Influence of cold work and recovery on the strength and toughness of iron
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well known that t h e r m a l and m e c h a n i c a l p r o c e s s i n g p r o d u c e m i c r o s t r u c t u r a l c h a n g e s in s t e e l s that s t r o n g l y influence the d e f o r m a t i o n and f r a c t u r e b e havior. Various thermomechanical processing treatm e n t s have b e e n d e v e l o p e d and have p r o v e d to be a t t r a c t i v e in p r o v i d i n g s u b s t a n t i a l s t r e n g t h i m p r o v e m e n t s without a c o n c o m i t a n t l o s s of t o u g h n e s s . One of the m o r e r e c e n t a d v a n c e s in this a r e a i s the p r o c e s s known a s i s o f o r m i n g in which d e f o r m a t i o n is a p p l i e d d u r i n g i s o t h e r m a l t r a n s f o r m a t i o n to p e a r l i t e . I r a n i et al. 1'2 have shown that the i s o f o r m i n g p r o c e s s can m a r k e d l y i m p r o v e the t o u g h n e s s of s e v e r a l low a l loy s t e e l s a s c o m p a r e d to the c o n v e n t i o n a l l y r o l l e d condition. I r a n i 1 c o n c l u d e d that the i m p r o v e m e n t in t o u g h n e s s was l a r g e l y due to the p r e s e n c e of a s u b g r a i n s t r u c t u r e in the f e r r i t e p h a s e . T h i s s u b s t r u c t u r e i s p r o d u c e d by a p o l y g o n i z a t i o n p r o c e s s at the i s o f o r m i n g t e m p e r a t u r e . I s o f o r m i n g a l s o c h a n g e s the m o r p h o l o g y of the p e a r l i t e s t r u c t u r e . The c e m e n t i t e p h a s e a p p e a r s a s a fine s p h e r o i d a l p r e c i p i t a t e d i s p e r s e d u n i f o r m l y throughout a f e r r i t e m a t r i x . It is w e l l known that l a m e l l a p e a r l i t e r e d u c e s the t o u g h n e s s of f e r r i t e - p e a r l i t e s t e e l s 3 a n d the e l i m i n a t i o n of p e a r l ite by an i s o f o r m i n g t r e a t m e n t would a l s o be e x p e c t e d to i m p r o v e t o u g h n e s s . Another f a c t o r of p o s s i b l e i m p o r t a n c e in the t h e r m o m e c h a n i c a l p r o c e s s i n g of s t e e l s i s the d e v e l o p m e n t of p r e f e r r e d o r i e n t a t i o n and the p r o d u c t i o n of an e l o n g a t e d g r a i n s t r u c t u r e . F o r e x a m p l e , in the d e f o r m a t i o n p r o c e s s i n g of r e f r a c t o r y m e t a l s it has b e e n shown that an i m p o r t a n t p a r a m e t e r g o v e r n i n g f r a c t u r e b e h a v i o r i s the g r a i n s h a p e . 4 The e l o n g a t e d n a t u r e of the g r a i n s t r u c t u r e p r o d u c e d by c o l d r o l l i n g i m p a r t s s o m e d e g r e e of a n i s o t r o p y to the mechanical properties. The p r e s e n t w o r k was u n d e r t a k e n in an a t t e m p t to p r o v i d e f u r t h e r definition of s o m e of the s t r u c t u r a l M. J. ROBERTS and W. JOLLEY are Research Engineer and Supervisor, respectively, Homer Research Laboratories, Bethlehem Steel Corp., Bethlehem, Pa. Manuscript submitted October 6, 1969. METALLURGICAL TRANSACTIONS
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