Phase changes in fatigued ball bearings
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high s t r e s s contact 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 may o c c u r a f t e r high n u m b e r s of o v e r r o l l i n g s . The p h e n o m e n o n is f r e q u e n t l y r e p o r t e d in the l i t e r a t u r e ~-12 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 in e t c h i n g c h a r a c t e r i s t i c s in m e t a l l o g r a p h i c s t u d i e s of the s t e e l . M e t a l c r y s t a l c h a r a c t e r i s t i c s , such as s u r f a c e e n e r g y or g r a i n b o u n d a r y m i s f i t e n e r g y , d e t e r m i n e the topography a f t e r etching. It is thus p o s s i b l e to choose a n e t c h a n t which p r e f e r e n t i a l l y a t t a c k s for i n s t a n c e g r a i n b o u n d a r i e s . When i l l u m i n a t e d in the light m i c r o s c o p e (LM), the g r a i n s then a p p e a r as b r i g h t a r e a s , confined by d a r k e r b o r d e r s . The r a t e of d i s s o l u t i o n d u r i n g etching m a y also v a r y with c r y s t a l d i r e c t i o n , which thus m a k e s it p o s s i b l e to d i s t i n g u i s h b e t w e e n i n d i v i d u a l g r a i n s . S i m i l a r l y , if s e v e r a l p h a s e s a r e p r e s e n t , they will g e n e r a l l y etch at d i f f e r e n t r a t e s . 13,14 In a low m a g n i f i c a t i o n m i c r o g r a p h of a fatigued b a l l b e a r i n g i n n e r r i n g the p r i n c i p a l f e a t u r e is a so c a l l e d D a r k E t c h i n g Region (DER). ~2 The DER a p p e a r s in the r e g i o n of m a x i m u m H e r t z i a n s h e a r s t r e s s a f t e r a high n u m b e r of r e v o l u t i o n s , depending on the load. It has b e e n s t u d i e d by, a m o n g s t o t h e r s , O ' B r i e n and King, ~ G e n t i l e e t ale and V a s i l c a and R a s z i l l e r . 3 The DER phase is a s s u m e d to be " t r o o s t i t e " by J o n e s , 4 w h e r e a s t e m p e r e d m a r t e n s i t e , ~ low t e m p e r a t u r e b a i n i t e 6 and f e r r i t e , 7-9 have a l s o b e e n s u g g e s t e d . G e n t i l e e t a l e found that n o n m e t a l l i c i n c l u s i o n s s o m e how might c a u s e the change in e t c h i n g c h a r a c t e r i s t i c s . J o n e s 4 found a new f e a t u r e within the DER a f t e r a high n u m b e r of r e v o l u t i o n s . He r e p o r t e d this o b s e r v a t i o n as " g r e y l i n e s " . A s i m i l a r s t r u c t u r e has b e e n o b s e r v e d by Kuroda, 6 G e n t i l e e t al e and B u s h e t a l . s Still other a u t h o r s have r e p o r t e d this phase as " w h i t e e t c h i n g a r e a s " , for i n s t a n c e M a r t i n e t a l , 7 T r i c o t e t al e and Buchwald and Heckel. ~~ " N o n - e t c h i n g b a n d s " , m a i n l y c h a r a c t e r i z e d by a high r e f l e c t a n c e in the light m i c r o s c o p e , were r e p o r t e d by Lurid, n while B o r g e s e 9 identified the phase as " d e f o r m a t i o n b a n d s " . Still a n o t h e r s t r u c
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