Hot workability of nimonic 115 as a function of strain rate
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I H E e x c e l l e n t high t e m p e r a t u r e s t r e n g t h and e n g i , n e e r i n g p r o p e r t i e s of m o d e r n n i c k e l b a s e s u p e r a l l o y s have been a c h i e v e d t h r o u g h l o n g - t e r m a d v a n c e s in m e l t i n g and a l l o y i n g t e c h n i q u e s . This h a s r e s u l t e d in c o m p l e x a l l o y s c o n t a i n i n g m a n y a l l o y i n g e l e m e n t s which m u s t be p r o p e r l y b a l a n c e d to give the d e s i r e d s t r u c t u r e s and p r o p e r t i e s . Unfortunately, i n c r e a s e s in high t e m p e r a t u r e s t r e n g t h , the n u m b e r of a l l o y i n g e l e m e n t s , and the t o t a l a l l o y content have a d v e r s e l y a f f e c t e d hot w o r k a b i l i t y . In m a n y c a s e s t h e s e a l l o y s can b e hot w o r k e d o n l y u n d e r n a r r o w l y r e s t r i c t e d conditions. The t e n s i o n t e s t h a s b e e n u t i l i z e d with m e a s u r a b l e s u c c e s s to i n d i c a t e p r e f e r r e d hot w o r k i n g c o n d i t i o n s . 1-3 Grant and c o - w o r k e r s 4,s u t i l i z e d high s t r a i n r a t e , t e n sion c r e e p r u p t u r e t e s t s and found p r e d i c t a b l e p a t t e r n s of d e f o r m a t i o n and f r a c t u r e which a r e s e n s i t i v e to s t r u c t u r e , t e m p e r a t u r e and s t r a i n r a t e . A d v a n t a g e s for such t e s t s a r e a s i m p l e t e s t b a r , a s i m p l e d e f i n a b l e s t r e s s s y s t e m , high s e n s i t i v i t y to s t r a i n s , and s t r a i g h t f o r w a r d n e s s of f r a c t u r e i n t e r p r e t a t i o n . Other t e s t p r o c e d u r e s have a l s o been u s e d b r o a d l y , and a r e well r e v i e w e d by T e g a r t . 6 The p r e s e n t s t u d y was c o n c e r n e d with N i m o n i c 115, a r e c e n t l y d e v e l o p e d a l l o y , and w a s i n i t i a t e d to d e t e r m i n e o p t i m u m hot w o r k i n g c o n d i t i o n s a s a function of s t r a i n r a t e and t e m p e r a t u r e t h r o u g h c r e e p r u p t u r e testing. P r e v i o u s s t u d i e s of this t y p e have u t i l i z e d the c a s t s t r u c t u r e ; ~'6 h o w e v e r , the p r e s e n t p r o g r a m w a s c o n c e r n e d with the a l l o y in a f i n e - g r a i n e d , hot w o r k e d condition. This condition w a s s e l e c t e d b e c a u s e of d i f f i c u l t i e s e x p e r i e n c e d in f o r g i n g t u r b i n e b l a d e s f r o m the w r o u g h t m a t e r i a l . The l a s t s t a g e s in such hot w o r k i n g o p e r a t i o n s a r e c r i t i c a l and have to be p e r f o r m e d under s t r i c t l y c o n t r o l l e d c o n d i t i o n s to i n s u r e a final u n i f o r m g r a i n s i z e and r e p r o d u c i b l e high temperature properties. NILS CEDERBLAD is with AB Uddeholm, Stainless and Creep Resisting Steels Section, Degerfors J~irnverk; S-693 00 Degerfors, Sweden. NICHOLAS J. GRANT is Director, Center for Materials Science and Eng
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