Fatigue behavior of maraging steel 300
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SWAM,
R. M. PELLOUX,
AND
N. J. GRANT
The c y c l i c s t r e s s - s t r a i n c u r v e s , the low c y c l e and high c y c l e fatigue l i v e s and the fatigue c r a c k growth r a t e s of a n n e a l e d (1 h 820~ and aged (3 h 480~ m a r a g i n g s t e e l 300 w e r e d e t e r m i n e d . I n c r e m e n t a l s t e p t e s t i n g and s t a b l e h y s t e r e s i s loop t i p m e a s u r e m e n t s w e r e u s e d to d e t e r m i n e the c y c l i c a-E c u r v e s . Both a n n e a l e d and aged m a r a g i n g s t e e l s w e r e found to c y c l i c a l l y soften at r o o m t e m p e r a t u r e o v e r a p l a s t i c s t r a i n r a n g e f r o m 0.1 to 20 pct. The S - N c u r v e s w e r e d e t e r m i n e d f r o m 10 to 107 c y c l e s to f a i l u r e b y p l a s t i c s t r a i n c o n t r o l l e d low c y c l e fatigue t e s t s p e r f o r m e d in a i r and load c o n t r o l l e d high c y c l e fatigue t e s t s p e r f o r m e d in d r y a r g o n . The t e s t r e s u l t s c o m p a r e d v e r y w e l l with the t h e o r e t i c a l l i f e t i m e p r e d i c t i o n s d e r i v e d f r o m T o m k i n s ' t h e o r y . F a t i g u e c r a c k growth r a t e s w e r e m e a s u r e d in a i r and d r y a r g o n f o r the a n n e a l e d and aged a l l o y s . C r a c k growth r a t e s of a n n e a l e d m a r a g i n g s t e e l w e r e found to b e equal to t h o s e of aged m a r a g i n g s t e e l at r a t e s b e t w e e n 10 -7 and 10 -s i n . / c y c l e . A s i g n i f i c a n t d i f f e r e n c e in c r a c k growth r a t e s in the two e n v i r o n m e n t s was found at low s t r e s s i n t e n s i t y f a c t o r r a n g e s , i n d i c a t i n g a high s u s c e p t i b i l i t y to c o r r o s i o n fatigue in the p r e s e n c e of w a t e r v a p o r . The m e c h a n i s m s of c y c l i c s o f t e n i n g in the two a l l o y s a r e d i s c u s s e d in t e r m s of d i s l o c a t i o n s r e a r r a n g e m e n t in the a n n e a l e d a l l o y and d i s l o c a t i o n - p r e c i p i t a t e i n t e r a c t i o n s in the aged alloy.
MARAG~Gs t e e l s
a r e a c l a s s of high a l l o y s t e e l s with a v e r y low c a r b o n content which use s u b s t i t u t i o n a l e l e m e n t s to p r o d u c e p r e c i p i t a t i o n - h a r d e n i n g in i r o n n i c k e l m a r t e n s i t e s . 1 The v e r y e f f e c t i v e h a r d e n i n g of c o b a l t and m o l y b d e n u m a d d i t i o n s to the i r o n - n i c k e l m a r t e n s i t e was d i s c o v e r e d b y D e c k e r , e t al. 2 and it r e s u l t e d in the d e v e l o p m e n t of 200, 250 and 300 g r a d e s of m a r a g i n g s t e e l s . All t h e s e s t e e l s with a c a r b o n c o n t e n t b e l o w 0.03 p c t contain 18 pct n i c k e l plus d i f f e r e n t a m o u n t s of c o b a l t , m o l y b d e n u m , and t i t a n i u m . The n o m i n a l c o m p o s i t i o n of m a r a g i n g s t e e l 300 is 18 p c t Ni, 9 p c t Co, 4.5 pct Mo, 0.6 p c t Ti,
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