Some effects of retained austenite on the fatigue resistance of carburized steel
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R. H RICHMAN and R. W. LANDGRAF are with the Scientific Research Staff, Ford Motor Company, Dearborn, Mich. 48121. Manuscript submitted July 19, 1974. METALLURGICAL TRANSACTIONS A
f e c t was i n t e r p r e t e d as a r e d u c t i o n in t e n s i l e b i a x i a l i t y in the s u r f a c e l a y e r s , which a c c o u n t e d for the i m p r o v e d ductility. The s u c c e s s of this a p p r o a c h for m o n o t o n i c d e f o r m a t i o n i n d i c a t e s that t h e r e is c o n s i d e r a b l e m e r i t in the t e c h n i q u e . As in the p r e v i o u s w o r k , we r e l y h e a v ily upon s e p a r a t e s i m u l a t i o n of c a s e and c o r e with bulk s p e c i m e n s . H e r e , h o w e v e r , we e m p h a s i z e c y c l i c flow p r o p e r t i e s . That is, c y c l i c d e f o r m a t i o n r e s p o n s e s a r e r e c o r d e d for high c a r b o n s t e e l s in an a t t e m p t to u n d e r s t a n d the f at i g u e b e h a v i o r of c a r b u r i z e d m e m bers.
M A T E R I A L S AND E X P E R I M E N T A L TECHNIQUES Materials The p r e s e n t i n v e s t i g a t i o n was co n d u ct ed with v a c uum m e l t e d and c a s t h e a t s of s t e e l that w e r e f o r m u l a t e d to c o r r e s p o n d to c a r b u r i z e d s u r f a c e l a y e r s on AISI 4027, a w i d e l y used s t e e l f o r a u t o m o t i v e d r i v e line c o m p o n e n t s such as g e a r s and s h a f t s . To f a c i l i t a t e c o m p a r i s o n s , the 4027 f o r c a r b u r i z i n g was also v a c u u m m e l t e d and c a s t in the l a b o r a t o r y . C o m p o s i t i o n s and a u s t e n i t i z i n g t e m p e r a t u r e s of s e v e n h e a t s spanning the r a n g e f r o m 0.27 to 1.25 pet c a r b o n a r e l i s t e d in Table I, and s o m e t y p i c a l m i c r o s t r u c t u r e s a r e g i v e n in Fig. 1. Since r e l a t i v e l y pure c o n s t i t u e n t s w e r e u s e d to m a k e the a l l o y s , g r a i n s i z e s a f t e r h e a t t r e a t m e n t w e r e f r e q u e n t l y l a r g e r than for normal commercial grades. Deleterious effects assoc i a t e d with c a r b u r i z i n g thus tend to be e x a g g e r a t e d in
Table I. Composition and Heat Treatment of Model Steels
89h Austenitizing
Steel
C
4027 4076 4086 4098 40108 40120 40126
0.27 0.76 0.86 0.98 1.08 1.20 1.26
Composition,Wt Pct Mn Si 0.80 0.78 0.78 0.78 0.78 0.78 0.78
0.28 0.26 0.24 0.26 0.24 0.26 0.26
Mo 0.25 0.25 0.25 0.25 0.26 0.24 0.23
Temperature oF (~ 1600 1600 1600 1700 1730 1800 1830
(871) (871) (871) (927) (943) (982) (999)
VOLUME 6A, MAY 1975-955
t h e s e s t e e l s . P r o b a b l y for the s a m e r e a s o n , t r a n s f o r m a t i o n m i c r o c r a c k i n g could not be c o m p l e t e l y e l i m i i n a t e d f r o m the s t e e l s with 0.98 and 1.08 p c t c a r b o n .
Why the m i c r o c r a c k i n g did not p e r s i s t to the h i g h e r c a r b o n c o n t e n t s is not known. Carburization
(a)
S p e c i m e n s of s i m u l a t e d 4027 w
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