Fracture resistance of low-carbon alloy irons
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T H E f r a c t u r e behavior of low alloy s t r u c t u r a l s t e e l is c h a r a c t e r i z e d by an abrupt b r i t t l e - t o - d u c t i l e t r a n sition which can occur over a n a r r o w t e m p e r a t u r e r a n g e , t y p i c a l l y l e s s than 75~ Below the t r a n s i t i o n t e m p e r a t u r e the l o w - e n e r g y r u p t u r e m e c h a n i s m is e i t h e r cleavage s e p a r a t i o n of g r a i n s along c r y s t a l line p l a n e s , b r i t t l e i n t e r g r a n u l a r s e p a r a t i o n or some combination of the two modes. In the t r a n s i t i o n f r o m b r i t t l e to ductile behavior the energy r e q u i r e d to f r a c t u r e a s p e c i m e n i n c r e a s e s r a p i d l y due to the i n c r e a s e in p l a s t i c deformation a s s o c i a t e d with c r a c k p r o p a g a t i o n . An engineering solution to p r e v e n t the b r i t t l e and often c a t a s t r o p h i c f a i l u r e of m e t a l s t r u c t u r e s is to specify the t r a n s i t i o n t e m p e r a t u r e to be l e s s than s e r v i c e t e m p e r a t u r e s by a r e a s o n a b l e factor of safety. L o w - t e m p e r a t u r e applications, t h e r e f o r e , have i n c r e a s e d demands for alloys with low t r a n s i t i o n t e m p e r a t u r e s without an accompanying reduction in yield s t r e n g t h s . In applications where ductile b e h a vior cannot be obtained, alloy s e l e c t i o n and design p r o c e d u r e s must be based on the b r i t t l e f r a c t u r e p r o p e r t i e s . Thus the mechanical and m e t a l l u r g i c a l f a c t o r s affecting the b r i t t l e s t r e n g t h and the b r i t t l e t o - d u c t i l e t r a n s i t i o n a r e of fundamental concern in the development and utilization of s t r u c t u r a l s t e e l s . Recent investigations 1- 3 have e m p h a s i z e d the i m p o r t a n c e of the m a t r i x phase in the f r a c t u r e p r o p e r t i e s of l o w - a l l o y s t e e l s , p a r t i c u l a r l y the b e n e f i c i a l effects of nickel aUoying. J o l l e y 1 concluded that the effects of nickel alloying a r e a s s o c i a t e d with changes in t e m p e r a t u r e and s t r a i n r a t e s e n sitivity of flow strength and in c r o s s slip b e h a v i o r . L e s l i e et al. 2 discount the t e m p e r a t u r e and s t r a i n r a t e s e n s i t i v e effects, however, and suggest that the p e r t i n e n t m e c h a n i s m is the enhancement of c r o s s L. R. HETTCHE is Head, Thermomechanical Effects Section, Metal Physics Branch, Metallurgy Division,Naval Research Laboratory, Washington, D . C . A . R . COX, formerly Exchange Scientist at the Naval Research Laboratory, is now Research Metallurgist, Royal Armament Research and Development Establishment, Fort Halstead, Sevenaks, Kent, United Kingdom. Manuscript submitted October 15, 1971. METALLURGICAL TRANSACTIONS
slip at high s t r a i n r a t e or low t e m p e r a t u r e or both. K r a f f t and G r a y s r e l a t e the t o u g h n e s s -
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