Fatigue behavior of a nial precipitation hardening medium carbon steel
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R. A. FOURNELLE
T h e fatigue b e h a v i o r of an F e - 0 . 3 0 C - 4 . 4 8 N i - l . 3 2 A 1 s t e e l t e m p e r e d to give t h r e e d i f f e r e n t m i c r o s t r u c t u r e s of the s a m e u l t i m a t e t e n s i l e s t r e n g t h h a s b e e n i n v e s t i g a t e d by light and e l e c t r o n m i c r o s c o p y , low and high c y c l e fatigue t e s t s , X - r a y line b r o a d e n i n g and s t r e s s r e l a x a t i o n m e a s u r e m e n t s . T h e t h r e e d i f f e r e n t heat t r e a t m e n t s p r o d u c e d the following s t r u c t u r e s : I) a c o n v e n t i o n a l quenched and t e m p e r e d m i c r o s t r u c t u r e with a high d e n s i t y of d i s l o c a t i o n s and e l o n g a t e d c a r b i d e s , H) a m i c r o s t r u c t u r e of high d i s l o c a t i o n d e n s i t y , c o a r s e c a r b i d e s and fine c o h e r e n t NiA1 p r e c i p i t a t e s and III) a highly t e m p e r e d m i c r o s t r u c t u r e with a r e c o v e r e d d i s l o c a t i o n s u b s t r u c t u r e , c o a r s e c a r b i d e s and fine c o h e r e n t NiA1 p r e c i p i t a t e s . In low c y c l e , s t r a i n c o n t r o l l e d fatigue c y c l i c s o f t e n i n g in T r e a t m e n t I was a c c o m p a n i e d b y a r e a r r a n g e m e n t of the d i s l o c a t i o n s u b s t r u c t u r e and a r e d u c t i o n in both the i n t e r n a l s t r e s s and l a t t i c e m i c r o s t r a i n . T r e a t m e n t II, which r e m a i n e d c y c l i c a l l y s t a b l e d u r i n g the i n i t i a l p o r t i o n of the fatigue life, showed l i t t l e change in the i n t e r n a l s t r e s s and d i s l o c a t i o n d e n s i t y and showed a s l i g h t i n c r e a s e in l a t t i c e m i c r o s t r a i n . T r e a t m e n t IH, which i n i t i a l l y c y c l i c a l l y h a r d e n e d , e x h i b i t e d a r i s e in i n t e r n a l s t r e s s , l a t t i c e m i c r o s t r a i n and d i s l o c a t i o n d e n s i t y . The b e h a v i o r of T r e a t m e n t s II and III is a t t r i b u t e d in p a r t to the p r e s e n c e of the fine NiA1 p r e c i p i t a t e s which a p p e a r to r e d u c e the t e n d e n c y of the t r a n s f o r m a t i o n induced d i s l o c a t i o n s u b s t r u c t u r e to r e a r r a n g e i t s e l f into a c e l l s t r u c t u r e d u r i n g f a t i g u e . In high c y c l e , s t r e s s c o n t r o l l e d fatigue T r e a t m e n t II showed the b e s t fatigue r e s i s t a n c e and T r e a t m e n t I the w o r s t . I m p r o v e m e n t in life was a t t r i b u t e d to i m p r o v e d r e s i s t a n c e to c r a c k i n i t i a t i o n .
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UENCHED, m e d i u m c a r b o n s t e e l s t e m p e r e d above C (473 K) a r e c y c l i c a l l y u n s t a b l e and fatigue s o f t e n , ~,2 Softening has b e e n a t t r i b u t e d to 1) the r e a r r a n g e m e n t of the t r a n s f o r m a t i o n induced d i s l o c a t i o n s u b s t r u c t u r e into a d i s l o c a t i o n c
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