The microstructure and fracture of a carburized steel
- PDF / 3,147,046 Bytes
- 9 Pages / 613 x 788.28 pts Page_size
- 106 Downloads / 182 Views
d e m o n s t r a t e d the b e n e f i t s of d e f o r m a t i o n - i n d u c e d t r a n s f o r m a t i o n of r e t a i n e d a u s t e n i t e and have shown that n o n r e f r i g e r a t e d s p e c i m e n s , i.e. those with large a m o u n t s of r e t a i n e d a u s t e n i t e , show s u p e r i o r fatigue r e s i s t a n c e c o m p a r e d to r e f r i g e r a t e d s p e c i m e n s with lower a u s t e n i t e content, and J o h n s o n 5 has r e v i e w e d other s t u d i e s that show the b e n e f i c i a l effect of r e tained a u s t e n i t e . Apple and K r a n s s , ~ however, have shown an opposite effect of r e t a i n e d a u s t e n i t e on high cycle b e n d i n g fatigue. Significantly i m p r o v e d fatigue r e s i s t a n c e was achieved in polished s p e c i m e n s with the lowest a m o u n t of a u s t e n i t e in the c a s e . The key to the r e s o l u t i o n of the a p p a r e n t l y d i v e r g e n t r e s u l t s noted above may lie to a large extent in the m o r p h o l o g i c a l d i f f e r e n c e s in the m a r t e n s i t e - a u s t e n i t e m i c r o s t r u c t u r e produced by v a r i a t i o n s in the c a r b o n content, alloying, a n d / o r heat t r e a t m e n t . F i g u r e 1 p r e s e n t s , s u p e r i m p o s e d on the i r o n - c a r b o n d i a g r a m , a m i c r o s t r u c t u r a l c l a s s i f i c a t i o ns'9 of the m a r t e n s i t e s that f o r m in F e - C a l l o y s . The plate m a r t e n s i t e s , by v i r t u e of f o r m i n g in higher c a r b o n a u s t e n i t e , a r e p r e s e n t in a s s o c i a t i o n with l a r g e r a m o u n t s of r e t a i n e d a u s t e n i t e than a r e the lath m a r t e n s i t e . Case m i c r o s t r u c t u r e s with plate m a r t e n s i t e a r e also unique in other r e s p e c t s . The m e c h a n i s m of m a r t e n s i t i c t r a n s f o r m a t i o n in h i g h - c a r b o n a u s t e n i t e p r o d u c e s an i r r a t i o n a l m a r t e n s i t e habit l e a d i n g to m i c r o c r a c k f o r m a t i o n at s i t e s of i m p i n g e m e n t between plates with d i f f e r e n t habit plane v a r i a n t s . i ~ Rauch and T h u r t l e 11 f i r s t identified m a r t e n s i t i c m i c r o c r a c k s as a p o s s i b l e cause of p r e m a t u r e f a i l u r e in c a r b u r i z e d , c o a r s e g r a i n e d steel, and J e n a and Heich 12 d e m o n s t r a t e d that m i c r o c r a c k s were p r e s e n t as well in f i n e - g r a i n e d m i c r o s t r u c t u r e s of 8620 s t e e l . A l a t e r s y s t e m a t i c study 13 of the r e l a t i o n s h i p between a u s t e n i t i c g r a i n size and m i c r o c r a c k i n g in F e - C plate m a r t e n s i t e showed that m i c r o c r a c k i n g did d e c r e a s e with d e c r e a s GEORGE KRAUSS is AMAX Foundation Professor of Physical ing a u s t e n i t i c g r a i n s i z e , but in a c c o r d with J e n a Metallurgy, Colorado School of Mines, Golden, CO 80401. and Heich, was p r e s e n
Data Loading...
