Matrix precipitation of Cr 2 N in a phosphorus-containing austenitic stainless steel
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croscopy w e r e p r e p a r e d f r o m the d i s c s by a single stage v e r t i c a l jet e l e c t r o p o l i s h i n g p r o c e s s . 12 The e l e c t r o l y t e c o n s i s t e d of 75 pct ethanol, 15 pct g l y c e r o l and 10 pct p e r c h l o r i c acid. The s p e c i m e n foils were e x a m i n e d in a J E M - 2 0 0 e l e c t r o n m i c r o s c o p e o p e r a t e d at 200 kV. D i c h r o m i u m n i t r i d e is an i n t e r s t i t i a l compound in which c h r o m i u m atoms f o r m a hexagonal close packed a r r a n g e m e n t and n i t r o g e n a t o m s occupy the octah e d r a l i n t e r s t i c e s . Since v e r y little has b e e n known about the p r e c i p i t a t i o n of Cr2N in 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 , the p r e v i o u s e x p e r i m e n t a l r e s u l t s 7-11 on the p r e c i p i t a t i o n of Cr2N in the 25 pct C r - 2 8 pct Ni-2 pct Mo a u s t e n i t i c s t e e l will b r i e f l y be s u m m a r i z e d in the following. In the s t e e l with 0.36 pct n i t r o g e n , only i n t e r g r a n u l a r p r e c i p i t a t i o n was o b s e r v e d at the aging t e m p e r a t u r e s between 700 and 900~ Intragranular p r e c i p i t a t i o n of Cr2N together with i n t e r g r a n u l a r p r e c i p i t a t i o n took place when the n i t r o g e n c o n c e n t r a tion exceeded 0.45 pct. The d e n s i t y of the i n t r a g r a n u l a r p r e c i p i t a t e s was only of the o r d e r of 101~ m ~ and t h e i r d i s t r i b u t i o n was quite i r r e g u l a r . The d e n s i t y of the i n t e r g r a n u l a r p r e c i p i t a t e s of Cr2N was v e r y low in c o m p a r i s o n with that of M2aC6 in 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 c o n t a i n i n g 0.10 pct c a r b o n . E v e n in the s t e e l with 0.63 pct n i t r o g e n , the d e n s i t y of the i n t r a g r a n u l a r p r e c i p i t a t e s r e m a i n e d of the o r d e r of 1017 m -3. It can be said that Cr2N is difficult to p r e c i p i tate in high c h r o m i u m - h i g h n i c k e l a u s t e n i t i c s t e e l s c o n t a i n i n g m o l y b d e n u m .12 One other c h a r a c t e r i s t i c f e a t u r e of the m i c r o s t r u c t u r e of the s t e e l s containing n i t r o g e n was that both p e r f e c t and faulted d i s l o c a t i o n loops w e r e f o r m e d with the d e n s i t y of 102o and 10 is m-s, respectively.% 12 Addition of 0.3 pct phosphorus greatly enhanced intrangranular precipitation of Cr2N and produced a homogeneous dispersion of the precipitates. Figures l(a) through l(d) show a series of the microstructures of this matrix precipitation of Cr2N. In Figs. l(a) and l(b), a grain boundary precipitate-free zone is observed as well as the precipitates of Cr2N at the grain boundary. It should be noted that the size of the intergranular precipitates was smaller than that of the matrix precipitates in the specimens aged at 600~ as can be seen in Figs. l(a) and l(b). The above observation strongly suggests that the denudatio
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