High temperature-high strain rate fracture of inconel 600
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T H E t o r s i o n t e s t for d e f o r m a t i o n p r o c e s s i n g s t u d i e s has m o s t often b e e n u s e d to m e a s u r e the flow s t r e s s at c o n s t a n t s t r a i n r a t e and t e m p e r a t u r e , or to d e t e r m i n e the o p t i m u m t e m p e r a t u r e for hot w o r k a b i l i t y by o b s e r v i n g the n u m b e r of t w i s t s to f a i l u r e . 1 T h i s r e s e a r c h i s c o n c e r n e d with the f r a c t u r e p r o c e s s in m e t a l s at high t e m p e r a t u r e s and at s t r e s s s t a t e s and s t r a i n r a t e s c o m p a r a b l e to those p r o d u c e d in hot working o p e r a t i o n s . Here the c a p a b i l i t i e s of the t o r s i o n t e s t r e m a i n r e l a t i v e l y unexploited. 2,s The object is to make a m i c r o s t r u c t u r a l study of the f r a c t u r e p r o c e s s over a wide t e m p e r a t u r e r a n g e in which d i f f e r e n t hot d e f o r m a t i o n and f r a c t u r e m e c h a n i s m s a r e e n c o u n t e r e d . The hot working conditions a r e s i m u l a t e d with a c o m b i n e d s t r e s s t o r s i o n m a c h i n e in which the s p e c i m e n m a y be s u b j e c t e d to t o r s i o n a l s h e a r s t r e s s e s with axial t e n s i l e or c o m p r e s s i v e s t r e s s e s s u p e r i m p o s e d on the t r a n s v e r s e s h e a r plane. T h i s type of t e s t i n g device is useful s i n c e p r a c t i c a l hot working p r o c e s s e s r a r e l y involve pure s h e a r , but involve complex s t r e s s s t a t e s . B r i d g m a n 4 f i r s t u s e d a c o m b i n e d s t r e s s t o r s i o n t e s t at room t e m p e r a t u r e , and showed l a r g e i n c r e a s e s in the s h e a r s t r a i n to f r a c t u r e of s t e e l s with high c o m p r e s s i v e s t r e s s e s s u p e r i m p o s e d on the t o r s i o n a l s h e a r s t r e s s e s . The a b i l i t y to v a r y the n o r m a l and s h e a r s t r e s s e s on the plane of f a i l u r e has not b e e n e m p l o y e d p r e v i o u s l y to study hot fracture.
To a s s i s t in m e t a l l o g r a p h i c s t u d i e s , s p e c i m e n s were given a g r a i n c o a r s e n i n g t r e a t m e n t at 1150~ for -~ hr r e s u l t i n g in an a v e r a g e g r a i n size of 0.25 m m .
Hot T o r s i o n Machine The t o r s i o n t e s t i n g m a c h i n e was c o n s t r u c t e d from a l a r g e lathe. D e t a i l s of the t e s t m a c h i n e have b e e n d e s c r i b e d e l s e w h e r e . 5 The b a s i c e l e m e n t s of the m a c h i n e a r e shown s c h e m a t i c a l l y in Fig. 1. The d y n a m o m e t e r m o u n t e d at the s t a t i o n a r y end of the s p e c i m e n c o n t a i n s s t r a i n gages suitably a r r a n g e d to m e a s -
Unit J
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Furnace H~
MATERIALS AND E X P E R I M E N T A L PROCEDURE Materials F r a c t u r e s t u d i e s were c a r r i e d out on hot r o l l e d Inconel 600. T h i s n i c k e l - b a s e alloy h a
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