Influence of heat treatment on the fatigue crack growth rates of a secondary hardening steel
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The r e l a t i o n s h i p s between m i c r o s t r u c t u r e and fatigue c r a c k propagation b e h a v i o r w e r e studied in a 5Mo-0.3C s t e e l . M i c r o s t r u c t u r a l d i f f e r e n c e s w e r e achieved by v a r y i n g the t e m p e r i n g t r e a t m e n t . The amounts, d i s t r i b u t i o n , and types of c a r b i d e s p r e s e n t were influenced by the t e m p e r i n g t e m p e r a t u r e . Optical m e t a l l o g r a p h y and t r a n s m i s s i o n e l e c tron m i c r o s c o p y were used to c h a r a c t e r i z e the m i c r o s t r u c t u r e s . Fatigue f r a c t u r e s u r faces were studied by scanning e l e c t r o n m i c r o s c o p y . F o r each heat t r e a t m e n t the fatigue c r a c k growth p r o p e r t i e s were m e a s u r e d under plane s t r a i n conditions using a compact tension f r a c t u r e toughness s p e c i m e n . The p r o p e r t i e s were r e p o r t e d using the e m p i r i c a l r e l a t i o n of P a r i s [ d a / d N = C ~ K ~ ] . It was found that s e c o n d a r y hardening did influence the fatigue crack growth r a t e s . In p a r t i c u l a r , i n t e r g r a n u l a r modes of f r a c t u r e during fatigue led to e x a g g e r a t e d fatigue c r a c k growth r a t e s for the t e m p e r i n g t r e a t m e n t p r o ducing peak h a r d n e s s . L i m i t e d t e s t i n g in a dry argon a t m o s p h e r e showed that the s e n s i tivity of fatigue c r a c k growth r a t e s to environment changed with heat t r e a t m e n t .
FATIGUE
c r a c k propagation behavior is v e r y i m portant in d e t e r m i n i n g the u s e f u l n e s s of high strength s t e e l s when they a r e subjected to cyclic loading. The c r a c k initiation period, which adds significantly to the life in conventional fatigue s t u d i e s , is often absent in the high strength m a t e r i a l s because of p r e e x i s t i n g flaws, such as inclusions. Because of i t s s u c c e s s f u l use in p r e d i c t i n g f r a c t u r e conditions under s t a t i c loading, l i n e a r e l a s t i c f r a c t u r e m e c h a n i c s has been applied to c r a c k propagation. P a r i s and his c o l leagues 1'2 have proposed an e m p i r i c a l equation r e lating the s t r e s s intensity, K, to c r a c k growth r a t e . They showed that the growth r a t e was r e l a t e d to the s t r e s s intensity range in the vicinity of the c r a c k tip. T h e i r r e l a t i o n is da
~-~ = C0(aKy" da where ~ is the change in crack length per cycle of loading, AK is the range of stress intensity being applied during cyclic loading, and Co and m are experimental constants. This relation has been used to characterize the behavior of many materials;a-6 investigations of the influence of steel purity and environmental effects have included it. 7'8 Some investigations have shown that specimen thickness can change fatigue c r a c k propagation r a t e s 9 the effect of t e m p e r i n g t e m p e r a t u r e on c r a c k growth has also been studied. ~
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