Development of a high strength stainless steel with improved toughness and ductility
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s t a i n l e s s s t e e l s contain as m a j o r a l l o y i n g e l e m e n t s carbon, c h r o m i u m , cobalt, and molybdenum. T h e s e s t e e l s , which a r e typified by A FC 77 and AF C 260, a r e d e s i g n e d b a s i c a l l y as a l l o y s f or u s e at e l e v a t e d t e m p e r a t u r e s . The c o m p o s i t i o n s of A FC 77 and AF C 260 used in this p a p e r a r e given in Ta ble I. Although t h e s e s t e e l s w e r e not d e s i g n e d for o p t i m u m t o u g h n e s s , e x c e l l e n t c o m b i n a t i o n s of s t r e n g t h and toughne s s can be obtained by c a r e f u l s e l e c t i o n of the a u s t e n i t i z i n g t e m p e r a t u r e ~ which c o n t r o l s the amount of r e t a i n e d a u s t e n i t e p r e s e n t a f t e r heat t r e a t m e n t . A F C 260 p o s s e s s e s good to u g h n e s s up to its m a x i m u m s t r e n g t h but is l i m i t e d to a t e n s i l e s t r e n g t h of 265 ksi. A F C 77 can be heat t r e a t e d to s t r e n g t h s of up to 290 k s i by t e m p e r i n g at 900 ~ to l l 0 0 ~ but its toughness is l i m i t e d by the s m a l l amount of r e t a i n e d a u s t e n i t e that can be m a i n t a i n e d at t h e s e high t e m p e r i n g t e m p e r a t u r e s . The object of this work was to p r o d u c e a s t a i n l e s s s t e e l with p r o p e r t i e s s u p e r i o r to A F C 260 and A F C 77 by m i n o r c o m p o s i t i o n a l v a r i a t i o n s d e s i g n e d to take advantage of the p r e v i o u s l y d e m o n s t r a t e d b e n e fits of r e t a i n e d a u s t e n i t e ' and fine a u s t e n i t e g r a i n
6 f e r r i t e to t r a n s f o r m to a u s t e n i t e , s A f t e r quenching to r o o m t e m p e r a t u r e s p e c i m e n s w e r e then cooled to - 1 0 0 ~ for 1 hr b e f o r e t e m p e r i n g in the r an g e 500 ~ to 1200~ f o r 2 + 2 hr. (2 hr at t e m p e r a t u r e followed by an a i r cool and then a f u r t h e r 2 hr at t e m p e r a t u r e . ) Some s p e c i m e n s w e r e given an i n t e r m e d i a t e 700~ t e m p e r b e f o r e t e m p e r i n g at h i g h e r t e m p e r a t u r e s . In s o m e c a s e s double a u s t e n i t i z i n g t r e a t m e n t s w e r e u s e d in which the s a m p l e s w e r e f i r s t a u s t e n i t i z e d at 1700~ o r 2200~ and then a u s t e n i t i z e d in the r a n g e 1900 ~ to 2200~ T h e s e double t r e a t m e n t s w e r e d e s i g n e d to v a r y au st en i t e g r a i n s i z e . Mechanical Testing The t e c h n i q u e s u s e d to d e t e r m i n e f r a c t u r e t o u g h n e s s , t e n s i l e , and c o m p r e s s i v e p r o p e r t i e s on the 0.6 in. plate have been p r e v i o u s l y d e s c r i b e d 1 and need not be r e p e a t e d h e r e except to say that all s p e c i m e n s w e r e t e s t e d in the longitudinal d i r e c t i o n and the t e n s i l e elongation v a l u e s w e r e m e a s u r e d on a 1 in. gage length.
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