The Effect of shock hardening on the impact resistance of low-carbon steel
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P r i o r to the p r e s e n t i n v e s t i g a t i o n v e r y little was known about the effect of shock loading on the s u b s e quent f r a c t u r e c h a r a c t e r i s t i c s of steel. A c c o r d i n g to the C o t t r e l l - P e t c h theory, 4 ' s ' n the i n c r e a s e in f r i c t i o n s t r e s s a i should cause the n o t c h - i m p a c t and s l o w - b e n d t r a n s i t i o n t e m p e r a t u r e s to r i s e . S i m u l t a n e o u s l y , the i n c r e a s e d twin d e n s i t y should p r o v i d e additional b a r r i e r s to m i c r o c r a c k p r o p a g a t i o n and effectively r e f i n e the g r a i n s i z e , t h e r e b y i n c r e a s i n g the m i c r o c l e a v a g e s t r e n g t h a] =f(d-i/2). Thus, the effects of i n c r e a s e d twin d e n s i t y would be expected to lower the v a r i o u s t r a n s i t i o n t e m p e r a t u r e s . P r e v i o u s l y , Rosoff 8 had found that a p r e s t r a l n of 0.4 pct u n d e r slow t e n s i l e loading at - 1 9 6 ~ produced a s u f f i c i e n t i n c r e a s e in twin d e n s i t y to lower the ductility t r a n s i t i o n of lowc a r b o n i r o n by 45~ The p r e s e n t i n v e s t i g a t i o n was conducted to d e t e r m i n e the effect of very high twin d e n s i t i e s achieved by shock loading upon the y i e l d and f r a c t u r e p r o p e r t i e s J. L. WATTERS,formerly with the Department of Materials Science, Stanford University, Stanford, Calif., is with the Homer Research Laboratories, Bethlehem Steel Corp., Bethlehem, Pa. T. R. WlLSHAW,formerly with the Department of Materials Science, Stanford University, is with the Department of Materials Science, Sussex University, Brighton, England. A. S. TETELMAN,formerly with the Department of Materials Science, Stanford University, is with the Materials Department, University of California at Los Angeles, Los Angeles,Calif. Manuscript submitted February 6, 1970. METALLURGICALTRANSACTIONS
of l o w - c a r b o n s t e e l . T w i n s w e r e p r o d u c e d by explosively d e f o r m i n g 1/2 in. thick p l a t e s of a c o m m e r c i a l h o t - r o l l e d l o w - c a r b o n s t e e l (AISI- 1009). S t a n d a r d Charpy V-notch t e s t b a r s w e r e m a c h i n e d , and m e t a l lographic e x a m i n a t i o n s w e r e made. The m a t e r i a l was t e s t e d in u n i a x i a l t e n s i o n , slow bend, and i n s t r u m e n t e d i m p a c t over the a p p r o p r i a t e t e m p e r a t u r e r a n g e s . The m a t e r i a l , s p e c i m e n g e o m e t r y , and t e s t p r o c e d u r e s r e m a i n e d c o n s t a n t throughout. The only v a r i a b l e s e m ployed w e r e those a s s o c i a t e d with the s h o c k - l o a d i n g procedure. The p r i n c i p a l r e s u l t o b s e r v e d was a l a r g e i n c r e a s e in yield strength, with little or no change in t r a n s i t i o n t e m p e r a t u r e . I n c r e a s e s in both the y i e l d s t r e n g t h and cleavage f r a c t u r e s t r e n g
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