The influence of martensite and ferrite on the properties of two-phase stainless steels having microduplex structures
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I ~ E C E N T s t u d i e s of t w o - p h a s e F e - N i - C r a l l o y s have shown that m a n y c o m p o s i t i o n s having a s u i t a b l y fine (microduplex) g r a i n size exhibit the p h e n o m e n o n of s u p e r p l a s t i c i t y at t e m p e r a t u r e s s i m i l a r to and in exc e s s of a p p r o x i m a t e l y o n e - h a l f the a b s o l u t e m e l t i n g t e m p e r a t u r e . 1'2 In addition to this u n u s u a l high t e m p e r a t u r e b e h a v i o r , m a n y of these a l l o y s have useful i n t e r m e d i a t e and low t e m p e r a t u r e p r o p e r t i e s . In p a r t i c u l a r , s t u d i e s of t w o - p h a s e i r o n - b a s e c o m p o s i t i o n s have led to the d e v e l o p m e n t of the s t a i n l e s s s t e e l IN-744 having a c o m p o s i t i o n of 26 pct C r , 6.5 pct Ni. 2 T h i s alloy has an e x c e l l e n t c o m b i n a t i o n of the m e c h a n i c a l p r o p e r t i e s coupled with v e r y good c o r r o sion r e s i s t a n c e . The p r e s e n t a u t h o r s have r e p o r t e d s t u d i e s of the d e f o r m a t i o n and f r a c t u r e c h a r a c t e r i s t i c s of IN-744. s The t e n s i l e d e f o r m a t i o n c h a r a c t e r i s t i c s s e e m e d c l o s e l y akin to the b e h a v i o r of f e r r i t i c s t a i n l e s s steel. H o w e v e r , the t w o - p h a s e alloy had a d u c t i l e - t o - b r i t t l e Charpy V-notch t r a n s i t i o n on the o r d e r of 300~ lower than that for a f e r r i t i c s t a i n l e s s s t e e l having the c o m p o s i t i o n of the f e r r i t e of the t w o - p h a s e a l l o y (app r o x i m a t e l y 32 pct C r - 3 pct Ni). In a s u b s e q u e n t publ i c a t i o n , 4 the m e c h a n i c a l b e h a v i o r of a s e r i e s of a l loys lying along a t i e - l i n e f r o m 32 pct C r - 3 pct Nib a l a n c e Fe to 19.5 pct C r - 9 pct N i - b a l a n c e Fe were i n v e s t i g a t e d . It was found that v e r y useful c o m b i n a t i o n s of s t r e n g t h and t o u g h n e s s could be o b t a i n e d in a l l o y s with m i c r o d u p l e x s t r u c t u r e s c o n t a i n i n g a p p r o x i m a t e l y 40 to 50 pct a u s t e n i t e . The i m p r o v e d toughn e s s of the two phase alloy was r e l a t e d to two fact o r s : 1) the g r e a t e r difficulty of cleavage c r a c k i n i t i a tion afforded by the g r a i n r e f i n e m e n t of the f e r r i t e phase which is e a s i l y a t t a i n e d in two-phase a l l o y s , and 2) blunting of p r o p a g a t i n g cleavage c r a c k s by s o f t e r , tougher, and t r a n s f o r m a b l e (to m a r t e n s i t e ) austenite particles. In a l a t e r i n v e s t i g a t i o n , 5 an alloy of 50 pct F e - 5 0 pct Cu having a m i c r o d u p l e x s t r u c t u r e was a n a l y z e d . Again it was found that the d u c t i l e - t o - b r i t t l e t r a n s i tion t e m p e r a t u r e was about 300~ lower than that of the t i e - l i n e iron phase. The i m p r o v e m e n t in toughH. W. HAY
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