Intergranular embrittlement of iron-carbon alloys by impurities
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grain boundaries at t e m p e r a t u r e s below ~ 870 K (~ 600~ However, some a s p e c t s , such as " o v e r a g i n g " b e h a v i o r , in which the d e g r e e of e m b r i t t l e m e n t p a s s e s through a maximum during i s o t h e r m a l aging, z' is, Is, zp cannot be e a s i l y r a t i o n a l i z e d on this b a s i s . The t e r m " t e m p e r e m b r i t t l e m e n t " is applied s p e c i f i c a l l y to the i n c r e a s e in d u c t i l e - b r i t t l e t r a n s i t i o n t e m p e r a t u r e in i m p u r e alloy s t e e l s which have been p r e v i o u s l y t e m p e r e d above ~ 870 K (~ 600~ and which a r e then cooled slowly through, or heated In, the t e m p e r a t u r e range ~ 820 to 620 K (~ 550 to 350~ For i s o t h e r m a l heating in this range the development of e m b r i t t l e m e n t exhibits C - c u r v e behavior on a t e m p e r a t u r e - t i m e plot, ~~176and the e m b r i t t l e m e n t can be r e m o v e d by r e h e a t i n g above ~ 870 K (~ 600~ although the t i m e for d e - e m b r i t t l e m e n t de~ends v e r y much on the e m b r l t t l t n g e l e m e n t s p r e s e n t ~ and on the t e m p e r a t u r e of reheating. ~ On the other hand, the t e r m "500~ (etc.) e m b r t t t l e m e n t " is g e n e r a l l y used to d e s c r i b e the anomalous reduction in toughness* which *Usually measured in a room temperature Charpy impact test, but also found in Klc measurements at room temperature ~ and below.4
o c c u r s in the range ~ 520 to 670 K (~ 250 to 400"C) when an as-quenched s t e e l is t e m p e r e d at s u c c e s s ively h ~ h e r t e m p e r a t u r e s . It has a l s o been d e m o n s t r a t e d ' that this phenomenon, like t e m p e r e m b r i t tlement, involves an upward shift in t r a n s i t i o n t e m p e r a t u r e for t e m p e r i n g above ~ 520K (~250~ wtth r e spect to the expected monotonic decrease in t r a n s i t i o n t e m p e r a t u r e with I n c r e a s e d t e m p e r i n g t e m p e r a t u r e (which one o b s e r v e s in a pure steelS). It has also been shown that the Impact notch toughness v a r i e s with the ttme of t e m p e r i n g , d e c r e a s i n g to a minimum value d e pending on t e m p e r a t u r e , zs Hence, in view of this s l m t l a x i t y to t e m p e r e m b r i t t l e m e n t , as well as the s i m i l a r i t i e s in f r a c t u r e mode and i m p u r i t y dependence mentioned e a r l i e r , t h e r e is r e a s o n to c o n s t r u c t the working hypothesis that the two phenomena a r e In r e a l i t y two a s p e c t s of a m o r e g e n e r a l phenomenon (or two m e m b e r s of one set). It has long been r e c o g n i z e d that the onset of 500~ e m b r i t t l e m e n t coincides wtth the p r e c i p i t a t i o n of c e mentite In the third stage of tempering, and e a r l y speculations c o n s i d e r e d that the e m b r i t t l e m e n t was connected with the f o r m a t i o n of cementtte p l a t e l e t s in p r i o r austenite g r a i n boundaries ~ or
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