Phase instabilities during high temperature exposure of 316 austenitic stainless steel
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T H E molybdenum-containing austenitic stainless steel of Type AISI 316 h a s found wide application in s t e a m g e n e r a t i n g p l a n t s a s piping and s u p e r h e a t e r tube m a t e r i a l . Due to its good m e c h a n i c a l p r o p e r t i e s at e l e vated t e m p e r a t u r e s , its excellent c o r r o s i o n r e s i s t a n c e and f a b r i c a b i l i t y , it i s a l s o e x t e n s i v e l y u s e d a s an e c o n o m i c a l s t r u c t u r a l m a t e r i a l and cladding alloy in c o n v e n t i o n a l and a d v a n c e d n u c l e a r r e a c t o r s . N u m e r o u s i n v e s t i g a t i o n s have been p u b l i s h e d on the m e c h a n i c a l p r o p e r t i e s and p r o p e r t y changes o c c u r r i n g d u r i n g e x p o s u r e to s t e a m plant and r e a c t o r e n v i r o n m e n t . I-1~ However, l i t e r a t u r e data on the m i c r o s t r u c r u r a l changes o c c u r r i n g d u r i n g l o n g - t i m e s e r v i c e e x p o s u r e and t h e i r c o r r e l a t i o n to p h y s i c a l , m e c h a n i c a l , and c h e m i c a l p r o p e r t y c h a n g e s a r e s c a r c e and in some i n s t a n c e s .contradictory. L o n g - t i m e e x p o s u r e of Type 316 s t a i n l e s s s t e e l to e l e v a t e d t e m p e r a t u r e s i s known to c a u s e d e c o m p o s i t i o n of the a u s t e n i t i c m a t r i x r e s u l t i n g in the f o r m a t i o n of s e v e r a l c a r b i d e and i n t e r m e t a l l i c p h a s e s . U n f o r t u n a t e l y , the t i m e / / t e m p e r a t u r e r a n g e s r e p o r t e d by diff e r e n t i n v e s t i g a t o r s for the f o r m a t i o n and s t a b i l i t y of the v a r i o u s p h a s e s a r e not c o n s i s t e n t . F o r i n s t a n c e , Wiegand and Doruk, 1~ p u b l i s h e d a p r e c i p i t a t i o n d i a g r a m r e p o r t i n g the p r e s e n c e of M2sC6, a, • and L a v e s p h a s e s . B l e n k i n s o p and Nutting 12 r e f e r r e d only to the f o r m a t i o n of the c a r b i d e and a p h a s e s in a s i m i l a r s t e e l h e a t - t r e a t e d in a c o m p a r a b l e t i m e / / t e m p e r a t u r e range. It i s known that t h e r m o m e c h a n i c a l p r e t r e a t m e n t s and s u b s e q u e n t aging t r e a t m e n t s of a u s t e n i t i c s t a i n l e s s s t e e l s can lead to i m p r o v e d h i g h - t e m p e r a t u r e t e n s i l e and c r e e p p r o p e r t i e s . One p a r t i c u l a r t r e a t m e n t for Type 316 s t a i n l e s s s t e e l w a s developed by G a r o f a l o e t a l . 13 who r e l a t e d the i m p r o v e m e n t s in m e c h a n i c a l p r o p e r t i e s to i n t e r a c t i o n s of d i s l o c a t i o n s with f i n e l y d i s p e r s e d c a r b i d e p a r t i c l e s . So far, a d e t a i l e d u n d e r s t a n d i n g of the effect of d e f o r m a t i o n and s u b s e q u e n t t h e r m a l t r e a t m e n t s on the s t r u c t u r a l s t a b i l i t y of Type 316 s t a i n l e s s s t e e l i s
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