The effect of hydrostatic pressure on the deformation behavior of maraging and HY-80 steels and its implications for pla
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R. J. SOBER,
AND
O. RICHMOND
E a r l i e r r e s u l t s showed that the d i f f e r e n c e b e t w e e n the t e n s i l e and c o m p r e s s i v e s t r e n g t h s of t e m p e r e d m a r t e n s i t e s is p r i m a r i l y a m a n i f e s t a t i o n of the g e n e r a l p r e s s u r e d e p e n d e n c e of flow s t r e s s in t h e s e m a t e r i a l s . H o w e v e r , the s a m e r e s u l t s a l s o showed that the v o l u m e e x p a n s i o n a f t e r d e f o r m a t i o n was much s m a l l e r than that p r e d i c t e d by the n o r m a l i t y flow r u l e of p l a s t i c i t y t h e o r y f o r m a t e r i a l s with s u c h p r e s s u r e d e p e n d e n c e . A d d i t i o n a l r e s u l t s now obtained on m a r a g i n g and HY-80 s t e e l s s u p p o r t t h e s e c o n c l u s i o n s . The r e s u l t s f o r a l l t h e s e m a t e r i a l s exhibit a s t r o n g , but not p e r f e c t , c o r r e l a t i o n b e t w e e n p r e s s u r e d e pendence, y i e l d s t r e s s , and v o l u m e e x p a n s i o n . The v o l u m e expansion, however, which i s b e l i e v e d to r e s u l t p r i m a r i l y f r o m the g e n e r a t i o n of new d i s l o c a t i o n s , i s v e r y s m a l l and d o e s not a p p e a r to be e s s e n t i a l to the p r e s s u r e d e p e n d e n c e . M o s t of the p r e s s u r e d e pendence, the p o r t i o n r e s p o n s i b l e for the d i s c r e p a n c y with the n o r m a l i t y flow r u l e , m a y be an effect on d i s l o c a t i o n motion. The r e s u l t s s u g g e s t that an a p p r o p r i a t e p l a s t i c i t y m o d e l would be one in which the o c t a h e d r a l s h e a r y i e l d s t r e s s i s l i n e a r l y dependent on the m e a n p r e s s u r e , but the volume change is n e g l i g i b l e in v i o l a t i o n of the n o r m a l i t y flow r u l e . Such a m o d e l has been p r o p o s e d p r e v i o u s l y for the p l a s t i c d e f o r m a t i o n of s o i l s . H o w e v e r , unlike that model, the p r e s e n t t h e o r y i n c l u d e s s t r a i n h a r d e n i n g .
IN the
i n i t i a l p a r t of this s t u d y the s t r e s s - s t r a i n c h a r a c t e r i s t i c s of t e m p e r e d AISI 4310 and 4330 s t e e l s w e r e e x a m i n e d in t e n s i o n and in c o m p r e s s i o n u n d e r h y d r o s t a t i c p r e s s u r e s up to 160 k s i 1 (1104 MPa), and v o l u m e - c h a n g e m e a s u r e m e n t s w e r e m a d e in t h e s e s a m e m a t e r i a l s a f t e r d e f o r m a t i o n at a t m o s p h e r i c p r e s s u r e . The y i e l d and flow s t r e s s e s i n c r e a s e d with h y d r o s t a t i c p r e s s u r e , but the r e l a t i v e m a g n i t u d e s by which the flow s t r e s s e s in c o m p r e s s i o n e x c e e d e d t h o s e in t e n s i o n (the s t r e n g t h d i f f e r e n t i a l , o r S - D effect) w e r e quite i n s e n s i t i v e to e i t h e r p r e s s u r e o r s t r a i n . A s p e c i f i c y i e l d condition d e r i v e d
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