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Fig. 3--Solubility products, K = wt pct R 9wt pet N, of CeN and LaN in 3,-Fe compared with the solubility products of other nitrides. T,K 1373 I

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c e r i u m and l an t h an u m in y - F e , s a t u r a t e d with r e s p e c t to n i t r o g e n of e s s e n t i a l l y 1 a t m , at 1473 K a r e equal to 0.58 and 0.45 wt pct, r e s p e c t i v e l y . Without f u r t h e r knowledge of the t e r n a r y s y s t e m s i n v o l v i n g i r o n , n i t r o gen, and r a r e - e a r t h , it i s in g e n e r a l not p o s s i b l e to deduce f r o m t h e s e data the s o l u b i l i t i e s of c e r i u m o r l an t h an u m in 7 - F e of z e r o n i t r o g e n c o n c e n t r a t i o n . No data on the s o l u b i l i t i e s of r a r e - e a r t h m e t a l s in y - F e a r e known to d at e. H o w e v e r , to p r o v i d e s o m e idea of p r e v i o u s l y r e p o r t e d s o l u b i l i t i e s of c e r i u m and l a n t h a num in i r o n the m o s t r e c e n t w o r k will be m e n t i o n e d . R i c h e r d 7 g i v e s the following v a l u e s for the s o l u b i l i t i e s in o r - i r o n at 873 K: about 0.40 wt pct Ce ( p r e s u m a b l y in e q u i l i b r i u m with Ce2Fe17 a c c o r d i n g to the r e c e n t w o r k of Buschow and van W i e r i n g e n s) and about 0.09 wt pct La (in e q u i l i b r i u m with lanthanum). B e c a u s e of the l a r g e d i f f e r e n c e b et w een the a t o m i c w e i g h t s of c e r i u m and l an t h an u m and the o t h e r n i t r i d e f o r m e r s c o n s i d e r e d h e r e , the v a r i o u s s o l u b i l i t y p r o d u c t s b a s e d on a t o m p e r c e n t a r e plotted as a function of the r e c i p r o c a l t e m p e r a t u r e in F i g . 4. It i s s e e n that, on an a t o m i c b a s i s , c e r i u m and l an t h an u m a r e about a s good n i t r i d e f o r m e r s in i r o n a s v a n a d i u m , while m o r e s t a b l e n i t r i d e s a r e f o r m e d with a l u m i n u m and n i o b i u m . In a s t e e l containing 30 to 40 ppm N about 0.4 to 0.5 wt pct Ce o r La would be n e e d e d to f o r m a s t a b l e n i t r i d e at 1273 K. Since n o r m a l r a r e - e a r t h additions to s t e e l , u s u a l l y in the f o r m of " m i s c h m e t a l " , a r e of the o r d e r of 200 to 450 g / 1 0 0 0 kg (-~ to 1 l b / t o n ) of s t e e l (0.05 to 0.1 wt pct) the r a r e - e a r t h n i t r i d e s will not be s t a b l e at heat t r e a t m e n t t e m p e r a t u r e s u n d er t h e s e co ndi t i ons . 1. L. S. Darken, R. P. Smith and E. W. Filer:AIME Trans., 1951, vol. 191, pp. 1174-79. 2. R. P. Smith: Trans. TMS-AIME, 1962, vol. 224, pp. 190-91. 3. G. J. W. Kor: Met. Trans., 1972,vol.3, pp. 2705-06. 4. N. S. Corney and E. T. Turkdogan: J. Iron Steel Inst., 1955, vol. 18