The mechanisms of improved creep strength in a new austenitic stainless steel
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s i n c e both the m a t r i x and the g r a i n b o u n d a r i e s m u s t be s t r e n g t h e n e d in o r d e r to r e a c h o p t i m a l c r e e p s t r e n g t h and ductility, s When r e g a r d i n g the a m o u n t s of v a n a d i u m and n i t r o gen r e s p e c t i v e l y to be added for a t t a i n i n g o p t i m a l p r o p e r t i e s , it was r e a l i z e d that a s t o i c h i o m e t r i c r a t i o would r e s u l t in the highest p r o p o r t i o n of n i t r i d e s a v a i l able for p r e c i p i t a t i o n . At the s a m e t i m e the r a t e of c o a l e s c e n c e of the n i t r i d e s would be kept at a m i n i m u m . EXPERIMENTAL 50 kg l a b o r a t o r y m e l t s of UHB STAINLESS 724LN with s t o i c h i o m e t r i c (designated alloy A) and n o n s t o i c h i o m e t r i c (designated alloy B) V : N - r a t i o s w e r e made in an induction f u r n a c e . The c o m p o s i t i o n s a r e shown in Table I. The ingots were hot forged and hot r o l l e d to 20 m m d i a m e t e r b a r s . Specimens for c r e e p t e s t s with a gauge length of 100 m m and 5 m m diam were p r e p a r e d f r o m t h e s e b a r s and solution t r e a t e d for 20 m i n at 1070~ T h r e e types of c r e e p t e s t s w e r e c a r r i e d out: - - R e c o r d i n g of s t r a i n for up to 400 h after which the f u r n a c e was switched off without unloading the s p e c i mens. - - R e c o r d i n g of s t r a i n up to f r a c t u r e . --Creep rupture tests. The f i r s t method was used for e x a m i n a t i o n of i n t e r n a l s t r u c t u r e s d u r i n g the s e c o n d a r y c r e e p stage. The other two methods were used to give data of the c r e e p p r o p e r t i e s of the new steel. S p e c i m e n s were also quenched f r o m the s e c o n d a r y c r e e p stage at 650~ and 700~ in o r d e r to make s u r e that the s t r u c t u r e s o b s e r v e d were s i g n i f i c a n t for c r e e p and did not r e s u l t f r o m cooling effect. RESULTS Creep Tests A typical c r e e p c u r v e for the s t e e l is shown in Fig. 1. A p p a r e n t l y t h e r e is no r e a l p r i m a r y c r e e p stage but a c o n s t a n t or p r o b a b l y slowly i n c r e a s i n g c r e e p r a t e is achieved a l m o s t f r o m the s t a r t . However, it is not obvious whether a t r u e s e c o n d a r y c r e e p stage is e v e r r e a c h e d . Steady state c r e e p will p r o b a b l y not o c c u r in the two alloys i n v e s t i g a t e d h e r e b e c a u s e of a s e l f g e n e r a t i n g p r e c i p i t a t i o n h a r d e n i n g r e a c t i o n which will VOLUME 6A, FEBRUARY 1975-279
a r e s i g n i f i c a n t l y s t r o n g e r than o r d i n a r y AISI 316L + 0.14N grade, at 650~ the m e a n c r e e p r a t e is a p p r o x i m a t e l y 10 t i m e s lower and the c r e e p r u p t u r e s t r e n g t h ~ m = c ( t R / 2 ) -- e (to) about 50 M N / m z higher. F r o m data p r e s e n t e d in the t R / 2 -- t o l i t e r a t u r e I~ the c r e e p p r o p e r t
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