Carbide refining heat treatments for 52100 bearing steel
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The life of t h r o u g h - h a r d e n e d 52100 a n t i - f r i c t i o n b e a r i n g components is improved if the e x c e s s c a r b i d e s , undissolved during austenitization, a r e s m a l l and uniformly d i s p e r s e d . One kind of c a r b i d e - r e f i n i n g heat t r e a t m e n t c o n s i s t s of 1) dissolving a l l c a r b i d e s , 2) i s o t h e r m a l l y t r a n s f o r m i n g the austenite to p e a r l i t e or bainite, and 3) austenitizing, quenching and t e m p e r i n g in the usual manner. Each step in this sequence of t r e a t m e n t s was investigated, and the behavior of p e a r l i t i c and bainitic m i c r o s t r u c t u r e s during subsequent austenitization was c o n t r a s t e d with the b e h a v i o r of f e r r i t e / s p h e r o i d i z e d - c a r b i d e m i c r o s t r u c t u r e s . It was shown that: 1) The usual hardening t r e a t m e n t s given s p h e r o i d i z e - a n n e a l e d b e a r i n g components r e s u l t in an inhomogeneous m i c r o s t r u c t u r e , p o s s i b l y due to the f a s t e r dissolution of c a r b i d e s near austenite grain b o u n d a r i e s . 2) Austenitization of p e a r l i t e or bainite p r o d u c e s v e r y uniform d i s p e r s i o n s of u l t r a - f i n e c a r b i d e s on the o r d e r of 0.1 g m d i a m e t e r or l e s s . 3) Specimens with u l t r a - f i n e c a r b i d e s tend to have more r e t a i n e d austenite. 4) The r a t e of c o a r s e n i n g of u l t r a - f i n e c a r b i d e s at austenitizing t e m p e r a t u r e s of 840~ and below, is slow enough so that conventional furnace heat t r e a t m e n t s a r e s a t i s f a c t o r y for producing this microstructure.
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has been r e c o g n i z e d for some t i m e that the r o l l i n g contact fatigue life of b e a r i n g s t e e l s such as 52100 is b e t t e r , the s m a l l e r the size of the e x c e s s c a r b i d e p a r t i c l e s p r e s e n t in the m i c r o s t r u c t u r e . However, b e cause soft s t e e l is needed for f a b r i c a b i l i t y , the m a t e r i a l c u s t o m a r i l y supplied to b e a r i n g m a n u f a c t u r e r s is s p h e r o i d i z e d to produce r a t h e r c o a r s e c a r b i d e s in a soft f e r r i t e m a t r i x . The c a r b i d e size and d i s p e r s i o n p r e s e n t a f t e r hardening heat t r e a t m e n t r e f l e c t s to a m a r k e d d e g r e e the size and d i s p e r s i o n of c a r b i d e s in the s p h e r o i d i z e d m a t e r i a l . Thus, n e a r l y all c o m m e r c i a l l y produced 52100 b e a r i n g components have c a r b i d e s which a r e too l a r g e for the b e s t fatigue life. The intent of the p r e s e n t work is to develop heat t r e a t m e n t s which could be applied to s p h e r o i d i z e d m i c r o s t r u c t u r e s to produce hardened b e a r i n g components with uniform d i s p e r s i o n s of fine c a r b i d e s .
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Little has been published of a s y s t e m a t i c nature on the effect o
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