Influence of γ′ precipitation on the creep strength and ductility of an austenitic Fe-Ni-Cr alloy
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HE b e h a v i o r of a u s t e n i t i c i r o n - n i c k e l - c h r o m i u m a l l o y s (such as Alloy 800) u n d e r s t r e s s at elevated t e m p e r a t u r e s is of p a r t i c u l a r i m p o r t a n c e for many i n d u s t r i a l a p p l i c a t i o n s . T h e s e a l l o y s a r e h a r d e n e d by solid s o l u t i o n effects and the p r e s e n c e of c a r b o n r e s u l t s in f u r t h e r h a r d e n i n g due to the f o r m a t i o n of c a r b i d e s . P r o v i d e d s u f f i c i e n t s t a b i l i z i n g e l e m e n t s , such as t i t a n i u m , a r e p r e s e n t then the f o r m a t i o n of c h r o m i u m c a r b i d e s will be s u p p r e s s e d . F u r t h e r high t e m p e r a t u r e s t r e n g t h e n i n g may be i n t r o d u c e d by the p r e c i p i t a t i o n of the i n t e r m e t a l l i c compound b a s e d on Ni3(A1Ti) which is i s o m o r p h o u s with the face c e n t e r e d cubic m a t r i x , and is known as ~,,1 if a m o u n t s of t i t a n i u m and a l u m i n u m a r e p r e s e n t up to the s o l u b i l i t y l i m i t s . The m a x i m u m s t r e n g t h e n i n g by 7 ' o c c u r s at a r o u n d 600~ It is e v i d e n t then that c a r b i d e s will always c o n t r i bute, to s o m e extent, to the high t e m p e r a t u r e s t r e n g t h but the f o r m a t i o n of ~' will depend upon the amount of t i t a n i u m and a l u m i n u m r e m a i n i n g in solid solution a f t e r the f o r m a t i o n of c a r b i d e s and n i t r i d e s . This in t u r n will play an i m p o r t a n t r o l e in the high t e m p e r a t u r e b e h a v i o r of the alloy. The p r e s e n t work was c a r r i e d out to d e t e r m i n e this effect.
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V a r i o u s Alloy 800 c a s t s c o n t a i n i n g high (~9 : 1) and low (~4 : 1) T i / ( C + N) r a t i o s w e r e i n v e s t i g a t e d and the a n a l y s e s a r e given in T a b l e I. A high T i / ( C + N) r a t i o c o r r e s p o n d e d to a high (Ti + A1) content, as s e e n in this t a b l e . S a m p l e s of the alloys w e r e t a k e n f r o m 16 and 25
A. PLUMTREEis Associate Professor, Department of Mechanical Engineering,University of Waterloo, Waterloo, Ontario, Canada, N2L 3G1. N-G. PERSSON is Senior Research Metallurgist,Tube Department, Research Centre, Sandvik A. B., S-81101 Sandviken 1, Sweden. Manuscript submitted December 29, 1975. METALLURGICALTRANSACTIONSA
m m b a r stock and s o l u t i o n heat t r e a t e d at 1150~ for 15 m i n followed by a w a t e r quench, r e s u l t i n g in a g r a i n size of 50 :~ 5 ~ m . S t r e s s r u p t u r e s p e c i m e n s were m a c h i n e d with threaded gripped p o r t i o n s and a r e d u c e d s e c t i o n of 20 m m x 4 m m d i a m . T e s t i n g was c a r r i e d out at 600 (+2)~ or 700 (+2)~ in a i r using a constant load. T h e t i m e to r u p t u r e and the r u p t u r e s t r a i n were r e c o r d e d as a function of the n o m i n a l stress. Optical m i c r o e x a m i n a t i o n
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