A study of grain boundary segregants in thermally embrittled maraging steel
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Auger e l e c t r o n s p e c t r o s c o p y was employed to d i r e c t l y d e t e r m i n e the cause of t h e r m a l e m b r i t t l e m e n t of 250 s e r i e s m a r a g i n g s t e e l . A drop in CVN e n e r g y with introduction of an e m b r t t t l i n g i n t e r m e d i a t e anneal was d i r e c t l y c o r r e l a t e d with a build-up of both Ti and C on the p r i o r austenite grain b o u n d a r i e s . By following grain boundary concentration of Ti as a function of time at a given t e m p e r a t u r e , the diffusion coefficient for Ti in the m a r a g tng s t e e l at 1600~ was d e t e r m i n e d . Finally, the f u r t h e r d r a s t i c drop in CVN e n e r g y on aging an e m b r i t t l e d s p e c i m e n was shown to be due to a s e g r e g a t i o n of B to the a l r e a d y e m b r i t t l e d p r i o r austenite g r a i n boundaries, causing a further loss of cohesion.
MARAGIN6s t e e l s ,
developed to achieve yield s t r e n g t h s is e x c e s s of 300,000 psi, undergo s e v e r e f r a c t u r e toughness degradation when heat t r e a t e d i m p r o p e r l y . When forged in l a r g e sections at r e l a t i v e l y high t e m p e r a t u r e s (above 2000~ and allowed to cool slowly (as in the c e n t e r of l a r g e forgings), the m a t e r i a l b e c o m e s t h e r m a l l y e m b r i t t l e d . 1-3 The f r a c t u r e s a r e t y p i c a l l y i n t e r g r a n u l a r and f r a c t u r e toughness as indicated by Charpy impact e n e r g y can be lowered by an o r d e r of magnitude. Kaltsh and Rack 3 were able to d e t e r m i n e the t e m p e r a t u r e range for t h e r m a l e m b r i t tlement, but were unable to identify the cause of the e m b r i t t l e m e n t . E x t r a c t i o n r e p l i c a s f r o m the f r a c t u r e s u r f a c e s of m a t e r i a l s given a 2400~ anneal and a subsequent i n t e r m e d i a t e anneal r e v e a l e d only a few Ti2S p a r t i c l e s , which were a p p a r e n t l y undissolved p a r t i c l e s f r o m a p r e v i o u s hot working operation. Under other conditions, Ti (C, N) p r e c i p i t a t e s were observed, but it was not c l e a r whether these were r e l a t e d to an e m b r i t t l i n g heat t r e a t m e n t or were s i m p l y p r e c i p i t a t e s that had not been d i s s o l v e d during annealing. This study was undertaken to examine d i r e c t l y the r o l e of s e g r e g a t i o n in the t h e r m a l e m b r i t t l e m e n t of a 250 s e r i e s maraging s t e e l . The cause of the e m b r i t tlement was identified using Auger e l e c t r o n s p e c t r o s copy to monitor grain boundary c h e m i s t r y as a function of the d e g r e e of e m b r i t t l e m e n t . The kinetics of the e m b r i t t l e m e n t were then examined and the r e s u l t s of aging an e m b r i t t l e d s p e c i m e n d e t e r m i n e d .
ing, used to c h a r a c t e r i z e the g r a i n boundary c h e m i s t r y , a r e both adequately reviewed e l s e w h e r e , so no a t tempt will be made
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