The process of crack initiation and effective grain size for cleavage fracture in pearlitic eutectoid steel

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P E A R L I T E plays an important r o l e in the ductile to b r i t t l e t r a n s i t i o n behavior of f e r r i t e / p e a r l i t e s t e e l s . l ' 2 In p a r t i c u l a r , p e a r l i t e tends to promote b r i t t l e f r a c ture, and b e c a u s e of this it has been studied extens i v e l y for a number of y e a r s , 2-7 with some a s p e c t s of the p r o c e s s now well e s t a b l i s h e d and documented. 8 F o r example, it has been o b s e r v e d that in t e n s i l e s p e c i mens of f e r r i t e / p e a r l i t e s t e e l s , p e a r l i t e c r a c k s lie p r e dominantly at 45 deg to the t e n s i l e axis, in p e a r l i t e colonies whose l a m e l l a e lie p a r a l l e l to the t e n s i l e a x i s . s,5'7 In other words, the f r a c t u r e p r o c e s s is initiated by the concentration of s t r e s s on the plane of maximum r e s o l v e d s h e a r s t r e s s , which in turn p r o motes c r a c k i n g of that cementite which lies in the d i r e c t i o n of maximum tensile s t r e s s . When the s h e a r s t r e s s b e c o m e s large enough, the s m a l l c r a c k s or holes in adjoining cementite l a m e l l a e can link up to f o r m a m a c r o s c o p i c c r a c k . This mechanism is known as a s h e a r cracking p r o c e s s in p e a r l i t e , after M i l l e r and Smith. 7 However, confusion has a r i s e n in attempting to c o r r e late this s h e a r cracking p r o c e s s to the subsequent p r o c e s s of b r i t t l e cleavage f r a c t u r e . Rosenfield et al 5 and Ohmori and T e r a s a k i 9 have c l a i m e d that cleavage f r a c t u r e in f e r r i t e / p e a r l i t e s t e e l s was init i a t e d from this type of p e a r l i t e c r a c k s , which they thought to be cleavage c r a c k s . On the other hand, T e t e l m a n and McEvily ~~ concluded, f r o m the e a r l y work of Bruckner, s that the initial holes in c e m e n t i t e s would link up to form fibrous c r a c k s , r a t h e r than cleavage Y. J. PARK, formerly with Carnegie-Mellon University, Pittsburgh, PA, is now with Technical Center, Association of American Railroads, Chicago, IL 60616. I. M. BERNSTEIN is Professor, Department of Metallurgy and Materials Science, Carnegie-Mellon University, Pittsburgh, PA 15 213. Manuscript submitted February 2, 1979. METALLURGICAL TRANSACTIONS A

c r a c k s , with these c r a c k s subsequently developing into fast running cleavage c r a c k s . Because of l i m i t e d f r a c t o g r a p h i c studies p e r f o r m e d to date, it was i m p o s s i b l e to r e s o l v e this conflict without further experimentation. Another i n t e r e s t i n g subject r e l a t e d to the p r o c e s s of cleavage f r a c t u r e in p e a r l i t i c s t e e l s is the r o l e of p r i o r austenite g r a i n s t r u c t u r e . It has been e s t a b l i s h e d for a c o n s i d e r a b l e time that p r i o r anstenite g r a i n size is the dominant m i c r o s t r u c t u r a l factor controlling the f r a c t u r e toughness of p e a r l i t