Influence of carbon content and ferrite morphology on the sensitization of duplex stainless steel
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r e m a r k a b l e r e s i s t a n c e of a u s t e n i t i c - f e r r i t i c duplex s t a i n l e s s s t e e l s to i n t e r g r a n u l a r c o r r o s i o n as a t e d with p r e c i p i t a t i o n of c h r o m i u m - r i c h c a r b i d e s has b e e n known for s o m e t i m e 1-3 Such a l l o y s have also been found r e s i s t a n t to i n t e r g r a n u l a r s t r e s s c o r r o s i o n c r a c k i n g in s e v e r a l aqueous e n v i r o n m e n t s following aging t r e a t m e n t s known to c a u s e s e n s i t i z a tion in fully 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 . 4-6 P r e v i o u s p u b l i c a t i o n s have d e s c r i b e d the m e c h a n i s m s of i n t e r g r a n u l a r c a r b i d e p r e c i p i t a t i o n %8 and i n t e r g r a n u l a r c o r r o s i o n of duplex s t a i n l e s s s t e e l s . 5,8 The model s t a t e s that the r e s i s t a n c e of the alloy to s e n s i t i z a t i o n should be r e l a t e d to the a m o u n t and d i s t r i b u t i o n of f e r r i t e - a u s t e n i t e ( a - y ) b o u n d a r y a r e a . 8 In this i n v e s t i g a t i o n we e x a m i n e the i n t e r g r a n u l a r c o r r o s i o n b e h a v i o r of duplex 308 s t a i n l e s s s t e e l with d i f f e r e n t f e r r i t e m o r p h o l o g i e s i n an a t t e m p t to d e t e r m i n e the f u n c t i o n a l r e l a t i o n s h i p b e t w e e n the i n t e r g r a n u l a r c o r r o s i o n r e s i s t a n c e of the alloy and its m i c r o s t r u c t u r e and c h e m i s t r y . PROCEDURE T h r e e c o m m e r c i a l l y p r o d u c e d h e a t s and six l a b o r a t o r y heats of 308 s t a i n l e s s s t e e l whose c o m p o s i t i o n s a r e l i s t e d in T a b l e I were used to e v a l u a t e the effects of aging t r e a t m e n t on the i n t e r g r a n u l a r c o r r o s i o n b e h a v i o r of wrought 308 s t a i n l e s s s t e e l . Heat L - B 7 was obtained in the f o r m of as cold d r a w n 0.114 cm d i a m w i r e . Heats L - P 1 and L - P 2 were obtained in the as cold d r a w n condition i n 91 c m lengths with d i a m e t e r s of 0.158 and 0.236 era, r e T. M. DEVINE, Jr. is Metallurgist, Metallurgy Laboratory, Corporate Research and Development Center, General Electric Company, Schenectady NY, 12301. Manuscript submitted October 23, 1979. METALLURGICAL TRANSACTIONSA
s p e c t i v e l y . The t h r e e l a b o r a t o r y heats w e r e cast as 13.6 kg t a p e r e d s q u a r e s , 7.6 c m 2 • 5.7 c m 2 • 28 cm long, hot forged at 1200~ to 3.8 x 6.4 cm, r e c t a n g u l a r c r o s s s e c t i o n , hot r o l l e d to 0.229 c m t h i c k n e s s at 1175~ and cold r o l l e d to 0.127 c m thick s h e e t . The sheet was t h e n a n n e a l e d at 1100~ w a t e r q u e n c h e d and s h e a r e d into t e s t coupons 17.8 x 1.3 c m . S a m p l e s were e l e c t r o p o l i s h e d in a s o l u t i o n of 60 pct H~PO4 + 40 pct H2SO4 at 40 to 60~ u s i n g ~ 2 a m p s / c m ~ for 10 m i n . Following e l e c t r o p
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