Temper embrittlement of Ni-Cr steel by Sn
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A. K. CIANELLI, formerly Research Fellow, Department of Metallurgy and Materials Science, Umverslty of Pennsylvania, Philadelphm, PA is now on the Engineering Staff, Mobil Corp., Paulsboro, NJ. A. H. UCISIK, formerly Post Doctoral Fellow, Department of Metallurgy and Materials Science, University of Pennsylvama, is now on the Staff, Maden Fakueltesi, Istanbul Teknik Universitesi, Instanbul, Turkey C J. McMAHON, Jr, is Professor, Department of Metallurgy and Materials Science, University of Pennsylvania. H. C. FENG, formerly with the Research Staff, LRSM, University of Pennsylvania, Is now deceased. Manuscript submitted October 12, 1976. METALLURGICAL TRANSACTIONS A
t i c a l heat t r e a t m e n t s w e r e f r a c t u r e d at ~ - 9 0 ~ in a v a c u u m of ~10 -l~ T o r r (10 -8 Pa) and a n a l y z e d by AES as d e s c r i b e d e l s e w h e r e . 5 T h e s e f r a c t u r e s u r f a c e s w e r e s u b s e q u e n t l y e x a m i n e d by s c a n n i n g e l e c t r o n m i c r o s c o p y (SEM), and the p e r c e n t i n t e r g r a n u l a r f r a c t u r e was d e t e r m i n e d . The l a t t e r data w e r e used to weight the AES data to r e f l e c t the c o m p o s i t i o n of g r a i n b o u n d a r i e s .5 Th e p e a k h ei g h t s w e r e c o n v e r t e d to at. pct by m e a n s of c a l i b r a t i o n c u r v e s . 5'6
RESULTS AND DISCUSSION The variation of transition temperature, hardness, and grain boundary composition in the low-C alloy, which was quenched before aging at 500~ is shown in Fig. I. The grain boundary concentrations of Sn and Ni and the corresponding embrittlement reach a steady state in 400 h. This behavior is consistent with equilibrium cosegregation of Sn and Ni and is thus similar to that observed in Sb-doped Ni-Cr steels.5'7 Embrittlement at 500~ of the high-C alloy in both the quenched and slow cooled conditions is depicted in Fig. 2. The steady state in quenched specimens is reached in about 240 h. The principal effect of the slow co o l i n g t r e a t m e n t a f t e r t e m p e r i n g was to soften the s t e e l c o n s i d e r a b l y . T h i s c a u s e d a g r e a t r e d u c tion in the m a x i m u m a t t a i n a b l e e m b r i t t l e m e n t , even though the f i n a l g r a i n b o u n d a r y c o m p o s i t i o n s w e r e a l m o s t the s a m e in the q u e n c h e d and slow cooled s p e c i m e n s . T h i s is an al o g o u s to the e f f e c t of h a r d n e s s o b s e r v e d in s t e e l s doped with P (Ref. 8) and Sb (Ref. 7). Slow cooling of the l o w - C alloy a f t e r t e m p e r i n g did not p r o d u c e m e a s u r a b l e softening, and the e m b r i t t l e m e n t in the slow c o o l e d and quenched s p e c i m e n s was s i m i l a r a f t e r 400 h aging. The transition temperatures after various treatm e n t s a r e p l o t t ed a g a i n s t the g r a i n b o u n d a r y c o n c e n t r a t i o n of Sn in s p e c i m e n s of s e v e r a l h a r d n e s s l e v e l s in F i g . 3. It is a p p a r e n t that: 1) T r a n s i t i
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