The effect of strain rate on pearlite morphology and microcracking
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The Effect of Strain Rate on Pearlite Morphology and Microcracking R. J. H E N R Y S T U D I E S of p e a r l i t e by o t h e r i n v e s t i g a t o r s l-s i d e n t i fied m o r p h o l o g i c a l f e a t u r e s r e s u l t i n g f r o m d e f o r m a tion but did not r e p o r t the p r e c i s e e f f e c t of i n c r e a s i n g s t r a i n r a t e on the m i c r o s t r u c t u r e . P u t t i c k ~ and B u t c h e r and P e t t i t 2 i n v e s t i g a t e d the slip m e c h a n i s m s a s s o c i a t e d with the d e f o r m a t i o n of p e a r l i t e . Danko and Stout 3 o b s e r v e d kinks in d e f o r m e d p e a r l i t e ; R o s e n f i e l d e t a l . 4 and B r u c k n e r s studied the f r a c t u r e c h a r a c t e r i s t i c s of p e a r l i t e s t e e l s . The p r e s e n t i n v e s t i g a t i o n was d es i g n e d to q u a n t i t a t i v e l y d e t e r m i n e the r e l a t i o n b e t w e e n m i c r o c r a c k i n g and s t r a i n r a t e f o r p e a r l i t e m i c r o structures. R. J. HENRY is Engineer, Alloy Development Section, Homer Research Laboratories, Bethlehem Steel Corporation, Bethlehem, Pa. Manuscript submitted May 19, 1969. METALLURGICAL TRANSACTIONS
H o t - r o l l e d 0.69 pct C and 0.90 pct Mn p l a t e s w e r e a u s t e n i t i z e d at 1600~ followed by s a l t bath i s o t h e r m a l t r e a t m e n t at 1250~ to p r o d u c e a c o m p l e t e l y p e a r l i t i c microstructure. Tensile samples were machined from the h e a t - t r e a t e d p l a t e s . The t e n s i l e s a m p l e s w e r e m e c h a n i c a l l y and e l e c t r o l y t i c a l l y p o l i s h e d 1 to b r i n g out e v i d e n c e of slip in the d e f o r m e d m i c r o s t r u c t u r e . P o l i s h i n g was followed by a light 2 pct nital etch to show the p e a r l i t i c m i c r o s t r u c t u r e . D u p l i cat e p o l i s h e d and e t c h e d s a m p l e s w e r e f r a c t u r e d at s t r a i n r a t e s of 0.98, 11.0, and 75.5 in. p e r in. p e r min. R e p l i c a s of the r e s u l t i n g f r a c t u r e f a c e s and the p o l i s h e d s u r f a c e of e a c h s p e c i m e n w e r e e x a m i n e d by m e a n s of t r a n s m i s sion e l e c t r o n m i c r o s c o p y . In addition, r e p r e s e n t a t i v e s a m p l e s w e r e e x a m i n e d with a s c a n n i n g e l e c t r o n m i c r o s c o p e . Q u a n t i t a t i v e m e a s u r e m e n t s of the a m o u n t of m i c r o c r a c k i n g within c e r t a i n p e a r l i t e c o l o n i e s w e r e m a d e by taking e l e c t r o n p h o t o m i c r o g r a p h s at 4000X of t w e l v e f i e l d s on the r e p l i c a s of s a m p l e s r e p r e s e n t i n g e a c h of the t h r e e s t r a i n r a t e s . Only those p e a r l i t e c o l o n i e s that w e r e v e r y n e a r the f r a c t u r e s u r f a c e and that c o n t a i n e d m i c r o c r a c k s w e r e p h o t o g r ap h e d. In a g r e e m e n t with the r e s u l t s of R o s e n f i e l d and Hahn, 8 it was o b s e r v e d
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