Effect of dissolved carbon on microcracking in martensite of an Fe-1.39 C alloy
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Effect of Dissolved Carbon on Microcracking in Martensite of an
An i n c r e a s e in the c a r b o n c o n t e n t of m a r t e n s i t e in s t e e l s a s a c c o m p l i s h e d by a l l o y i n g a n d / o r h e a t t r e a t m e n t m a y i n f l u e n c e m i c r o c r a c k i n g in two w a y s : m i c r o c r a c k i n g m a y i n c r e a s e 4 b e c a u s e of i n c r e a s e d l a t t i c e b r i t t l e n e s s due to h i g h e r t e t r a g o n a l d i s t o r t i o n , 5 o r it m a y be i n c r e a s e d b e c a u s e of a m o r p h o l o g y c h a n g e ~'6 that m i g h t a f f e c t p l a t e i n t e r a c t i o n o r i m p i n g e m e n t . T h i s c o m m u n i c a t i o n p r e s e n t s the r e s u l t s of e x p e r i m e n t s p e r f o r m e d to f u r t h e r e v a l u a t e the e f f e c t of c a r b o n on m i c r o c r a c k i n g a n d to r e c o n c i l e the s i g n i f i c a n t d i f f e r e n c e s r e p o r t e d ''2 f o r the m i c r o c r a c k i n g b e h a v i o r of AISI 52100 s t e e l a u s t e n i t i z e d at t e m p e r a t u r e s a b o v e and b e l o w A c r e . T h e a p p r o a c h t a k e n c o n Table I. Structural Characterization as a Function of Step-Annealing of Fe-1,39C
Austenit]zing Pct Carbon Pct Temperature, Content of Martensite Austenite ~ Austemtet c/a Transformed 1850" 1655 1600 1575 1500 1470 1415 1350
1.39 1.30 1.21 1.18 1.05 1.01 0.92 0.83
1.063 1.054 1.048 1.046 1.044 1.042 1.035 1.034
Pct Cementite
66.5 78 85 87 88 92 91 94
Mlcrocracking Sensitivity Sv, mm"1'
3.9 6 6.5 12 15 17.5 20
18 17 13 9 10 4.5 1.5 0.15
*Entirely austemte phase. All other specimenswere first homogemzedat 1850~ J'Obtained from Fe-Cequdibrium diagram.
Fe-1.39 C Alloy M . G. M E N D I R A T T A , ' J .
S A S S E R , A N D G. K R A U S S
R E C E N T . c o n t r i b u t i o n s ''z to the l i t e r a t u r e h a v e a d v a n c e d c o n f l i c t i n g p r o p o s a l s to a c c o u n t f o r t h e p r e s e n c e of m i c r o c r a c k s in t h e p l a t e m a r t e n s i t e of AISI 52100 s t e e l . One a p p r o a c h ' e x p l a i n s m i c r o c r a c k i n g a s a surface phenomenon that develops during metallog r a p h i c p r e p a r a t i o n to r e l i e v e the high l o c a l r e s i d u a l s t r e s s e s in a s - q u e n c h e d m a r t e n s i t e . A l t h o u g h m i c r o c r a c k i n g c o u l d be r e v e a l e d in a s p e c i m e n a u s t e n i t i z e d at 2200~ (1204~ no c r a c k i n g w a s o b s e r v e d in s a m p l e s of 52100 o r h y p e r e u t e c t o i d s t e e l s w h e n t h e y w e r e a u s t e n i t i z e d at l o w e r t e m p e r a t u r e s w h e r e a s u b s t a n t i a l a m o u n t of c a r b o n r e m a i n e d u n d i s s o l v e d in t h e f o r m of c a r b i d e p a r t i c l e s . T h e o t h e r a p p r o a c h i d e n t i f i e s a s a d i r e c t c a u s e of m i c r o c r a c k i n g the i m p i n g e m e n t of m a r t e n s i t e p l a t e s a w i t h i n the bulk of a t r a n s f o r m i n g s p e c i m
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