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BINARYh i g h - C r

f e r r i t i c s t a i n l e s s s t e e l s have the following d e s i r a b l e p r o p e r t i e s c o m p a r e d with the widely used C r - N i a u s t e n i t i c s t a i n l e s s s t e e l s : 9 Good g e n e r a l c o r r o s i o n r e s i s t a n c e (as do the austenitics) 9 R e s i s t a n c e to f a i l u r e by s t r e s s - c o r r o s i o n c r a c k i n g 9 Lower raw m a t e r i a l cost b e c a u s e of the a b s e n c e of Ni as an a l l o y i n g e l e m e n t . The h i g h - C r f e r r i t i c s t a i n l e s s s t e e l s , however, have found little o r no use a s m a t e r i a l s of c o n s t r u c t i o n . The r e a s o n m a y be found in Fig. 1, which shows a welded c o m m e r c i a l alloy c o n t a i n i n g 26 pct Cr. In the a s welded condition, this alloy is b r i t t l e , c r a c k s when d e f o r m e d and is s u b j e c t to i n t e n s e i n t e r g r a n u l a r c o r r o sion in the a r e a of the weld. The p r o b l e m is s c h e m a t i c a l l y i l l u s t r a t e d in Fig. 2. As Cr c o n t e n t i n c r e a s e s , c o r r o s i o n r e s i s t a n c e i n c r e a s e s , z but the a s - w e l d e d d u c t i l i t y d e c r e a s e s . With this l i m i t a t i o n , only two types of h i g h - c h r o m i u m f e r r i t i c s t a i n l e s s s t e e l s have s e e n l i m i t e d use in c h e m i cal plant c o n s t r u c t i o n . One of t h e s e (Type 4 3 0 ) c o n r a i n i n g 17 pct Cr is the m o r e - w i d e l y used. It r e p r e s e n t s a c o m p r o m i s e b e t w e e n c o r r o s i o n r e s i s t a n c e and ductility. The other (Type 446) c o n t a i n i n g 26 pct Cr has found l i m i t e d u s e ; while it has good c o r r o s i o n r e s i s t a n c e , its d u c t i l i t y is poor. The p r o b l e m s a s s o c i ated with welded o r h e a t - t r e a t e d f e r r i t i c s t a i n l e s s s t e e l s have b e e n r e v i e w e d in the l i t e r a t u r e , z-4 R e s e a r c h at Du Pont o v e r the l a s t decade has b e e n d i r e c t e d towards the d e v e l o p m e n t of h i g h - C r f e r r i t i c s t a i n l e s s s t e e l a l l o y s that would have in the a s - w e l d e d condition both h i g h - c o r r o s i o n r e s i s t a n c e and good ductility. Initial r e s e a r c h s t u d i e s included the effects of c o m p o s i t i o n , heat t r e a t m e n t , cooling r a t e s , and m i c r o s t r u c t u r e on the p r o b l e m s of i n t e r g r a n u l a r c o r r o s i o n and poor d u c t i l i t y when h i g h - C r f e r r i t i c s t a i n l e s s s t e e l s were welded. These r e s u l t s have b e e n d e s c r i b e d e l s e w h e r e b y the author and other i n v e s t i g a t o r s in the field, s-8 Du Pont r e s e a r c h e r s concluded f r o m t h e i r exp e r i m e n t a l work that good a s - w e l d e d c o r r o s i o n r e s i s t a n c e and d u c t i l i t y depend on the c o n t r o l of i n t e r s t i t i a l s J. J. DEMO is a Consultant E. I. du Pont Co., EngineeringD