Microcracking sensitivity in Fe-C plate martensite
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b e e n known for s o m e t i m e x-s that m i c r o c r a c k s , defined for the p u r p o s e s of this paper a s c r a c k s c o n fined to individual plates of m a r t e n s i t e , a r e p r o d u c e d when m e d i u m - or h i g h - c a r b o n s t e e l is quenched to f o r m m a r t e n s i t e . A r e c e n t i n v e s t i g a t i o n has e m p h a s i z e d that m i c r o c r a c k i n g i s a s s o c i a t e d only with the a c i c u l a r or plate m a r t e n s i t e that f o r m s in h i g h - c a r b o n F e - C a l l o y s ,8 and a s e r i a l polishing technique has shown that the m i c r o c r a c k s r e s u l t d i r e c t l y f r o m the i m p i n g e m e n t of l a r g e plates of m a r t e n s i t e . 7 A u s t e n i t e g r a i n size has b e e n the m a j o r s t r u c t u r a l f a c t o r a s s o c i a t e d with m i c r o c r a c k i n g . D a v e n p o r t et al.,4 as well as Ranch and T h u r t l e , s who studied m i c r o c r a c k ing in c a s e - h a r d e n e d s t e e l s , found that the d e g r e e of m i c r o c r a c k i n g i n c r e a s e s with i n c r e a s i n g a u s t e n i t i c g r a i n size. S e v e r a l i n v e s t i g a t o r s 3'4 have b e l i e v e d that quench s e v e r i t y i n f l u e n c e s the f o r m a t i o n of m i c r o c r a c k ing and that this m a y , in t u r n , be r e s p o n s i b l e for the g r o s s c r a c k i n g of quenched s t e e l s . On the o t h e r hand, the i n v e s t i g a t i o n by Ranch and T h u r t l e 8 showed that at l e a s t t h r e e different quench s e v e r i t i e s , f r o m q u e n c h i n g in a c a u s t i c solution to q u e n c h i n g in 350~ s a l t , had no effect on m i c r o c r a c k f o r m a t i o n in c a s e - h a r d e n e d s a m p l e s . They also i n d i c a t e d that the c a r b o n c o n t e n t (based on a l i m i t e d n u m b e r of o b s e r v a t i o n s ) m u s t be e u t e c t o i d or g r e a t e r before m i c r o c r a c k s a p p e a r , and that l a r g e 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 appear to r e d u c e the n u m b e r of m i c r o c r a c k s . The purpose of the p r e s e n t paper is to r e p o r t the r e s u l t s of a s y s t e m a t i c m e t a l l o g r a p h i c i n v e s t i g a t i o n p e r f o r m e d to explore f u r t h e r the effects of g r a i n s i z e , q u e n c h i n g conditions, c a r b o n content, and r e t a i n e d a u s t e n i t e on m i c r o c r a c k i n g in pure F e - C plate m a r t e n s i t e .
E X P E R I M E N T A L PROCEDURE The complete c h e m i c a l a n a l y s i s of the t h r e e a l l o y s u s e d in the i n v e s t i g a t i o n is given in T a b l e I. S a m p l e s of t h e s e alloys were cut f r o m plate p r o c e s s e d f r o m a 300 lb ingot. The ingot was made from pure e l e c A. R. MARDER and A. O. BENSCOTER are Supervisor and Metallographer, respectively Alloy Development Section, Homer Research Laboratories, Bethlehem Steel Corp., Bethlehem, Pa. G. KRAUSS is Associate Professor, De
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