Effect of hydrogen on the mechanical properties of iron-base superalloys
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EXPOSUREto hydrogen has been known to c a u s e c a t a s t r o p h i c f a i l u r e in m e t a l s and a c o n s i d e r a b l e a m o u n t of work has b e e n done to d e t e r m i n e the c a u s e f o r this f a i l u r e mode. 1"3 M a t e r i a l s u s e d in hydrogen e n v i r o n m e n t s have included 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 , which show a g e n e r a l c o r r e l a t i o n between r e s i s t a n c e to h y d r o g e n e m b r i t t l e m e n t and a u s t e n i t e s t a b i l i t y , e'3 However, t h e i r yield s t r e n g t h s a r e t y p i cally 200 to 450 MPa, l i m i t i n g t h e i r a p p l i c a t i o n to low p r e s s u r e hydrogen environments. Iron base austenitic s u p e r a l l o y s , n o r m a t l y a s s o c i a t e d with m o d e r a t e t e m p e r a t u r e a p p l i c a t i o n s , can be p r e c i p i t a t i o n s t r e n g t h ened to h i g h e r s t r e n g t h s (700 to 1000 MPa) and r e p r e s e n t potential c a n d i d a t e s f o r high s t r e n g t h , h y d r o g e n e n v i r o n m e n t a p p l i c a t i o n s at a m b i e n t t e m p e r a t u r e s . T h e extent of t h e i r hydrogen c o m p a t i b i l i t y , 4'~ however, i s not a s s t r a i g h t f o r w a r d as with the l o w e r s t r e n g t h a u s t e n i t i c s t e e l s . ~ In p a r t i c u l a r , A286, a c o m m e r c i a l i r o n b a s e s u p e r a l l o y , exhibits a change f r o m d u c t i l e d i m p l e d f r a c t u r e to l e s s e x t e n s i v e d i m p l e fracture" c o m b i n e d with a c o n s i d e r a b l e a m o u n t of s e c o n d a r y c r a c k i n g a f t e r hydrogen c h a r g i n g . 4 However, with s l i g h t c h e m i s t r y m o d i f i c a t i o n s , the extent of d u c t i l i t y loss can be m i n i m i z e d5 and has r e s u l t e d in the g r a n t ing of a patent 6 for a h y d r o g e n c o m p a t i b l e a l l o y . T h e b a s i c A286 alloy c o n s i s t s of 15 pct Cr, 25 pct Ni, s m a l l a m o u n t s of Ti, Mo, Mn, and A1, with the b a l a n c e b e i n g iron. S t r e n g t h e n i n g is due to p r e c i p i tation of Nia(At, Ti), a n o r d e r e d fcc phase known a s V'. The aging c h a r a c t e r i s t i c s have b e e n r e p o r t e d by s e v e r a l a u t h o r s 7-t2 d e s c r i b i n g the p r e c i p i t a t i o n s e q u e n c e l e a d i n g to the f o r m a t i o n of Ni~Ti, a hexagonal p h a s e known a s 7. V a r i a t i o n s in the T i and A1 c o n t e n t s a r e thought to c o n t r o l the e q u i l i b r i u m or o v e r a g e d p r e c i p i t a t e t~ as well as the k i n e t i c s of aging. In this work an a t t e m p t was made to obtain an alloy s t r o n g e r than A286 by v a r y i n g the Ti/A1 r a t i o for c o n s t a n t Ni content, the Ni content for c o n s t a n t Ti/A1 r a t i o and the s u b s t i t u t i o n of Mn for Ni. By o b s e r v i n g the effect of these v a r i a t i o n s on the a g i n g r e s p o n s e , s e v e r a l c a n d i d a t e s w e r e c h o s e n to d e t e r m
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