Effect of homogenization on sulfide inclusions in ferrous alloys
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T H E m o r p h o l o g i c a l m o d i f i c a t i o n of s u l f i d e i n c l u s i o n s d u r i n g h e a t t r e a t m e n t h a s b e e n the subject of only a few p u b l i s h e d s t u d i e s . F l o w e r s and K u r u s i w e r e the f i r s t to r e p o r t t h a t MnS I n c l u s i o n s c o a r s e n e d d u r i n g b o x a n n e a l i n g of F e - 3 wt p e t St a l l o y . The a l l o y was p r o c e s s e d in the f o r m of thin f o i l s in o r d e r to o b t a i n a v e r y fine d i s p e r s i o n of s p h e r i c a l b i n s i n c l u s i o n s in the m a t r i x . T h e f o i l s w e r e c o a t e d with m a g n e s i a and w e r e h e a t e d in d r y h y d r o g e n f r o m 590~ to a t e m p e r a t u r e b e l o w 1200~ at a r a t e of 28~ Detailed examination b y e l e c t r o n m i c r o s c o p y showed that the i n c l u s i o n s w e r e r a n d o m l y d i s t r i b u t e d in the m a t r i x and at the g r a i n b o u n d a r i e s b e f o r e h e a t t r e a t m e n t . A f t e r h e a t t r e a t m e n t c o a r s e n i n g and c o a l e s c e n c e took p l a c e , the a v e r a g e i n c l u s i o n d i a m e t e r i n c r e a s e d and the n u m b e r of r e m a i n i n g i n c l u s i o n s d e c r e a s e d . S i m s d i s c u s s e d s u l f i d e f o r m a t i o n in s t e e l z and gave an a c c o u n t of the effect of h e a t t r e a t m e n t on s u l f i d e i n c l u s i o n s in r e s u l f u r i z e d s t e e l . The s t e e l was h o t - r o l l e d p r i o r to the high t e m p e r a t u r e t r e a t m e n t and in t h i s c o n dition the s u l f i d e i n c l u s i o n s w e r e e l o n g a t e d . However, t h e y s p h e r o i d i z e d a f t e r h o m o g e n i z a t i o n at 1315~ f o r 24 h without a n y change in t h e i r n u m b e r o r a v e r a g e size. Keh and Van V l a c k s e x a m i n e d the e f f e c t of h e a t t r e a t m e n t on i n c l u s i o n s in i r o n - s u l f u r a l l o y s b e t w e e n 850~ and 1300~ f o r d i f f e r e n t t i m e s . In the a s - c a s t condition, the F e S i n c l u s i o n s a p p e a r e d g l o b u l a r f o r s u l f u r l e v e l s l o w e r than 0.17 wt p e t and in the f o r m of i n t e r g r a n u l a r f i l m s a s well a s g l o b u l a r f o r s u l f u r l e v e l s h i g h e r than 0.17 wt pct. A f t e r h e a t t r e a t m e n t at a r e l a t i v e l y high t e m p e r a t u r e f o r a s u f f i c i e n t l y long t i m e , followed b y quenching, the s u l f i d e i n c l u s i o n s w e r e found a l o n g g r a i n b o u n d a r i e s o r a t g r a i n c o r n e r s . It was pointed out that s p h e r o i d i z a t i o n of sulfide i n c l u s i o n s within the g r a i n s took p l a c e d u r i n g the h e a t t r e a t m e n t b y d i s s o l u tion, diffusion t h r o u g h the m a t r i x and r e d e p e s i t i o n at l o w e r e n e r g y s i t e s such a s g r a i n b o u n d a r i e s , g r a i n
corners or larger spherical inclusions with a consequent decrease in interfacial
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