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has b e e n r e c o g n i z e d for n e a r l y fifty y e a r s that high c a r b o n m a r t e n s i t e s in tool s t e e l s , o r n e a r the s u r f a c e of c a r b u r i z e d p a r t s , a r e s u s c e p t i b l e to the f o r m a t i o n of m i c r o c r a c k s 1-3 which can lead to p r e m a t u r e f a i l u r e . T h e s e m i c r o c r a c k s a r e o b s e r v e d both i n s i d e and along the edges of the m a r t e n s i t e p l a t e s . It has b e e n p r o posed that m i c r o c r a c k s do not exist in the i n t e r i o r of the s p e c i m e n s but only a p p e a r when a s u r f a c e is p r e p a r e d and the local s t r e s s s t a t e t h e r e b y u n b a l a n c e d . 4 However, c a r e f u l a n n e a l i n g and polishing e x p e r i m e n t s have d e m o n s t r a t e d that m i c r o c r a c k s a r e indeed f o r m e d in the s p e c i m e n i n t e r i o r . 5 In a r e c e n t s e r i e s of p a p e r s M a r d e r , B e n s c o t e r , and K r a u s s have shox~n that: 1) m i c r o c r a c k s only occ u r in plate m a r t e n s i t e s , 6 2) they a r e f o r m e d by the impingem~ent of one plate upon a n o t h e r 7 and 3) in b i n a r y F e - C alloys the m i c r o c r a c k s e n s i t i v i t y i n c r e a s e s a s the c a r b o n content is i n c r e a s e d f r o m 1.0 to 1.4 pct. 8 C o n t r a r y to the r e s u l t s of D a v e n p o r t , Roff, and Bain 2 and R a u s c h and T h u r t l e , 3 M a r d e r e t a l . 8 did not obs e r v e any dependence of m i c r o c r a c k s e n s i t i v i t y upon the p r i o r a u s t e n i t e g r a i n s i z e . M a r d e r and B e n s c o t e r 7 s u g g e s t e d that m i c r o c r a c k i n g is r e l a t e d to the t e t r a gonality of the m a r t e n s i t e ; the l a r g e r the t e t r a g o n a l i t y (thus higher c a r b o n content) the g r e a t e r will be the tendency for m i c r o c r a c k i n g . O t h e r f a c t o r s that do not a p p e a r to have b e e n cons i d e r e d when a t t e m p t i n g to account for the v a r i a t i o n in m i c r o c r a c k i n g p r o p e n s i t y f r o m one s t e e l to a n o t h e r , but which could be i m p o r t a n t in this i n t e r n a l f r a c t u r e p r o b l e m , a r e : a) M s t e m p e r a t u r e : s i n c e c r a c k s a r e f o r m e d when the s t r a i n i m p o s e d by the i m p i n g i n g plate cannot be a c c o m m o d a t e d by p l a s t i c d e f o r m a t i o n it is not u n r e a s o n a b l e to a n t i c i p a t e that the M s t e m p e r a t u r e may i n f l u e n c e m i c r o c r a c k i n g . At low M s t e m p e r a t u r e s , the m a r t e n s i t e will be s t r o n g e r and p l a s t i c a c c o m m o d a t i o n will be m o r e difficult; b) the habit plane of the plate m a r t e n s i t e which can be e i t h e r {225~y o r {259}7: s i n c e the a v e r a g e plate v o l u m e s 9'~~ and angle of i m p i n g e m e n t u a r e different f o r t h e s e two k i n d s of plate m a r t e n s i t e s , a l a r g e effect on