Precipitation of VC in ferrite and pearlite during direct transformation of a medium carbon microalloyed steel
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s t r e n g t h l e v e l s a r e r e a c h e d to that of e q u i v a l e n t s e c o n d a r y h a r d e n i n g s t e e l s with the a c c o m p a n y i n g s a v ings of c o n s i d e r a b l e e n e r g y and c o s t in p r o d u c t i o n . Van den Steinen et al ~~ have given s o m e e v i d e n c e that the i n c r e a s e d s t r e n g t h e n i n g of m i c r o a l l o y e d m e d i u m c a r b o n s t e e l s is due to the p r e c i p i t a t i o n of a l l o y c a r b i d e s in f e r r i t e . In the p r e s e n t i n v e s t i g a t i o n , the p r e c i p i t a t e d i s p e r s i o n s which a r i s e in a continuo u s l y cooled 0.45 wt p c t C s t e e l containing 0.1 wt p c t V a r e e x a m i n e d in d e t a i l with the a i d of thin foil t r a n s m i s s i o n e l e c t r o n m i c r o s c o p y , Two a u s t e n i t i z i n g t e m p e r a t u r e s and two cooling r a t e s w e r e u s e d in o r d e r to d e t e r m i n e the l i k e l y i m p o r t a n c e of t h e s e p a r a m e t e r s in i n d u s t r i a l heat t r e a t m e n t p r a c t i c e . EXPERIMENTAL Steel of the composition shown in Table I was supplied as hot forged bar. In order to provide different cooling rates during air cooling from the austenitizing temperature and also to provide a comparison with industrial h e a t t r e a t m e n t p r o c e d u r e , 50 m m long c y l i n d r i c a l s p e c i m e n s of two d i a m , 40 and 5 m m r e s p e c t i v e l y , w e r e m a c h i n e d . A 3 m m d i a m hole was d r i l l e d f r o m one end to the c y l i n d e r s ' c e n t e r s in o r d e r to a c c o m m o d a t e a t h e r m o c o u p l e . The s p e c i m e n s w e r e induction h e a t e d to a u s t e n i t i z i n g t e m p e r a t u r e s of 1250 and 1310~ ( i n d i c a t e d by o p t i c a l p y r o m e t r y on the s p e c i m e n s u r f a c e s ) , held at t e m p e r a t u r e 10 s and s u b s e q u e n t l y a l l o w e d to cool in a i r to r o o m t e m p e r a t u r e . The t e m p e r a t u r e at the c e n t e r of the s p e c i m e n s was m o n i t o r e d continuously d u r i n g h e a t i n g and c o o l i n g by a thermocouple. M e a s u r e m e n t s of m a c r o h a r d n e s s w e r e m a d e using a W o l p e r t Httrte/pr~ifer D i a T e s t e r 2 while m i e r o h a r d n e s s m e a s u r e m e n t s w e r e m a d e with a R e i c h e r t i n s t r u m e n t fitted to a R e i c h e r t MeA o p t i c a l m i c r o s c o p e . Both i n s t r u m e n t s w e r e fitted with V i c k e r s - t y p e d i a mond p y r a m i d i n d e n t e r s . D i s c s p e c i m e n s of 3 m m d i a m , for t r a n s m i s s i o n e l e c t r o n m i c r o s c o p y at 100 kV, w e r e m a c h i n e d f r o m
Table I. Composition of the Experimental Material in Wt Pct
G. L. DUNLOP is Research Associate and C.-J. CARLSSON and C Si G. FR1MODIG were formerly undergraduate students, Department of Physics, Chalmers University of Technology, GSteborg, Sweden. 0.45 0.28 Manuscript submitted May 31, 1977. ISSN0360
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