w e v e r , e l i m i n a t i o n of m i c r o s e g r e g a t i o n (homogenization) r e q u i r e s prolonged t r e a t m e n t at such a high t e m p e r a t u r e that at l e a s t some change in c h e m i c a l Composition is likely and sulphide i n c l u s i o n s , u s u a l l y p r e s e n t in c o m m e r c i a l wrought s t e e l , change f r o m a h a r m f u l elongated shape to r o w s of r e l a t i v e l y s m a l l and innocuous s p h e r o i d a l p a r t i c l e s . R e c e n t l y , a method of m a r k e d l y r e d u c i n g the s e v e r i t y of m i c r o s t r u c t u r a l b a n d i n g by a s h o r t - t i m e , h i g h - t e m p e r a t u r e n o r m a l i z i n g t r e a t m e n t was developed, s T h i s t r e a t m e n t m a k e s it p o s s i b l e to p r e p a r e s a m p l e s of the s a m e s t e e l in a v i r t u a l l y unbanded v e r s u s a s e v e r e l y banded condition, with negligible d i f f e r e n c e in c h e m i c a l c o m p o s i t i o n , g r a i n size and i n c l u s i o n morphology. MATERIAL AND PROCEDURE F o r t h i s i n v e s t i g a t i o n , it was d e s i r a b l e to s e l e c t a s t e e l c o m p o s i t i o n l i k e l y to be s e v e r e l y banded and yet one which i s r e a s o n a b l y r e p r e s e n t a t i v e of both c a r b o n and l o w - a l l o y s t e e l s . M a n g a n e s e is the only a l l o y i n g e l e m e n t p r e s e n t in a p p r e c i a b l e a m o u n t in all such s t e e l s and is also an e l e m e n t which c a u s e s p e r s i s t e n t b a n d i n g in wrought s t e e l . 4 M i c r o s t r u c t u r a l banding is u s u a l l y m o s t pronounced in m o d e r a t e l y l o w - c a r b o n R. A. GRANGE is Staff Scientist, Edgar C. Bain Laboratory for Fundamental Research, U.S. Steel Corp., Research Center, Monroeville, Pa. This pap
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