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P. C. BECKER,

AND

O. VINGSBO

D u r i n g r o l l i n g c o n t a c t fatigue of b a l l b e a r i n g s s t r u c t u r a l c h a n g e s m a y occur below the raceway, i n the r e g i o n of m a x i m u m s h e a r s t r e s s . A great n u m b e r of 6309 type b e a r i n g s w e r e t e s t e d at two s t r e s s levels for v a r y i n g n u m b e r s of r e v o l u t i o n s , and the r e s u l t i n g s t r u c t u r a l changes in the i n n e r r i n g s w e r e studied by o p t i c a l and e l e c t r o n m i c r o s c o p y . The p r e s e n t o b s e r v a t i o n s have b e e n c o m p a r e d to p r e v i o u s l y r e p o r t e d o b s e r v a t i o n s , which a r e often d e s c r i b e d in u n s y s t e m a t i c t e r m i n o l o g i e s and s o m e t i m e s a p p a r e n t l y cont r a d i c t o r y . A unified t e r m i n o l o g y is worked out, on the b a s i s of which, the s t r u c t u r a l changes a r e d e s c r i b e d . The following f e a t u r e s occur, in c h r o n o l o g i c a l o r d e r . i) A f e r r i t i c phase, containing an i n h o m o g e n e o u s l y d i s t r i b u t e d e x c e s s c a r b o n content, c o r r e s p o n d i n g to that of the p a r e n t m a r t e n s i t e . A m i x t u r e of this phase with r e s i d u a l m a r t e n s i t e c o n s t i t u t e s the well known d a r k etching r e g i o n . ii) D i s c - s h a p e d r e g i o n s of f e r r i t e , t h e r m o d y n a m i c a l l y s t a b l e , about 0.1 /zm thick, i n clined by about 30 deg to the r a c e w a y , and sandwiched b e t w e e n c a r b i d e r i c h d i s c s . T h e l a t t e r a r e c o n s t i t u t e d by v e r y s m a l l c a r b i d e p a r t i c l e s , and a r e not n e c e s s a r i l y compact. iii) A second set of l a r g e r d i s c - s h a p e d r e g i o n s about 10 ~ m thick, of p l a s t i c a l l y d e f o r m e d f e r r i t e in t h e r m o d y n a m i c e q u i l i b r i u m , f o r m i n g an angle of about 80 deg to the raceway. T r a n s f o r m a t i o n m e c h a n i s m s a r e p r o p o s e d . P a r t i c u l a r a t t e n t i o n is paid to s h o r t r a n g e c a r b o n diffusion, induced by the cyclic s t r e s s e s .

IT

is well r e c o g n i z e d that m a r k e d m i c r o - s t r u c t u r a l changes can o c c u r in s t e e l b a l l b e a r i n g s , s u b j e c t e d to cyclic s t r e s s e s . The p h e n o m e n o n has b e e n r e p o r t e d f r e q u e n t l y and is m a i n l y c h a r a c t e r i z e d by a s u b s t a n t i a l change i n etching c h a r a c t e r i s t i c s of the s t e e l . U n f o r t u n a t e l y , however, d i f f e r e n t i n v e s t i g a t o r s have used d i f f e r e n t t e r m s to identify the v a r i o u s s t r u c t u r a l e n t i t i e s o b s e r v e d . Also with few exceptions, m o s t a u t h o r s have l i m i t e d t h e m s e l v e s to optical m e t a l l o graphic t e c h n i q u e s , while those who have u s e d the e l e c t r o n m i c r o s c o p e , have s t u d i e d the s