Embrittlement of a continuously cooled Fe-25 Cr alloy
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i s known that the d u c t i l e - b r i t t l e i m p a c t t r a n s i t i o n t e m p e r a t u r e i s higher for a f e r r i t i c c h r o m i u m s t e e l in the a i r c o o l e d as o p p o s e d to the w a t e r - q u e n c h e d condition. P l u m t r e e and G u l l b e r g 1 showed that the p r e s e n c e of l a r g e r a m o u n t s of h e t e r o g e n e o u s l y d i s t r i b u t e d p r e c i p i t a t e s , m a i n l y Cr2N at the g r a i n b o u n d a r i e s , c a u s e d an i n c r e a s e in the d u c t i l e - b r i t t l e i m p a c t t r a n s i t i o n t e m p e r a t u r e of v a r i o u s F e - 2 5 p c t C r s t e e l s . T h i s effect was shown by i n c r e a s i n g the c a r b o n and n i t r o g e n content of the s t e e l s t e s t e d in t h e w a t e r quenched condition. H o w e v e r , t h i s i n c r e a s e d t e n d e n c y t o w a r d s b r i t t l e n e s s could a l s o be i n d u c e d b y a i r cooling m a t e r i a l with a c o n s t a n t c a r b o n and n i t r o gen content. In o r d e r to r e s o l v e w h e t h e r 475~ e m b r i t t l e m e n t was p l a y i n g a s i g n i f i c a n t r o l e d u r i n g v e r y s l o w cooling, the p r e s e n t w o r k w a s u n d e r t a k e n . T h e c a u s e of 475~ e m b r i t t l e m e n t i s the p r e c i p i t a t i o n of a body c e n t e r e d cubic c h r o m i u m - r i c h p h a s e known a s a l p h a p r i m e . 2-5 T h e t e m p e r a t u r e l i m i t s f o r t h e a p p e a r a n c e of t h i s p h a s e can be a s high a s 550~ %7 and a s low a s 385~ f o r long h e a t i n g t i m e s . 6's Howe v e r , a n o t h e r type of e m b r i t t l e m e n t has b e e n r e p o r t e d s i m i l a r to that o b s e r v e d by P l u m t r e e and G u l l b e r g , 1 w h i c h h a s been a t t r i b u t e d to the i n t e r g r a n u l a r p r e c i p i t a t i o n of c h r o m i u m c a r b o n i t r i d e s a f t e r e x p o s u r e in the t e m p e r a t u r e r a n g e of 540~ to 760~ 9-z2 o r on f u r n a c e cooling t h r o u g h t h e r a n g e of 900~ to 600~ E X P E R I M E N T A L PROCEDURE A l l the e x p e r i m e n t s w e r e c a r r i e d out on an F e - 2 5 p c t C r a l l o y of the c o m p o s i t i o n g i v e r / i n T a b l e I which w a s m e l t e d and c a s t u n d e r high v a c u u m . T h i s a l l o y w a s f i r s t hot f o r g e d to 20 m m and then cold r o l l e d to 14.2 m m (i.e. 50 pct r e d u c t i o n of a r e a ) . I n i t i a l l y the cold r o l l e d m a t e r i a l w a s h e a t t r e a t e d
a t 850~ f o r 30 m i n . One b a t c h of s p e c i m e n s w a s w a t e r quenched f r o m 850~ a s e c o n d w a s a i r cooled and a t h i r d b a t c h was f u r n a c e c o o l e d at the r a t e of 30~ to 400~ and then a i r c o o l e d . T h e w a t e r quenched s p e c i m e n s w e r e d i v i d e d into t h r e e s e p a r a t e g r o u p s - t h e f i r s t was s u b s e q u e n t l y a g e d at 475~ the s e c o n d group w a s a g e d at 550~ and t h e l a s t w a
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