Effect of minor alloying element variation on the properties of alloy 800
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G. COLOMBE
A p p l i c a t i o n of A l l o y 800 in s t e a m g e n e r a t o r tubing of f a s t r e a c t o r s , w h e r e continuous s e r v i c e t e m p e r a t u r e of the o r d e r of 550~ is e x p e r i e n c e d , has b e e n a n a l y z e d with r e s p e c t to s m a l l v a r i a t i o n s in i t s c h e m i c a l c o m p o s i t i o n . S e v e r a l l a b o r a t o r y m e l t s of A l l o y 800 have b e e n p r e p a r e d and t h e i r m i c r o s t r u c t u r a l and m e c h a n i c a l p r o p e r t y c h a n g e s d u r i n g s i m p l e a g i n g and c r e e p t e s t s at 500 to 600~ have b e e n s t u d i e d . It h a s been found that in the a b o v e t e m p e r a t u r e r a n g e p r e c i p i t a t i o n of M23C6 on the g r a i n b o u n d a r i e s is i n d e p e n d e n t of the T i : C r a t i o g e n e r a l l y s p e c i f i e d f o r A l l o y 800. G a m m a p r i m e p r e c i p i t a t i o n o c c u r r e d in a l l o y s c o n t a i n i n g a s low a s 0.5 p c t T i + A1 a f t e r 1000 h of a g i n g and was a c c o m p a n i e d with a c r e e p d u c t i l i t y d e c l i n e . Upon ~\ p r e c i p i t a t i o n c r e e p r a t e was r e t a r d e d and its r e a c c e l e r a t i o n for t e s t t i m e s up to 8500 h a t 550~ was not o b s e r v e d . B a s e d on the findings, i n c r e a s e d T i c o n c e n t r a t i o n at the e x p e n s e of A1 within the s p e c i f i e d c h e m i c a l c o m p o s i t i o n r a n g e with c a r b o n content of 0.030 to 0.050 has b e e n s u g g e s t e d . S U C C E S S F U L a p p l i c a t i o n of the A l l o y 800 in cons t r u c t i o n of high t e m p e r a t u r e c h e m i c a l and f o s s i l f u e l l e d p l a n t s , h a s l e d to i t s c o n s i d e r a t i o n f o r s o d i u m cooled f a s t r e a c t o r s . T h e o v e r r i d i n g i m p o r t a n c e of the s t r e s s c o r r o s i o n h a s guided the n u c l e a r i n d u s t r y to r e v i s e the n o r m a l A l l o y 800 c o m p o s i t i o n . In p a r t i c u l a r the c a r b o n s p e c i f i c a t i o n h a s b e e n r e d u c e d f r o m 0.1 p c t m a x i m u m to 0.03 p c t m a x i m u m and a m i n i m u m v a l u e for T i : C o r T i : C + N is s p e c i f i e d to ensure adequate stabilization against sensitization c o r r o s i o n . 1 M o r e r e c e n t l y s t r i n g e n t c o n t r o l of T i and A1 content h a s b e e n sought to c o u n t e r the c r e e p d u c t i l i t y d e c l i n e o b s e r v e d in long d u r a t i o n t e s t s . The o r i g i n a l ASTM d e s i g n a t i o n s of G r a d e s I and II f o r A l l o y 800, now a r e g e n e r a l l y r e f e r r e d to a s A l l o y 800 and 800 H. The p r i n c i p a l d i f f e r e n c e b e t w e e n the two a l l o y s i s the c a r b o n content. A l l o y 800 h a s -
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