Residual stresses in ground steels
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AND
J. B. COHEN
A new X - r a y method for the e v a l u a t i o n of t h r e e d i m e n s i o n a l ( r e s i d u a l ) s t r e s s s t a t e s is d e m o n s t r a t e d by s t u d i e s of the effect of g r i n d i n g on A r m e o i r o n and a m e d i u m c a r b o n steel. Although the p e n e t r a t i o n depth of the C r - r a d i a t i o n employed in this study is only 5 ~ m , t h e r e is evidence of r e s i d u a l s t r e s s e s n o r m a l to the s u r f a c e ( n o r m a l and s h e a r components). In the p a s t it has been a s s u m e d that these s t r e s s c o m p o n e n t s can be n e glected. Shear s t r e s s e s n o r m a l to the s u r f a c e a r e s m a l i in A r m c o i r o n , but s i g n i f i c a n t (• 60 MPa) in steel. F r o m the sign of the s h e a r s , the d i r e c t i o n of final g r i n d i n g can be d e t e r m i n e d . Cooling d e c r e a s e s the t e n s i l e s t r e s s e s p a r a l l e l to the s u r f a c e in steel; s u r p r i s i n g l y , the opposite r e s u l t is found in A r m c o iron.
DURINGm a c h i n i n g (milling, t u r n i n g or
the p e n e t r a t i o n depth of the X - r a y b e a m . It is m o r e grinding) the s e l e c t i v e than m e c h a n i c a l methods, b e c a u s e the s t r a i n s n e a r - s u r f a c e r e g i o n of the workpiece is d e f o r m e d p l a s t i c a l l y . As v o l u m e e l e m e n t s in this r e g i o n a r e a r e a v e r a g e d over only those g r a i n s or s u b g r a l n s of a extended when the tool p a s s e s by, the c o n s t r a i n t s of phase o r i e n t e d to diffract. B e c a u s e of the s e l e c t i v i t y the bulk should i n t r o d u c e c o m p r e s s i v e s t r e s s e s n e a r of the X - r a y m e a s u r e m e n t the s t r e s s e s have to be c a l culated f r o m l a t t i c e s t r a i n s with X - r a y e l a s t i c c o n s t a n t s , 17 the s u r f a c e . Indeed, this does o c c u r for shallow, slow, which depend on the c r y s t a l l o g r a p h i c d i r e c t i o n (hkl} and w e l l - l u b r i c a t e d cuts with a s h a r p tool. But extensive s t u d i e s ~-'~ of the s t r e s s p a t t e r n have shown that the the p o l y c r y s t a l l i n e p r o p e r t i e s (coupling of g r a i n s , texactual s i t u a t i o n is u s u a l l y much m o r e complex: If ture, p l a s t i c deformation). The s t r e s s e s evaluated a r e s t r o n g work h a r d e n i n g o c c u r s only n e a r the s u r f a c e r e p r e s e n t a t i v e of the v o l u m e diffracting, and even when this p r o d u c e s t e n s i l e r e s i d u a l s t r e s s e s , due to the no m a c r o s c o p i c s t r e s s e s can be detected by d e f l e c g r e a t e r e l a s t i c r e l a x a t i o n of this r e g i o n c o m p a r e d to tion 15'21'22 Ioeal s t r e s s e s can be detected by X - r a y s . the bulk. Local c o m p r e s s i v e p l a s t i c d e f o r m a t i o n due D u r i n g g r i n d i n g t h e r e a r e n o r m a l and t a n g e n t i a l f o r c e s to the
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