Equilibrium studies on a chi phase-strengthened ferritic alloy
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F E R R I T I C a l l o y s with i m p r o v e d high t e m p e r a t u r e p r o p e r t i e s a r e of i n t e r e s t f o r u s e in a n u m b e r of a p p l i c a t i o n s s u c h a s cladding and duct m a t e r i a l s and s t e a m g e n e r a t o r c o m p o n e n t s for liquid m e t a l - c o o l e d fast reactors, 1 pressure vessels for coal gasification, 2 h e a t e x c h a n g e r s and tubing for high t e m p e r a t u r e g a s cooled r e a c t o r s , S and r e a c t o r t u b e s f o r p e t r o c h e m i c a l r e f i n i n g . A n u m b e r of f e r r i t i c a l l o y s b a s e d on the a d d i t i o n of c h r o m i u m and m o l y b d e n u m have been r e p o r t e d ~ to have i m p r o v e d s t r e n g t h s at e l e v a t e d t e m p e r a t u r e s , and b e t t e r c o r r o s i o n r e s i s t a n c e and r e s i s t a n c e to s t r e s s c o r r o s i o n c r a c k i n g and t e m p e r e m b r i t t l e m e n t than c a r b o n s t e e l s . ' M a j o r e m p h a s i s has b e e n on the 2 1//4 C r - 1 - M e and 9 C r - 1 Me a l l o y s . T h e m a i n i m p e t u s behind the d e v e l o p m e n t of high strength ferritic alloys is their appreciably lower mat e r i a l s c o s t than that of the a u s t e n i t i c s t e e l s o r n i c k e l b a s e a l l o y s now in use in m o s t high t e m p e r a t u r e a p plications. E f f o r t s to i m p r o v e the p r o p e r t i e s of f e r r i t i c s t e e l s involve m o d i f i c a t i o n s in the c h r o m i u m a n d / o r m o l y b denum c o m p o s i t i o n s and the a d d i t i o n of o t h e r c o m p o n e n t s . Huet and c o w o r k e r s 4 have d e v e l o p e d a Chi p h a s e - s t r e n g t h e n e d f e r r i t i c a l l o y with a r u p t u r e s t r e n g t h c o m p a r a b l e to that of type 316 a u s t e n i t i c s t a i n l e s s s t e e l at 700~ T h i s f e r r i t i c a l l o y has the c o m p o s i t i o n F e - 1 3 pct C r - l . 5 pct M o - 3 . 5 pct T i s t a b i l i z e d by the a d d i t i o n of 2 pct TiO2 o r 1 pct Y203. A t 500~ and above the elongation of n e u t r o n - i r r a d i a t e d 316 SS d e c r e a s e s to 2.5 pct 5 w h e r e a s the " H u e t " a l l o y e x h i b i t s an e l o n g a t i o n of 5 p c t at the s a m e t e m p e r a t u r e a f t e r i r r a d i a t i o n . S i m u l a t i o n t e s t s i n d i c a t e that the a l l o y is c o n s i d e r a b l y m o r e r e s i s t a n t to i r r a d i a t i o n s w e l l i n g than 316 SS and e x h i b i t s e x c e l l e n t r e s i s t a n c e to c o r r o s i o n by liquid s o d i u m . The e x c e l l e n t high t e m p e r a t u r e s t r e s s - r u p t u r e p r o p e r t i e s of the q u a t e r n a r y a l l o y is a t t r i b u t e d to the f o r m a t i o n of a Chi (or X) p h a s e p r e c i p i t a t e the high t e m p e r a t u r e m i c r o s t r u c t u r e of which is s t a b i l i z e d by the oxide d i s p e r s o i d . I. C. I. OKAFOR and O. N. CARLSON are Graduate Assistant and Professor and Senior Metallurgi
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