The effect of cyclic stressing on the oxidation of a low-carbon steel
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B. S. HOCKENHULL,
AND
P. HANCOCK
T h e e f f e c t on o x i d a t i o n of c y c l i c a l l y s t r e s s i n g a low c a r b o n s t e e l was s t u d i e d at t e m p e r a t u r e s of 500 and 570~ It was found that if the c y c l i c s t r a i n was above a t h r e s h o l d v a l u e which c o r r e s p o n d e d to the f r a c t u r e s t r a i n of the s u r f a c e s c a l e , then the oxide g r o w t h r a t e i n c r e a s e d both at 500 and at 570~ T h e oxide f r a c t u r e s t r a i n s u n d e r fatigue c o n d i t i o n s w e r e found to be in the r a n g e 2.7 - 5.1 • 10 "4. The m e c h a n i s m of the p r o c e s s i s d i s c u s s e d in r e l a t i o n to o x i d a t i o n k i n e t i c s and the m e c h a n i c a l p r o p e r t i e s of the g r o w i n g s u r f a c e s c a l e s .
IN m a n y
industrial situations, steel components are s u b j e c t e d to c y c l i c s t r e s s i n g in an o x i d a t i v e e n v i r o n m e n t at e l e v a t e d t e m p e r a t u r e s . P r e v i o u s w o r k h a s i n d i c a t e d the d e l e t e r i o u s effect of o x i d a t i o n e n v i r o n m e n t s on fatigue life. H o w e v e r , t h e r e i s l i t t l e i n f o r m a t i o n on the i n f l u e n c e of c y c l i c s t r e s s i n g on the m e c h a n i c a l b e h a v i o r of the oxide s c a l e and on the oxide g r o w t h k i n e t i c s and m o r p h o l o g y . A c h t e r and Shahinian 1 have shown in fatigue t e s t s on a n u m b e r of m a t e r i a l s that the c r a c k p r o p a g a t i o n r a t e i s s t r o n g l y a f f e c t e d by the e n v i r o n m e n t and have a s s o c i a t e d t h i s with a d s o r p t i o n p r o c e s s e s in the g r o w ing c r a c k . T h i s h a s b e e n c o n f i r m e d by T h o m p s o n e t al.,2 W a d s w o r t h and Hutchings 3 and o t h e r s . White 4 has a l s o d e m o n s t r a t e d the influence of e n v i r o n m e n t on the high s t r a i n fatigue p r o p e r t i e s of a l o w - a l l o y s t e e l at e l e v a t e d t e m p e r a t u r e s . A l l of t h e s e c a s e s d e a l with the p r o p a g a t i o n r a t h e r than the i n i t i a t i o n of a c r a c k and i n e v i t a b l y t h i s cont r a s t s l o w - f r e q u e n c y h i g h - s t r a i n t e s t i n g with h i g h e r f r e q u e n c y , low s t r a i n c y c l i n g . In the l a t t e r c a s e , the influence of the m e c h a n i c a l p r o p e r t i e s and the i n t e g r i t y of the oxide s c a l e b e c o m e s i n c r e a s i n g l y i m p o r t a n t a s i n i t i a t i o n b e c o m e s the d o m i n a n t p r o c e s s . F o r e x a m p l e , Hockenhull e t a l . 5 have shown that f a t i g u e c r a c k i n i t i a t i o n b e h a v i o r in an a n o d i c a l l y o x i d i z e d a l u m i n u m a l l o y i s s t r o n g l y d e p e n d e n t upon the r e s i d u a l s t r e s s s i t u a t i o n in the o x i d e . B r u c e and H a n c o c k 6 have shown that the m e c h a n i c a l s t a b i l i t y of the oxide f i l m , f o r
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