On the formation of a liquid phase during cooling of steel
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6 shows that the t r a n s f o r m a t i o n of f e r r i t e to a u s t e n i t e s t a r t s j u s t b e f o r e the c o m p l e t i o n of s o l i d i f i c a t i o n . The l a t t e r r e a c t i o n s t a r t s w h e r e the liquid p h a s e is p r e s ent. Fig. 7 shows the s t r u c t u r e a f t e r this t r a n s f o r m a -
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Small a m o u n t s of a liquid phase in a solid m e t a l l i c m a t e r i a l can c a u s e s e v e r e p r a c t i c a l p r o b I e m s d u r i n g hot d e f o r m a t i o n of s t e e l . F o r instance, it may be r e s p o n s i b l e for such p h e n o m e n a as hot s h o r t n e s s and r e d s h o r t n e s s . The p r e s e n t note will d i s c u s s s o m e c a s e s w h e r e such a liquid phase can f o r m or grow as a r e s u l t of cooling. It is s u g g e s t e d that such a phenomenon would be p a r t i c u l a r l y d a n g e r o u s . The c a s e to be c o n s i d e r e d can be i l l u s t r a t e d by the F e - S s y s t e m . The F e - S p h a s e d i a g r a m shows a m e t a t e c t i c r e a c t i o n at the i r o n - r i c h c o r n e r , Fig. 1.1 The s o l i d i f i c a t i o n p r o c e s s for an alloy with the c o m p o s i t i o n of x o w i l l s t a r t with a p r i m a r y p r e c i p i t a t i o n of f e r r i t e and will be c o m p l e t e d at T 1 if t h e r e is no s e g r e g a t i o n . At the m e t a t e c t i c t e m p e r a t u r e the r e a c t i o n a - - y + L o c c u r s , r e s u l t i n g in p a r t i a l r e m e l t i n g . The new liquid phase will be s u l f u r - r i c h . The f o r m a t i o n of a s u l f u r r i c h liquid due to the t r a n s f o r m a t i o n of f e r r i t e to aus t e n i t e has now b e e n e x a m i n e d by u n i d i r e c t i o n a l s o l i d ification and t r a n s f o r m a t i o n s e q u e n c e in a s p e c i m e n w h e r e the constant r a t e e x p e r i m e n t was i n t e r r u p t e d by quenching. Alloy 1 is a F e - N i - S alloy containing 3 pct Ni and 0.07 pct S. Fig. 2 shows how d e n d r i t e s of f e r r i t e g r o w into the m e l t in alloy 1. The s p e c i m e n s o l i d i f i e d c o m p l e t e l y to f e r r i t e as can be s e e n at the r i g h t hand s i d e of Fig. 2. All the sulfur is then d i s s o l v e d and no sulfide p a r t i c l e s can be s e e n h e r e . Fig. 3 shows how a u s t e n i t e g r o w s into f e r r i t e . Thin s t r i n g s of f e r r i t e r e m a i n at the r i g h t hand side of the m i c r o graph. The application of high m a g n i f i c a t i o n r e v e a l e d that a thin f i l m of YeS often f o r m e d in the boundary between this f e r r i t e and the s u r r o u n d i n g austenite. Fig. 4 was taken a f t e r a r e p o l i s h and shows a typical c a s e of the YeS film. It is evident that the FeS f i l m in this c a s e is f o r m e d b e c a u s e the s o l u b i l i t y of s u l f u r is l o w e r in a u s t e n i t e than in f e r r i t e . The s a m e phenomenon m a y be e x p e c t e d at s u f f i c i e n t
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