The nitriding of Fe-V alloys
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T H E n i t r i d i n g of i r o n b a s e alloys c o n t a i n i n g s t r o n g n i t r i d e - f o r m i n g e l e m e n t s has been one of the c u r r e n t r e s e a r c h fields in r e c e n t y e a r s . The work on the n i t r i d i n g k i n e t i c s of Fe-Nb, ~ F e - V , 2'a F e - C r , 4 and F e - T i (Refs, 5, 6) alloys showed that the p a r a b o l i c r a t e law is obeyed. On the other hand, an a n o m a l y was o b s e r v e d in the F e - M o s s y s t e m which was l a t e r a t t r i b u t e d to the p r e s e n c e of oxygen in the n i t r i d i n g a t m o s p h e r e . 7 The h a r d n e s s of the n i t r i d e d r e g i o n i n c r e a s e s due to the f o r m a t i o n of c l u s t e r s of n i t r o g e n and s u b s t i t u t i o n a l solute a t o m s , and the t r a n s i t i o n to the bulk h a r d n e s s is s h a r p in F e - T i s'6 a l l o y s while it is g r a d u a l in F e - V (Refs. 2, 3), F e - C r 4 and F e - M o 6 alloys. The t r a n s i t i o n b e c o m e s m o r e g r a d u a l with d e c r e a s i n g the s u b s t i t u tional solute content, and this was a t t r i b u t e d to the slow diffusivity of s u b s t i t u t i o n a l s o l u t e s b e c o m i n g the r a t e c o n t r o l l i n g p r o c e s s . 3 The highest h a r d n e s s v a l u e of 1300 VHN (12.75 GNm -2) o b s e r v e d in the F e - T i a a l loys o c c u r s at about 3 wt pct Ti and it was a t t r i b u t e d to the s o l u b i l i t y l i m i t of Ti in f e r r i t e . Although t h e r e is i n f o r m a t i o n on the n i t r i d i n g k i n e t i c s of F e - V 3 a l l o y s , the p r e s e n t i n v e s t i g a t i o n was u n d e r taken to c o m p l e t e the study of the k i n e t i c s of n i t r i d i n g in this alloy with a l a r g e r r a n g e of working t e m p e r a t u r e s , to i n v e s t i g a t e much higher v a n a d i u m contents, and to check the p r o p o s i t i o n s r e g a r d i n g the m a x i m u m h a r d n e s s a t t a i n a b l e in n i t r i d e d alloys.
m i c r o s c o p e after t h e s p e c i m e n s had b e e n sectioned, polished and etched with 4 vol pct Nital. The t h i c k n e s s of the n i t r i d e d l a y e r m e a s u r e d optically, obeyed a p a r abolic t i m e d e p e n d e n c e as shown i n Figs. i and 2. A Tukon m i c r o h a r d n e s s t e s t e r was used with a 1.96 N (200 g) load e m p l o y i n g a Knoop i n d e n t e r . The h a r d n e s s of the n i t r i d e d l a y e r was o b s e r v e d to i n c r e a s e with i n c r e a s i n g v a n a d i u m content up to about 4 wt pct V, with no f u r t h e r i n c r e a s e above that value, Fig. 3. It is also n o t i c e d that the t r a n s i t i o n f r o m the h a r d n e s s value of the n i t r i d e d l a y e r to that of u n n i t r i d e d m a t r i x was g r a d ual for the 1.06 wt pct V alloy, while it was s h a r p for the other alloys with higher V c o n t e n t s for n i t r i d i n g t e m p e r a t u r e s of 773 K to 973 K, Fig. 4. On the other hand, at higher t e m p e r
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