The effects of solution and intermediate heat treatments on the notch-rupture behavior of Inconel 718
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INCONEL
718 is a widely used n i c k e l - b a s e s u p e r a l l o y p o s s e s s i n g high s t r e n g t h up to 704~ It is s t r e n g t h ened by a c o h e r e n t body c e n t e r e d t e t r a g o n a l p r e c i p i tate, y" Ni 3 (Cb, A1, Ti) but also c o n t a i n s a FCC y ' phase, c a r b i d e s , and o r t h o r h o m b i c NizCb 5 phase. 1 Inconel 718 is unique as it is v i r t u a l l y the only s u p e r alloy that is e a s i l y welded in both the a n n e a l e d and aged conditions. However, its f o r m a b i l i t y a f t e r the c o n v e n t i o n a l 954~ (1 h) s o l u t i o n heat t r e a t m e n t (SHT) is often i n a d e q u a t e . This heat t r e a t m e n t , in m a n y c a s e s , c a n n o t fully s t r e s s r e l i e v e or r e c r y s t a l l i z e this alloy, w h e r e a s a 1038~ heat t r e a t m e n t will a c c o m p l i s h t h i s . However, the 1038~ heat t r e a t m e n t r e n d e r s the alloy n o t c h - b r i t t l e in s t r e s s - r u p t u r e , a condition which, in a l m o s t e v e r y a p p l i c a t i o n , cannot be t o l e r a t e d . It g e n e r a l l y has b e e n a s s u m e d that g r a i n size and the p r e s e n c e of the u s u a l l y a c i c u l a r shaped 6 phase a r e p r i m a r y f a c t o r s c o n t r o l l i n g the notch p r o p e r t i e s of I n c o n e l 718. This i n v e s t i g a t i o n was i n i t i a t e d to v e r i f y the n o t c h - r u p t u r e p r o p e r t i e s and to d e t e r m i n e , m o r e c o m p l e t e l y , the effects of g r a i n s i z e and 5 phase d i s t r i b u t i o n . I. EXPERIMENTAL
The b a r s t o c k was m a c h i n e d into c y l i n d r i c a l notched and s m o o t h b a r s p e c i m e n s with gage l e n g t h s of 29.2 ram. The notch d i a m e t e r and gage d i a m e t e r of the notched s p e c i m e n s w e r e n o m i n a l l y 4.53 m m and 6.40 m m r e s p e c t i v e l y ; the K t of t h e s e s p e c i m e n s was a p p r o x i m a t e l y 3.5. The n o m i n a l gage d i a m e t e r of the s m o o t h b a r s p e c i m e n s was 4.51 ram. F o l l o w i n g m a chining, all s p e c i m e n s w e r e g i v e n a SHT in a r g o n for a p p r o p r i a t e t i m e s at v a r i o u s t e m p e r a t u r e s . After s o l u t i o n heat t r e a t i n g , s o m e s p e c i m e n s then r e c e i v e d an i n t e r m e d i a t e heat t r e a t m e n t (IHT) in a r g o n and all s p e c i m e n s w e r e aged in a r g o n as follows: 718~ (8 h), f u r n a c e cool at 55~ to 621~ 621~ (8 h) a i r cool. E v a l u a t i o n of the notch s t r e s s - r u p t u r e p r o p e r t i e s of this a l l o y in the v a r i o u s heat t r e a t e d c o n d i t i o n s was conducted by t e s t i n g the s p e c i m e n s at 690 M N / m z and 649~ This c o m b i n a t i o n of s t r e s s and t e m p e r a t u r e is one of the m o s t s e v e r e with r e s p e c t to notch d u e t i l i t y . 2 Smooth b a r t e s t s w e r e conducted u n d e r the s a m e c o n d i t i o n s on s e l e c t i v e l y heat
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