Effect of grain size, cold work, loading frequency and temperature on fatigue limit of mild steel
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THE
fatigue l i m i t of a m a t e r i a l , g e n e r a l l y , i s a function of many p h y s i c a l - m e t a l l u r g i c a l , t e c h n o l o g i c a l and t e s t i n g conditions f a c t o r s , e.g. type of l a t t i c e , c o m p o s i t i o n , heat t r e a t m e n t , s u b s t r u c t u r e , g r a i n s i z e , p r e v i o u s cold d e f o r m a t i o n , l o a d i n g f r e q u e n c y , t e m p e r a t u r e and e n v i r o n m e n t conditions, s h a p e and d i m e n s i o n s of the s p e c i m e n s , c h a r a c t e r i s t i c s of l o a d ing etc. The influence of m a n y of t h e s e f a c t o r s on f a t i g u e l i m i t i s d e s c r i b e d f o r low f r e q u e n c y l o a d i n g in t e r m s of p h y s i c a l - m e t a l l u r g i c a l i n t e r p r e t a t i o n s , w h i l e f o r h i g h - f r e q u e n c y l o a d i n g t h e s e i n f l u e n c e s have not b e e n o b s e r v e d s y s t e m a t i c a l l y . T h e p r e s e n t w o r k d e s c r i b e s the i n f l u e n c e s of g r a i n s i z e and p r e v i o u s cold d e f o r m a t i o n on the f a t i g u e l i m i t of m i l d s t e e l at l o w - f r e q u e n c y loading (83.3 Hz) and at h i g h - f r e q u e n c y l o a d i n g (23 kHz) in d i f f e r e n t t e m perature conditions. 1. E X P E R I M E N T A L Low c a r b o n s t e e l , which had a c o m p o s i t i o n of 0.07 p c t C, 0.27 p c t Mn, 0.03 p c t Si, 0.013 p c t P, 0.018 p c t S, 0.07 pct C r and 0.0064 p c t N was r e c e i v e d in t h e f o r m of 20 m m d i a m h o t - r o l l e d b a r s . A f t e r 7 p c t cold w o r k by t e n s i l e d e f o r m a t i o n the b a r s w e r e a n n e a l e d at 870~ 920~ o r 970~ f o r 6 h, f u r n a c e c o o l e d a t 60~ p e r hour to 100~ and f i n a l l y a i r c o o l e d to r o o m t e m p e r a t u r e . T h e s e v a r i o u s heat t r e a t m e n t s p r o d u c e d d i f f e r e n t g r a i n s i z e s , G1, G2 o r G3 r e s p e c t i v e l y , with d i a m e t e r s i d e n t i f i e d in T a b l e I. G r a i n s i z e , m e a s u r e d by a m e t h o d due to H i l l i a r d 1 w a s r e p r e s e n t a t i v e f o r m o r e then 90 pct of the o b s e r v e d g r a i n s and a c c u r a t e to • m m . Some of the b a r s with g r a i n s i z e G1 w e r e s u b j e c t e d to 5 o r 10 p c t cold d e f o r m a t i o n by d r a w i n g . F o r h i g h - f r e q u e n c y t e s t i n g , an u l t r a s o n i c r e s o n a n t s y s t e m with a p p r o p r i a t e d i s p l a c e m e n t g a g e s , shown s c h e m a t i c a l l y in F i g . 1, w a s u s e d . The whole s y s t e m w a s d e s i g n e d for a f r e q u e n c y of 23 kHz. The s h a p e and d i m e n s i o n s of the s p e c i m e n s , shown in F i g . 2, ANTON PUSKAR is associated with the Department of Mechanical Technology, University of Transport, Zilina, Czechoslovakia. Manuscript submitted December 5, 1975. METALLURGICAL TRANSACTIONS A
w e r e c a l c u l a t e d by B o c k and W l i s t e n b e r g . 2 The s p e c i m e n s
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