The role of microstructure on the strength and toughness of fully pearlitic steels
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r e l a t i o n of m i c r o s t r u c t u r e to m e c h a n i c a l p r o p e r t i e s in c a r b o n s t e e l s has b e e n the s u b j e c t of cons i d e r a b l e r e s e a r c h . It is well known for e x a m p l e that i n c r e a s i n g the c a r b o n content i n c r e a s e s a s t e e l ' s s t r e n g t h , but u s u a l l y only at the expense of f r a c t u r e t o u g h n e s s . More s p e c i f i c a l l y , however, t h e r e is cons i d e r a b l e debate as to how m i c r o s t r u c t u r a l v a r i a t i o n s affect m e c h a n i c a l p r o p e r t i e s . We intend to e x a m i n e the o r i g i n of such v a r i a t i o n s for fully p e a r l i t i c e u t e c told s t e e l s . It is g e n e r a l l y a g r e e d that i n high c a r b o n n e a r - e u t e e t o i d s t e e l s , it is the p e a r l i t e r a t h e r than p r o e u t e c told f e r r i t e that c o n t r o l s s t r e n g t h , and r e f i n i n g the p e a r l i t e i n t e r l a m e l l a r s p a c i n g r e s u l t s in an i n c r e a s e in y i e l d s t r e n g t h . 1-3 However, the f r a c t u r e p r o c e s s i n p e a r l i t i c s t e e l s is l e s s well u n d e r s t o o d . T h i s is due, in p a r t , to the difficulty in i s o l a t i n g different m i c r o s t r u c t u r a l v a r i a b l e s . T h e r e have been conflicting r e s u i t s r e p o r t e d in the l i t e r a t u r e as to the effects of p e a r l i t e i n t e r l a m e l l a r spacing, 4'5 p e a r l i t e colony s i z e , 1 and p r i o r - a u s t e n i t i c g r a i n s i z e 6 on the toughness of s t e e l . D i f f e r e n c e s in c a r b o n level, alloy content, and p r o c e s s i n g conditions a m o n g the s t e e l s e x a m i n e d m a k e d e f i n i t i v e e v a l u a t i o n of the l i t e r a t u r e r e s u l t s difficult. The p r e s e n t work was u n d e r t a k e n , t h e r e f o r e , to c l a r i f y the s t r u c t u r e - p r o p e r t y r e l a t i o n s h i p s in f u l l y p e a r l i t i c s t e e l , and to identify which of the above c r i t i c a l m i c r o s t r u c t u r a l f e a t u r e s c o n t r o l the d e f o r m a t i o n and f r a c t u r e p r o c e s s e s . EXPERIMENTAL Material The m a t e r i a l used for this i n v e s t i g a t i o n was supplied by the A s s o c i a t i o n of A m e r i c a n R a i l r o a d s as hot J. M. HYZAKis Metallurgist,Air Force Materials Laboratory, (LLN), Wright Patterson Air Force Base, Dayton, OH 45433. I. M. BERNSTEIN is Associate Professor, Department of Metallurgyand MaterialsScience, Carnegie-MellonUniversity, Pittsburgh, PA 15213. Manuscript submitted September 23, 1975. METALLURGICALTRANSACTIONSA
r o l l e d r a i l s t e e l stock. The c h e m i c a l c o m p o s i t i o n was a n a l y z e d to be: C-0.81 wt pct, M n - 0 . 8 7 wt pct, P - 0 . 0 1 8 wt pct, S-0.013 wt pct, Si-0.17 wt pct and F e - B a l a n c e . Both s t a n d a r d ASTM Charpy and t e n s i l e b l a n k s were cut from the stock so that the f r a c t u r e p l a n
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