The nitridation of chromium and Cr-Ti alloys
- PDF / 1,379,949 Bytes
- 7 Pages / 612 x 792 pts (letter) Page_size
- 51 Downloads / 227 Views
THOUGHc h r o m i u m
and i t s a l l o y s f o r m n i t r i d e s and e m b r i t t l e f r o m t h e n i t r o g e n in hot a i r , l i t t l e a t t e n tion h a s b e e n given the g a s - m e t a l r e a c t i o n , known a s n i t r i d a t i o n , w h e r e n i t r o g e n i s the s o l e g a s d u r i n g h i g h t e m p e r a t u r e e x p o s u r e . In 1962, H a g e l ~ u s e d a m i c r o b a l a n c e to follow the weight g a i n e d by i o d i d e c h r o m i u m , with and without 0.9 wt pct Y, in n i t r o g e n at 76 m m Hg b e t w e e n 650 ~ and 1000~ A Cr2N l a y e r g r e w i n w a r d at a p a r a b o l i c r a t e on the s u r f a c e , and no i n t e r n a l n i t r i d e s w e r e o b s e r v e d . Seybolt and H a m a n 2 s t a t e d that the r a t e of Cr2N f o r m a t i o n i s c o n t r o l l e d by n i t r o g e n diffusion t h r o u g h Cr2N and that t h i s r a t e at 1000~ i s s o m e 30 t i m e s f a s t e r than the r a t e of oxidation to Cr2Os. C h r o m i u m - b a s e a l l o y s a l s o exhibit a l m o s t 100 t i m e s g r e a t e r s o l u b i l i t y f o r n i t r o g e n than f o r oxygen and internal nitridation is a characteristic feature. Henderson et al.3 n i t r i d e d c h r o m i u m a l l o y s with up to 2 wt p c t of Ti, Ce, A1, and T a and found low b r i t t l e - d u c t i l e t r a n sition temperatures as more stable nitrides replaced Cr2N and a p p a r e n t l y s c a v e n g e d n i t r o g e n f r o m the m a t r i x . Ryan and J o h n s t o n e 4 p r e p a r e d C r - T i - N a l l o y s by adding n i t r o g e n d i r e c t l y to the m e l t and found high s t r e n g t h s and d u c t i l i t i e s . H o w e v e r , s o m e of the TiN p a r t i c l e s o b s e r v e d w e r e 1 to 2 # in width, and f i n e r s i z e s a r e d e s i r a b l e f o r effective d i s p e r s i o n s t r e n g t h ening. The o b j e c t i v e s of t h i s i n v e s t i g a t i o n w e r e : 1) to e s t a b l i s h m i c r o s c o p i c a l l y the n i t r i d a t i o n k i n e t i c s of h i g h p u r i t y c h r o m i u m and C r - T i a l l o y s , up to 5 wt p c t Ti; 2) to e x a m i n e the n i t r i d e s which f o r m and t h e i r effect on a l l o y h a r d n e s s ; 3) to p r o v i d e n i t r o g e n diffusion c o e f ficients for this system. THEORY Since i n t e r n a l o x i d a t i o n and i n t e r n a l n i t r i d a t i o n a r e e q u i v a l e n t f u n d a m e n t a l l y , e a r l i e r s t u d i e s of t h e f o r m e r were utilized for working relationships. Easily adapted i s W a g n e r ' s 5 a n a l y s i s which can be s t a t e d a s
= 27(Dt) ~/2
[1 ]
w h e r e ~ i s the depth of the i n t e r n a l - n i t r i d e zone, 7 is a c o n s t a n t at a given t e m p e r a t u r e , c o m p o s i t i o n , and g a s p r e s s u r e , D i s the diffusion c o e f f i c i e n t f o r n i t r o gen in the i n t e r n a l - n i t r i d e zone, and t i s e x p o s u r e JERRY L. ARNOLD is Research Metallurgist, Armco Steel Corporation, Middletown, Ohio
Data Loading...
