Adiabatic instability in the orthogonal cutting of steel
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THE m a r t e n s i t e
r e v e r s i o n to a u s t e n i t e d u r i n g d e f o r m a t i o n in F e - N i - C s t e e l s has been o b s e r v e d u n d e r a v a r i e t y of conditions, i n c l u d i n g h a m m e r i n g , t e n s i o n and c o m p r e s s i o n t e s t i n g , and shock h y d r o s t a t i c l o a d ing. 1'2 M o r e r e c e n t i n v e s t i g a t i o n s have shown that r e v e r s i o n d u r i n g heavy, r a p i d s h e a r i n g o c c u r s only when enough heat is g e n e r a t e d to r a i s e the t e m p e r a t u r e above A s . s L o c a l i z e d u n s t a b l e flow can lead to high t e m p e r a t u r e s and steep g r a d i e n t s . How high the r i s e in t e m p e r a t u r e is not u s u a l l y c l e a r . But with m a r t e n site r e v e r s i o n in these s p e c i a l s t e e l s it should be p o s s i b l e to p r o d u c e a m e t a l l o g r a p h i c indication that at l e a s t the A s t e m p e r a t u r e is r e a c h e d . One p r o c e s s in which t h e r m a l effects a r e not fully u n d e r s t o o d is orthogonal cutting, r e p r e s e n t e d in Fig. 1. Chips a r e t o r m e d by high s t r a i n r a t e flow over cutting tools. D i s c o n t i n u o u s chips r e s u l t when chip growth is i n t e r r u p t e d by s e p a r a t i o n on a p l a n e o r c u r v e d s u r f a c e ahead of the a d v a n c i n g tool. It has been suggested that t h e r m a l l y - a i d e d or a d i a b a t i c i n s t a b i l i t i e s , f r o m the i n t e r a c t i o n of s t r a i n r a t e , flow s t r e s s , and t e m p e r a t u r e , could be r e s p o n s i b l e for the s e r r a t e d or d i s c o n t i n u o u s chip b e h a v i o r in the m a c h i n i n g of t i t a n i u m and steel. 4 Most i n v e s t i g a t i o n s of d i s c o n t i n u o u s chip s e p a r a t i o n , however, have cons i d e r e d only the v a r i a t i o n s of s t r e s s e s on g r o w i n g c r a c k s in the r e g i o n b e t w e e n the tip of the c u t t i n g tool and the f r e e s u r f a c e of the s a m p l e and have n e glected t h e r m a l a s p e c t s . 5'6 Yet it is r e c o g n i z e d that d e f o r m a t i o n in o r t h o g o n a l cutting is confined to a n a r row s h e a r zone when the chip s t a r t s s l i d i n g up the face of the cutting tool. 7 Conditions, t h e r e f o r e , a r e f a v o r a b l e for some d e g r e e of heating, and so this p r o c e s s was chosen for study with the t e m p e r a t u r e i n d i c a t i n g m e t a l l u r g i c a l s t r u c t u r e change. S a m p l e s of t e m p e r e d m a r t e n s i t i c F e - 1 8 . 5 Ni-0.52 C s t e e l were cut at low s p e e d s , below 0.333 in. p e r sec, with d i f f e r e n t rake a n g l e s and depths of cut. R e v e r J. C. LEMAJ[RE, formerly National Science Foundation Trainee, Massachusetts Institute of Technology, Cambridge,Mass., is now with the United States Naval Reserve. W.A. BACKOFENis Professor, Department of Metallurgyand Materials Science, Massachusetts Institute of Tec
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