The strength differential in a maraging steel
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KALISH
AND
Table I. Flow Strengths at 0.2 Pct Strain in 18Ni (350) Maragin 9 Steel
in a
Tensile Yield, ksi
Heat
Compressive Yield, ksi
Treatment
Longitud.
Transv.
Longitud.
Transv.
Unaged 950~ 1100~
119 331 243
130 325 237
130 365 254
151 369 237
H. J. RACK
THE
s t r e n g t h d i f f e r e n t i a l (SD) b e t w e e n the u n i a x i a l c o m p r e s s i v e and t e n s i l e flow s t r e s s e s h a s been d o c u m e n t e d f o r a v a r i e t y of m i c r o s t r u c t u r e s and p r o c e s s ing c o n d i t i o n s in c a r b o n s t e e l s . '-4 V a r i o u s t h e o r i e s of the m e c h a n i s m of the s t r e n g t h d i f f e r e n t i a l have b e e n s c r u t i n i z e d 2-5 in t e r m s of the e x p e r i m e n t a l e v i d e n c e and, at t h i s t i m e , only t h e h y p o t h e s i s of H i r t h and Cohen 5 a p p e a r s to be c o n s i s t e n t with the r e p o r t e d o b s e r v a t i o n s . T h e y p r o p o s e 5 that t h e SD i s a t t r i b u t a b l e to nonlinear elastic interactions between interstitial solute a t o m s and d i s l o c a t i o n s . The s e v e r e t e t r a g o n a l d i s t o r tion of the i r o n l a t t i c e a r o u n d an i n t e r s t i t i a l c a r b o n a t o m p r o d u c e s l o c a l e l a s t i c s t r a i n s which a r e n o n l i n e a r . T h e r e f o r e , the a t o m i c f o r c e - d i s p l a c e m e n t r e l a t i o n s h i p s a r e not the s a m e in c o m p r e s s i o n and t e n s i o n and the c a r b o n / d i s l o c a t i o n i n t e r a c t i o n e n e r g y c o n t a i n s a d i f f e r e n c e in the n o n l i n e a r p o r t i o n . A m o d e l c a l c u l a tion showed that t h e p r o p o s e d i n t e r a c t i o n can a c c o u n t for an a p p r e c i a b l e p a r t of the SD effect. H i r t h and Cohen S a l s o s u g g e s t e d that t h e n o n l i n e a r e l a s t i c i n t e r a c t i o n t h e o r y n e e d not be r e s t r i c t e d to t e t r a g o n a l d e f e c t s such a s i n t e r s t i t i a l c a r b o n in bee i r o n but could, in p r i n c i p l e , be e x t e n d e d to the i n t e r a c t i o n s of d i s l o c a t i o n s with l a r g e l o c a l s t r a i n f i e l d s f r o m o t h e r d e f e c t s . M a r a g i n g s t e e l s offer s e v e r a l m i c r o s t r u c t u r e s that cannot be d e v e l o p e d in c a r b o n - s t e e l s a n d t h e r e b y m a y give s o m e d i s c r i m i n a t i n g e x p e r i m e n t a l i n f o r m a t i o n to s u p p o r t t h e n o n l i n e a r e l a s t i c i n t e r a c t i o n h y p o t h e s i s . In t h i s s t u d y , we s e l e c t e d an 18Ni (350) M a r a g i n g s t e e l , of c o m p o s i t i o n 18.5 N i - l l . 9 C o - 4 . 7 M o - l . 5 T i - 0 . 0 9 A10.008 C, for e x a m i n i n g the SD effect. The a l l o y w a s 5 9 o b t a i n e d a s a -~ m . p l a t e . The p l a t e w a s s o l u t i o n i z e d a t 1500~ (1 h r ) and a i r c o o l e d to r o o m t e m p e r a t u r e . T e s t s p e c i m e n s w e r e m a
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