Hardness of tempered martensite in carbon and low-alloy steels
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C. R. HRIBAL,
AND
L. F. PORTER
This paper p r e s e n t s the r e s u l t s of a s y s t e m a t i c study of the effect of carbon, manganese, phosphorus, silicon, nickel, chromium, molybdenum, and vanadium on the h a r d n e s s of m a r t e n s i t e in low to medium carbon s t e e l s t e m p e r e d for one hour at 100~ (56~ i n t e r v a l s in the r a n g e 400 to 1300~ (204 to 704~ R e s u l t s show that the as-quenched h a r d n e s s depends s o l e l y on carbon content. On t e m p e r i n g , the effect of carbon on h a r d n e s s d e c r e a s e s m a r k e d l y with i n c r e a s i n g t e m p e r i n g t e m p e r a t u r e . Studies of c a r b o n - 0 . 5 manganese s t e e l s showed that the i n c r e m e n t a l i n c r e a s e in h a r d n e s s f r o m 0.5 pct manganese a f t e r a given t e m p e r i n g t r e a t m e n t was independent of carbon content. Based on this r e sult, studies of the effects of the other alloying e l e m e n t s w e r e made using a 0.2 or 0.3 pct carbon, 0.3 to 0.5 pct manganese s t e e l b a s e composition. The h a r d n e s s of the r e s u l t ing t e m p e r e d m a r t e n s i t e was a s s u m e d to be due to a given alloy addition, and when two or m o r e alloying e l e m e n t s w e r e added, t h e i r effects w e r e a s s u m e d to be additive. Each of the seven alloying e l e m e n t s i n c r e a s e d the h a r d n e s s of t e m p e r e d m a r t e n s i t e by v a r y i n g amounts, the i n c r e a s e being g r e a t e r as m o r e of each element was p r e s e n t . Nickel and phosphorus have s u b s t a n t i a l l y the s a m e effect at all t e m p e r i n g t e m p e r a t u r e s . Manganese has e s s e n t i a l l y the s a m e hardening effect at any t e m p e r a t u r e in the range 700 (371~ to 1300~ s i l i c o n is most effective at 600~ (316~ chromium at 800~ (427~ molybdenum at 1000 to l l 0 0 ~ (538 to 592~ and vanadium at 1200~ (649~ Using the data obtained, a p r o c e d u r e is e s t a b l i s h e d for calculating the h a r d n e s s of t e m p e r e d m a r t e n s i t e for carbon and alloy s t e e l compositions in the range studied and for any combination of t e m p e r i n g time and t e m p e r a t u r e .
T
HE i m p o r t a n c e of developing a t e m p e r e d m a r t e n s i t e s t r u c t u r e to p r o v i d e s t r e n g t h and toughness in h i g h - s t r e n g t h s t e e l s has long been recognized. I F o r a s t e e l of given carbon content and alloy composition, the final s u b c r i t i c a l heat t r e a t m e n t (tempering t r e a t ment) e s t a b l i s h e s the h a r d n e s s of the s t e e l and for t e m p e r e d m a r t e n s i t e s , the h a r d n e s s can be used to e s t i m a t e t e n s i l e strength. In addition to carbon and alloy content, such f a c t o r s as the p r i o r austenite g r a i n s i z e influence strength. This l a s t - m e n t i o n e d factor is beyond the scope of the p r e s e n t paper. Although t h e r e has been a c o n s i d e r a b l e body of inf o r
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