Fracture toughness of AISI M2 and AISI M7 high-speed steels
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T H E toughness of h i g h - s p e e d s t e e l is a p r o p e r t y of c o n s i d e r a b l e p r a c t i c a l i m p o r t a n c e and g e n e r a l l y i m p l i e s some m e a s u r e of the ability of the s t e e l to a b s o r b impact loads without significant m a c r o s c o p i c p l a s t i c deformation or c a t a s t r o p h i c f a i l u r e . A cutting tool which is r e q u i r e d to maintain p r e c i s e dimensional t o l e r a n c e s , frequently under conditions of i n t e r m i t t e n t cutting involving r e p e a t e d impact loading, cannot be p e r m i t t e d any l a r g e amount of i r r e v e r sible (nonelastic) d e f o r m a t i o n . F o r this r e a s o n , ductility alone is of questionable significance as a c r i t e r i o n of adequate tool p e r f o r m a n c e . The l i t e r a t u r e on toughness t e s t s for h i g h - s p e e d s t e e l is modest. P r o b a b l y the most c o m p r e h e n s i v e studies of toughness of high-speed s t e e l s have been r e p o r t e d by Grobe and Roberts.l,2 T h e i r definition of the "toughness concept" is: 1) The a b i l i t y to d e f o r m b e f o r e b r e a k i n g (plastic deflection from the bend test). 2) The a b i l i t y to r e s i s t p e r m a n e n t deformation (yield strength from the bend test). An a l t e r n a t i v e definition has been advanced by Weigand 3 who defines toughness as the amount of p l a s t i c deformation energy m e a s u r e d in a s t a t i c bend or t o r s i o n t e s t . Weigand's findings d e m o n s t r a t e that t h e r e i s not n e c e s s a r i l y a positive c o r r e l a t i o n between the toughness, so-defined, and tool life in i n t e r m i t t e n t cutting. In addition he finds no c o r r e l a tion between toughness, so-defined, and grain s i z e over a range of i n t e r c e p t grain s i z e s f r o m 5 to 20. Dulls 4 finds that at grain s i z e s finer than 20, duct i l i t y as m e a s u r e d in the bend t e s t i n c r e a s e s and t h e r e is a coincident i m p r o v e m e n t in i n t e r m i t t e n t cutting p e r f o r m a n c e . At the p r e s e n t time, it is g e n e r a l l y a g r e e d that toughness of h i g h - s p e e d s t e e l s is an i m p o r t a n t p r o p e r t y . An adequate definition of toughness, however, is s t i l l lacking. The influence of such s t r u c t u r a l p a r a m e t e r s as grain s i z e as well as the m o r phology and d i s t r i b u t i o n of c a r b i d e s on toughness is still controversial. A. R. JOHNSON is Staff Metallurgist,Climax MolybdenumCompany of Michigan,Ann Arbor, MI 48105. Manuscript submitted August ] 1, 1976. METALLURGICALTRANSACTIONS A
Recent y e a r s have w i t n e s s e d the development of a new way of viewing the concept of toughness as e m bodied in the a r t and s c i e n c e of f r a c t u r e m e c h a n i c s . F r a c t u r e mechanics has been p a r t i c u l a r l y s u c c e s s f u l in evaluating the p r o p e n s i t y of a m a t e r i a l to undergo b r i t t l e f a i l u r e when subjec
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