Effect of internal boundaries on the yield strengths of spheroidized steels
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T H E r o l e of c e m e n t i t e p a r t i c l e s in the strengthening of s p h e r o i d i z e d p l a i n c a r b o n s t e e l s h a s been w i d e l y studied. ~-16 The s p h e r o i d i z a t i o n of c e m e n t i t e is c o n v e n t i o n a l l y a c h i e v e d b y a u s t e n i t i z i n g , quenching, and t e m p e r i n g . It h a s b e e n shown that the m i c r o s t r u c t u r a l p a r a m e t e r c o n t r o l l i n g the s t r e n g t h of q u e n c h e d - a n d t e m p e r e d low c a r b o n s t e e l s is the f e r r i t e g r a i n s i z e . ~'9 In c o n t r a s t to the p r o n o u n c e d i n t e r g r a n u l a r p l a c e m e n t of s p h e r o i d a l c a r b i d e in low c a r b o n s t e e l s , the inc r e a s e d v o l u m e f r a c t i o n of the s e c o n d p h a s e in high c a r b o n s t e e l s r e s u l t s in a p r e d o m i n a n t l y i n t r a g r a n u l a r p l a c e m e n t of the c e m e n t i t e p a r t i c l e s , and a c c o r d ingly the y i e l d s t r e n g t h s of t h e s e s t e e l s have been i n t e r p r e t e d on the b a s i s of d i s l o c a t i o n - p a r t i c l e i n t e r a c tions a c c o r d i n g to e i t h e r the Orowan bowing and b y p a s s i n g m o d e l 8 o r the A n s e l l and L e n e l p i l e - u p m o d e l (Refs. 5, 9, 11), with o r without an a d d i t i v e t e r m for g r a i n b o u n d a r y s t r e n g t h e n i n g . 8 However, s p h e r o i d i z e d s t e e l s y i e l d in the i n h o m o g e n e o u s L[iders mode, and the v a l i d i t y of u s i n g d i s l o c a t i o n m o d e l s b a s e d on q u a s i s t a t i c , homogeneous c o n d i t i o n s i s doubtful. M o r e o v e r , the d a t a used b y T y s o n 6 to show the a p p l i c a b i l i t y of the Orowan m o d e l a r e the s a m e a s that used by A n s e l l and Lenel s to e x p e r i m e n t a l l y v e r i f y t h e i r m o d e l ; both r e l y on the r e s u l t s of R o b e r t s et al ~ on high c a r b o n s t e e l s s p h e r o i d i z e d b y the q u e n c h - a n d - t e m p e r p r o c e d u r e . This p r o c e d u r e r e s u l t s in a s u b g r a i n n e t w o r k c o n n e c t ing the c e m e n t i t e p a r t i c l e s , a s r e v e a l e d by e l e c t r o n m i c r o s c o p y , ls'~7'~a and the p r e s e n c e of the s u b g r a i n b o u n d a r i e s o b s c u r e s an e v a l u a t i o n of the d i r e c t r o l e of the c e m e n t i t e p a r t i c l e s in the d e f o r m a t i o n p r o c e s s and n e c e s s i t a t e s a r e a s s e s s m e n t of p r e v i o u s r e s u l t s . L. ANAND is with the Research Center, U.S. Steel Corporation, Monroeville, Pa. J. GURLAND is with the Division of Engineering, Brown University, Providence, R.I., where L. Anand was formerly affiliated. Manuscript submitted May 19, 1975. METALLURGICAL TRANSACTIONS A
The p r e s e n t w o r k a t t e m p t s to e l u c i d a t e the s t r e n g t h ening e f f e c t s a s s o c i a t e d with g r a i n b o u n d a r i e s , s u b g r a i n b o u n d
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