Corrosion fatigue of type 4140 high strength steel
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ALTHOUGHv a r i o u s
fatigue s t u d i e s of high s t r e n g t h s t e e l s have been r e p o r t e d , the data u s u a l l y r e f e r to a i r ; only s e l d o m a r e o t h e r e n v i r o n m e n t s m e n t i o n e d . T h o m s o n , 1 studying the fatigue b e h a v i o r of AISI 52100 and 5160 s t e e l s , p o i n t e d to the d a m a g i n g e f f e c t s of i n c l u s i o n s in the s t e e l and of m o i s t u r e in a i r . He doubted the e x i s t e n c e of a r e a l fatigue l i m i t for high s t r e n g t h s t e e l s . W a d s w o r t h 2 r e p o r t e d an i m p r o v e m e n t in the fatigue l i m i t of 0,5 p c t C s t e e l when t e s t e d in v a c u u m i n s t e a d of in a i r . Mantel e t a l . 3 r e p o r t e d that the fatigue life of high s t r e n g t h SAE 52100 s t e e l w a s s e n s i t i v e l y l o w e r e d by m o i s t u r e in a i r , n i t r o g e n and a r g o n and r e c o m m e n d e d that h u m i d i t y of the t e s t e n vironment be closely controlled. Dahlberg 4 confirmed that w a t e r v a p o r in a i r can g r e a t l y i n c r e a s e r a t e of fatigue c r a c k p r o p a g a t i o n in a high s t r e n g t h 4340 s t e e l . L i e t a l . 5 s i m i l a r l y r e p o r t e d that fatigue c r a c k growth of 0.45 p c t C, low a l l o y high s t r e n g t h s t e e l w a s a c c e l e r a t e d by m o i s t u r e in a i r o r in a r g o n , the effect of which d e c r e a s e d with f r a c t u r e t o u g h n e s s of the s t e e l . I n c r e a s e d m o i s t u r e beyond a s p e c i f i c r e l a t i v e h u m i d i t y (R.H.) of 40 pct a p p e a r e d to have no f u r t h e r d a m a g i n g e f f e c t , s i m i l a r to the effect of m o i s t u r e d e s c r i b e d by J o h n s o n and W i l l n e r 8 for high s t r e n g t h s t e e l under c o n d i t i o n s of s t a t i c loading. The l a t t e r i n v e s t i g a t o r s r e p o r t e d that the c r a c k g r o w t h r a t e i n c r e a s e s with r e l a tive h u m i d i t y , b e c o m i n g i d e n t i c a l to the r a t e in w a t e r at o r a b o v e 60 p c t R.H. Hanna e t a l . 7 in s i m i l a r s t a t i c l o a d t e s t s of high s t r e n g t h AISI 4340 and 300 M s t e e l s a l s o found m o i s t u r e in a r g o n to be d a m a g i n g , a dew point of - 5 ~ (-21~ (or r e l a t i v e h u m i d i t y of 3 pct) b e i n g s u f f i c i e n t to i n i t i a t e a c r a c k . Duquette and Uhlig s found no effect of m o i s t u r e in oxygen (93 pct R.H.) on the fatigue l i m i t of a 0,18 p c t C m i l d s t e e l (AISI 1015, R B = 77); at a p p l i e d s t r e s s e s above the f a t i g u e l i m i t , the fatigue life in m o i s t 02 w a s a c t u a l l y i n c r e a s e d o v e r that in d r y 02. To the c o n t r a r y , in a r gon at s t r e s s e s above the fatigue l i m i t , m o i s t u r e d e c r e a s e d the fatigue l i f e . F o r a m o d e r a t e s t r e n g t h AISI 1035 s t e e l (87 k s i t e n s i l e s t r e n g t h ) , M e h d i z a d e h e t a l . 9 found th
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