Fracture resistance of various carburized steels
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I M P R O V E M E N T S in the m e c h a n i c a l p r o p e r t i e s of c a r b u r i z e d components depend in a complex way upon both m e t a l l u r g i c a l and m e c h a n i c a l f a c t o r s . The m e t a l l u r g i c a l f a c t o r s include both c o m p o s i t i o n a l and m i c r o s t r u c t u r a l g r a d i e n t s in the c a r b u r i z e d case. The m e c h a n i c a l f a c t o r s include: 1) a r e s i d u a l s t r e s s g r a d i e n t a r i s i n g f r o m the t r a n s f o r m a t i o n k i n e t i c s at r e l a t i v e locations within the case, 2) an applied S t r e s s g r a d i e n t depending on s p e c i m e n g e o m e t r y and the loading o r i e n t a t i o n , and 3) the s t r e s s i n t e r action b e t w e e n the case and the core as the core is loaded into the p l a s t i c r a n g e ( c a s e - c o r e i n t e r a c t i o n ) . The m a n n e r in which these g r a d i e n t s i n t e r a c t d e t e r m i n e s the m e c h a n i c a l p r o p e r t i e s of the component. E f f o r t s a r e being made to e v a l u a t e the m e c h a n i c a l p r o p e r t i e s of v a r i o u s c a r b u r i z e d EX s t e e l s intended as a l t e r n a t e choices for many of the m o r e c o m m o n g r a d e s . It has b e e n d e m o n s t r a t e d that the EX g r a d e s can have equal h a r d e n a b i l i t y . However, for s t e e l s having s i m i l a r h a r d e n a b i l i t y , the a l t e r n a t e EX s t e e l m u s t also exhibit s i m i l a r o r s u p e r i o r m e c h a n i c a l behavior. The p r e s e n t i n v e s t i g a t i o n e v a l u a t e s the m e c h a n i c a l p r o p e r t i e s of v a r i o u s p a i r s of s t e e l s having s i m i l a r h a r d e n a b i l i t y . The t e s t s used in the i n v e s t i g a t i o n were 1) a fatigue t e s t of a s p e c i m e n having a geomet r y intended to r e p r e s e n t that of a gear tooth, 2) a p l a n e - s t r a i n f r a c t u r e toughness test, and 3) a t e n s i l e t e s t of the core. Although the length of the tooth and the angle of loading of the fatigue t e s t s p e c i m e n were s e l e c t e d to f a c i l i t a t e the use of e x i s t i n g test equipment, the i m p o r t a n t d i m e n s i o n s of the s p e c i m e n r e s e m b l e those used by Hans B r u g g e r . 1 The B r u g g e r s p e c i m e n was designed to r e s e m b l e the root r a d i u s of an a c t u a l g e a r tooth, and the angle of loading r e s e m b l e d that e x p e r i e n c e d in s e r v i c e . The f r a c t u r e toughness t e s t was used to evaluate the r e l a t i v e f r a c -
D. E. DIESBURGand G. T. ELDISare SeniorResearchMetallurgistand ReSearch Supervisor,respectively,ClimaxMolybdenumCompanyof Michigan,Ann Arbor,M148105,a subsidiaryof AMAX,Inc. Thispaperis basedon a presentationmadeat a symposiumon "Carburizing and Nitriding:Fundamentals,ProcessesandProperties"held at the Cincinnati Meetingof The MetallurgicalSocietyof AIME,November11 and 12, 1975under the sponsorshipof the Heat TreatmentComm
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