Static strain aging in an Fe-25 pct Cr alloy
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gion and s u b s e q u e n t l y unloaded c o m p l e t e l y . Aging was then c a r r i e d out in an oil bath for v a r i o u s t i m e s at 100 ~ 124 ~ 135 ~ and 150~ After aging, e a c h s p e c i men was m o u n t e d in the I n s t r o n m a c h i n e and r e s t r a i n e d at r o o m t e m p e r a t u r e (23~ Since t h e s e t e s t s i n v o l v e d r e m o v i n g the s p e c i m e n f r o m the t e n s i l e m a c h i n e , the m a c h i n e - s p e c i m e n a l i g n m e n t was d i s t u r b e d . As a c o n s e q u e n c e , the l o w e r y i e l d s t r e s s (%) a f t e r aging was u s e d in the s t r a i n aging p a r a m e t e r given b y A ~ / ~ o r 2 (% - Crp)/(% + ~p). The t e r m Aa o r (as -- ap) is the d i f f e r e n c e b e t w e e n the l o w e r y i e l d s t r e s s , % , and the flow s t r e s s on p r e s t r a i n i n g , _ap, w h e r e a s ~ is the a r i t h m e t i c a v e r a g e of the two [ = 89(% + ap)]. A c c o r d i n g to H a r t l e y ' s m o d i f i c a t i o n 7 of the C o t t r e l l - B i l b y r e l a t i o n , 8 the r a t e of i n c r e a s e of the y i e l d s t r e s s due to aging is r e l a t e d to the r a t e of s e g r e g a t i o n of i n t e r s t i t i a l s to d i s l o c a t i o n s t h r o u g h the equation: [1]
A a / ~ = g + St als
Static Strain Aging in an Fe-25 Pct Cr Alloy R. G U L L B E R G AND A. P L U M T R E E S T R A I N aging in f e r r o u s m a t e r i a l s , p a r t i c u l a r l y f e r r i t e , h a s r e c e i v e d much a t t e n t i o n and h a s b e e n the s u b j e c t of r e c e n t r e v i e w s b y B a i r d 1 and Hall. 2 Most of the w o r k h a s , quite n a t u r a l l y , c e n t e r e d upon aging in r e l a t i v e l y p u r e i r o n due to its i n d u s t r i a l i m p o r t a n c e . G e n e r a l l y , a d d i t i o n s of s m a l l a m o u n t s of s t r o n g n i t r i d e and c a r b i d e f o r m e r s to r e l a t i v e l y p u r e i r o n m a r k e d l y r e d u c e the s t a t i c s t r a i n aging p o t e n t i a l at t e m p e r a t u r e s below 200~ s'4 y e t on c o n s i d e r i n g c h r o m i u m a d d i t i o n s , E d w a r d s et al. s showed that 6 p c t C r was n e e d e d to s u p p r e s s s t r a i n aging. G a r o f a l o et al. 6 found that s t r a i n aging i n c r e a s e d above 12 p c t Cr but it was not c l e a r if t h i s effect was due to c a r b i d e o r to the c h r o m i u m a t o m s , a s no a c t i v a t i o n e n e r g y d e t e r m i n a tion was m a d e . R e c e n t l y , i n t e r e s t h a s b e e n shown in f e r r i t i c s t e e l s containing b e t w e e n 18 and 28 p c t Cr with a s m a l l i n t e r s t i t i a l content. The p r e s e n t s t u d y was c o n ducted to obtain a b e t t e r u n d e r s t a n d i n g of the s t r a i n aging m e c h a n i s m in t h i s group of s t e e l s and was b a s e d on s a m p l e s containing a p p r o x i m a t e l y 25 p c t C r . Two m e l t s of f e r r i t i c c h r o m i u m s t e e l w e r e p r e p a r e d b y v a c
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