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T H E f o r m a t i o n of o x i d e s u n d e r n e a t h the s u r f a c e of a m e t a l is c a l l e d " i n t e r n a l o x i d a t i o n " . T h i s p r o c e s s will t a k e p l a c e d u r i n g heat t r e a t i n g a m e t a l in an e n v i r o n m e n t which contains oxygen o r i t s compounds, if 1) the m e t a l has a c e r t a i n s o l u b i l i t y f o r oxygen, 2) the m e t a l contains a l l o y i n g e l e m e n t s with a h i g h e r affinity f o r oxygen than the p a r e n t m e t a l , and 3) the diffusion r a t e of oxygen in the p a r e n t m e t a l is h i g h e r than that of such a l l o y i n g e l e m e n t s . C o m m e r c i a l s t e e l s fulfill t h e s e r e q u i r e m e n t s , a s i r o n h a s a c e r t a i n s o l u b i l i t y for oxygen, the diffusion r a t e of oxygen is g e n e r a l l y h i g h e r than that of m o s t a l l o y i n g e l e m e n t s , and s e v e r a l a l l o y i n g e l e m e n t s have a h i g h e r affinity for oxygen than i r o n . The oxygen content n e c e s s a r y f o r this p r o c e s s to o c c u r is p r o v i d e d b y the heat t r e a t m e n t m e d i a , e i t h e r a s f r e e oxygen o r f r o m CO2, H20 and so f o r t h in g a s e ous a t m o s p h e r e s o r f r o m o x i d e s , e.g. such a s in s a l t b a t h s . O x i d e s w i l l be f o r m e d if a s t e e l is t r e a t e d in a m e d i u m with an oxygen a c t i v i t y h i g h e r than that of the l o w e s t oxide at heat t r e a t m e n t t e m p e r a t u r e s . I n t e r n a l o x i d a t i o n d u r i n g c a r b u r i z i n g h a s b e e n known f o r about f o r t y y e a r s . 1 Since so f a r the oxide l a y e r was r e m o v e d by p o s t heat t r e a t m e n t m a c h i n i n g o r g r i n d i n g t h i s phenomenon and i t s h a r m f u l effect on m e c h a n i c a l p r o p e r t i e s and a s to the life of c a s e hardened parts remained academic. Since p o s t h e a t t r e a t m e n t m a c h i n i n g o r g r i n d i n g should be a v o i d e d if p o s s i b l e - o n e c o n o m i c a l c o n s i d e r a t i o n s - t h i s oxide containing l a y e r and i t s effect on the life of e n g i n e e r i n g c o m p o n e n t s has b e c o m e of inc r e a s i n g i m p o r t a n c e . H o w e v e r , this oxide l a y e r can be c o n s i d e r e d a s a d a m a g e d zone, which in g e n e r a l has a m o r e o r l e s s g r e a t effect on the p r o p e r t i e s of a part. The a i m of the i n v e s t i g a t i o n s d i s c u s s e d below was to find out if t h e r e a r e p r a c t i c a l p o s s i b i l i t i e s of a v o i d i n g o r at l e a s t r e d u c i n g t h i s p h e n o m e n o n to an RUTH CHATTERJEE-FISCHER, is Senior Research Scientist Stiftung Institut for Haerterei-Technik, Bremen, Federal Republic of Germany. This paper is based on a presentation made at a symposium on "Carburizing and Nitriding: Fundamentals, Processes and Properties" held at the Cincinnati Meeting of The Metallurgical Society of A/ME, November 11 and 12, 1975