Optimization of strength and toughness in two high-strength stainless steels
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WEBSTER
It has been found p o s s i b l e to i n c r e a s e the s t r e n g t h and t o u g h n ess of two h i g h - s t r e n g t h s t a i n l e s s s t e e l s , A F C 77 and A F C 260, by a u s t e n i t i z i n g at t e m p e r a t u r e s that a r e in the r a n g e w h e r e both a u s t e n i t e and 5 f e r r i t e a r e s t a b l e . The 5 f e r r i t e is then r e m o v e d by i s o t h e r m a l t r a n s f o r m a t i o n in the r a n g e 1800 ~ to 2000~ T h i s technique r e s u l t s in a g r e a t e r solution of c a r b i d e s and i n t e r m e t a l l i c p a r t i c l e s and c o n s e q u e n t l y in a g r e a t e r amount of r e t a i n e d a u s tenite than is p o s s i b l e at a u s t e n i t i z i n g t e m p e r a t u r e s below the 5 - f e r r i t e r a n g e . In addition, the technique p e r m i t s o p t i m u m m e c h a n i c a l p r o p e r t i e s to be obtained o v e r a w i d e r c o m p o s i tional r a n g e .
PREVIOUS work ~ has d e m o n s t r a t e d the i n c r e a s e d toughne s s and s t r e s s - c o r r o s i o n r e s i s t a n c e that m ay be p r o d u c e d in AFC 77 by c o n t r o l l e d a m o u n t s of r e t a i n e d a u s t e n i t e . The high a u s t e n i t i z i n g t e m p e r a t u r e s u s e d to i n s u r e the p r e s e n c e of a u s t e n i t e in the final s t r u c t u r e a l s o i n c r e a s e the al l o y content of the m a t r i x through g r e a t e r solution of c a r b i d e s and i n t e r m e t a l l i c p a r t i c l e s . A l i m i t on the m a x i m u m a u s t e n i t i z i n g t e m p e r a t u r e that can be used is set by the f o r m a t i o n of 5 f e r r i t e , which is known to l o w e r ductility and t o u g h n e s s in s t a i n l e s s s t e e l s . H o w e v e r , in A F C 77 and A F C 260 it has been found p o s s i b l e to r e m o v e the f e r r i t e i s o t h e r m a l l y at t e m p e r a t u r e s in the r a n g e 1800 ~ to 2000~ This allows u s e of a u s t e n i t i z i n g t e m p e r a t u r e s that a r e h i g h e r than n o r m a l , g r e a t e r r e t e n t i o n of a u s t e n i t e and, in g e n e r a l , i m p r o v e d m e c h a n i c a l p r o p e r t i e s . The object of the p r e s e n t work is to d o c u m e n t the c h a n g e s in m i c r o s t r u c t u r e and m e c h a n i c a l p r o p e r t i e s p r o d u c e d by this t e c h nique. MATERIAL Two steels--AFC 77 and AFC 260--were studied in this investigation. Both steels were developed by Crucible Steel Company under Air Force contracts. 2'3 9 The AFC 260 was obtained as -~ in. thick plate and the AFC 77 both as 3 in.-diam bar and 0.090 in.-thick sheet. Analysis showed the AFC 77 sheet, nominally 0.15 pct C, to be only 0.115 pct C, which is outsiJe the recommended range (0.13 to 0.18 pet). However, in this study the low carbon content was an advantage in that it provided a more severe test of the 5-ferrite removal technique than would be provided by material within the AFC 77 composition range. The chemical compositions of the m
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