High-rate carburizing in a glow- discharge methane plasma
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MANY
i n v e s t i g a t i o n s on v a c u u m c a r b u r i z i n g in r e c e n t y e a r s 1-a have shown that s i g n i f i c a n t s a v i n g in both p r o c e s s i n g t i m e and m a t e r i a l r e s o u r c e s a r e p o s s i b l e with a vacuum, o r r e d u c e d p r e s s u r e , method. The r e d u c e d p r o c e s s t i m e r e s u l t s chiefly f r o m the h i g h e r t e m p e r a t u r e s p e r m i s s i b l e , and the s a v i n g s in n a t u r a l gas, o r m e t h a n e , a c c r u e f r o m a) the r e d u c e d p r o c e s s t i m e , b) e l e c t r i c a l l y h e a t e d r a t h e r than g a s - f i r e d f u r n a c e s , c) e l i m i n a t i o n of the c o n v e n t i o n a l c a r r i e r gas and, f i n a l l y d) the r e d u c e d p r e s s u r e in t h e f u r n a c e . Even l o w e r p r e s s u r e s might be e m p l o y e d w e r e it not f o r the p r o b l e m of c a s e u n i f o r m i t y . T h a t is, at l o w e r p r e s s u r e s it i s difficult to o b t a i n c a s e s of u n i f o r m depth on p a r t s of i r r e g u l a r g e o m e t r y b e c a u s e the n e c e s s a r y c a r b o n p o t e n t i a l cannot be m a i n t a i n e d in deep r e c e s s e s and blind h o l e s without i n t e r m i t t e n t l y r e c h a r g i n g the f u r n a c e with c a r b u r i z i n g g a s . T h i s p a p e r d e m o n s t r a t e s that t h i s p r o b l e m can b e o v e r come by c a r b u r i z i n g at low p r e s s u r e in a g l o w - d i s c h a r g e m e t h a n e p l a s m a . The p r o c e s s , which m a y be c a l l e d p l a s m a c a r b u r i z i n g , s i n g i f i c a n t l y r e d u c e s both p r o c e s s t i m e and the n a t u r a l gas r e q u i r e m e n t s without s a c r i f i c i n g c a s e depth o r c a s e u n i f o r m i t y . It is a l s o shown that the high c a r b u r i z i n g r a t e o b s e r v e d a r i s e s chiefly f r o m the i n c r e a s e d r a t e at which c a r bon e n t e r s the s t e e l s u r f a c e , r a t h e r than f r o m a n o m a l o u s l y high diffusion r a t e s . PLASMA CARBURIZING
a s u i t a b l e p r e s s u r e in the r a n g e of 0.1 to 2.7 k P a (1 to 20 T o r r ) a g l o w - d i s c h a r g e p l a s m a * i s e s t a b l i s h e d *Plasma-A gaseous state of matter consisting of ions, electrons and both charged and neutral species of atoms and molecules, but unlike a gas is a good conductor of electricity
b e t w e e n the two by a p p l i c a t i o n of a few h u n d r e d v o l t s . U n d e r t h e s e conditions a " t i g h t " thin p l a s m a , a s shown in F i g . 2, c o m p l e t e l y e n v e l o p s the w o r k p i e c e and c l o s e l y follows i t s contour even though the s u r f a c e m a y be quite i r r e g u l a r . While 500 to 1000 V m a y be r e q u i r e d to s t r i k e the p l a s m a , depending upon the p r e s s u r e , a p p r o x i m a t e l y 400 V will s u s t a i n the p l a s m a once i n i t i a t e d in a m e t h a n e a t m o s p h e r e at 2.7 k P a (20 T o r r ) . It
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