The influence of the ferrite to austenite transformation on the formation of sulfidesx
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s h o r t n e s s in s t e e l i s g e n e r a l l y connected with b r i t t l e n e s s at high t e m p e r a t u r e s . T o w a r d s the end of the last c e n t u r y it was s u g g e s t e d that this b r i t t l e n e s s i n s t e e l was c a u s e d by s u l f u r . The g e n e r a l l y a c c e p t e d m e c h a n i s m of this i s a p r e c i p i t a t i o n of sulfide f i l m s in the g r a i n b o u n d a r i e s . Van Vlack e t al 1'2 showed that a m o r e o r l e s s continuous n e t w o r k of sulfide m e l t was f o r m e d i n s t e e l with high S and low Mn content at high t e m p e r a t u r e s . With dec r e a s i n g t e m p e r a t u r e and i n c r e a s i n g Mn content this t e n d e n c y was lowered. P r e c i p i t a t e d oxides did not wet the g r a i n b o u n d a r i e s . T h e s e s t u d i e s explained why a high content of Mn r e d u c e s the t e n d e n c y to hot s h o r t n e s s i n s t e e l . Van Vlack e t al studied the change of sulfides d u r i n g long h e a t - t r e a t m e n t but did not cons i d e r , for example, the p o s s i b i l i t y of sulfide p r e c i p i t a t i o n d u r i n g cooling a s a c o n s e q u e n c e of t r a n s f o r m a t i o n f r o m 5 - F e to a u s t e n i t e o r the d e c r e a s e d s o l u b i l i t y of S in a u s t e n i t e with d e c r e a s i n g t e m p e r a ture. The l a t t e r p h e n o m e n o n was studied by J o s e f s s o n e t al, 3 who showed that the d e c r e a s e in s o l u b i l i t y of S i n a u s t e n i t e with d e c r e a s i n g t e m p e r a t u r e could c a u s e the f o r m a t i o n of sulfide f i l m s i n the a u s t e n i t e g r a i n b o u n d a r i e s . T h e s e f i l m s caused a b r i t t l e n e s s in the m a t e r i a l d u r i n g hot work. As the s o l u b i l i t y of S in 5 - F e is c o n s i d e r a b l y higher than that in a u s t e n i t e , the f o r m a t i o n of sulfide f i l m s in the a u s t e n i t e g r a i n b o u n d a r i e s s e e m s p o s s i ble. T h i s f o r m a t i o n should o c c u r even if t h e r e is a solid s t a t e t r a n s f o r m a t i o n f r o m f e r r i t e to a u s t e n i t e or if the m a t e r i a l u n d e r g o e s a p e r i t e c t i c t r a n s f o r mation. The p u r p o s e of the p r e s e n t study is to a s c e r t a i n the conditions for sulfide f o r m a t i o n d u r i n g s o l i d i f i c a tion of a s t e e l in which the p r i m a r y solid phase is ferrite. In the late 1930's, Sims 4 i n t r o d u c e d a s y s t e m for c l a s s i f i c a t i o n of sulfide i n c l u s i o n s , which has often b e e n u s e d i n works d e a l i n g with this p r o b l e m . In this HASSE FREDRIKSSON is Professor and JOHAN STJERNDAHL is research assistant, Department of Casting of Metals, The Royal Institute of Technology, S-10044 Stockholm 70, Sweden. Manuscript submitted September 21, 1976. METALLURGICALTRANSACTIONSA
and s e v e r a l other i n v e s t i g a t i o n s ,5-8 it is e m p h a s i z e d that Type II s u l f i d
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