On the formation of carbide coatings by chromizing carbon steels
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THE a p p l i c a t i o n of a
c h r o m i u m diffusion coating on c a r b o n s t e e l s has c o n s i d e r a b l e t e c h n i c a l i m p o r t a n c e . Usually, c h r o m i u m is t r a n s f e r r e d to the s t e e l by m e a n s of s o m e c h r o m i u m - h a l o g e n gas which d e c o m p o s e s on the s u r f a c e and f o r m s a l a y e r of c h r o m i u m r i c h c a r b i d e s if the s t e e l contains c a r b o n . The c a r bide l a y e r will grow in t h i c k n e s s by diffusion of c h r o m i u m i n w a r d s and by diffusion of c a r b o n o u t w a r d s . The c a r b i d e l a y e r was f i r s t r e p o r t e d by S a m u e l and Lockington. 1 M e n z i e s and M o r t i m e r 2 identified CrTC~ in the l a y e r and K a r y a k i n a and D u b i n i n3 found Cr23C6 and, in addition, an e x t e r n a l l a y e r of Cr2(N, C). K o m e m , W e i s s and Niedzwiedz 4 c o n f i r m e d the e x i s t e n c e of t h e s e t h r e e compounds and for l o w - c a r b o n s t e e l s they r e p o r t e d a l a y e r of m e t a l l i c c h r o m i u m b e t w e e n CrTCz and Cr2(N, C), a l a y e r which l a t e r d i s a p p e a r s . They d i s c u s s e d the m e c h a n i s m of growth of the c a r b i d e layer m qualitative t e r m s . The p u r p o s e of the p r e s e n t work was to a n a l y z e in m o r e d e t a i l the growth of the c o m p l e x c a r b i d e l a y e r and to c o m p a r e with the e q u i l i b r i u m phase d i a g r a m . In o r d e r to obtain r e l i a b l e e x p e r i m e n t a l i n f o r m a t i o n , a s e r i e s of l o n g - t i m e c h r o m i z i n g h e a t - t r e a t m e n t s were made at 1000~ with c a r b o n s t e e l s r a n g i n g f r o m 0.2 to 1.65 wt pct c a r b o n . The t r e a t m e n t was made in a m o l t e n s a l t s y s t e m . T h i s technique has b e e n studied by C h a m p e l l e t al s and m o r e r e c e n t l y by Enghag. 6 EXPERIMENTAL The c h r o m i z i n g of the s t e e l s a m p l e s was c a r r i e d out in a s a l t m e l t c o n t a i n i n g 40 mole pct s o d i u m c h l o r i d e , 40 mole pct p o t a s s i u m c h l o r i d e and 20 mole pct c h r o m i u m d i c h l o r i d e . Sodium c h l o r i d e and p o t a s s i u m c h l o r i d e (E. Merck, p r o a n a l y s i grade) w e r e d e h y d r a t e d at 400~ u n d e r v a c u u m (
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