Structure and mechanical properties of Fe--Cr--Mo--C alloys with and without boron

Abstract

Nonconventional heat treatments were designed to improve the mechanical properties of these martensitic steels. Results show that the as-quenched structures of both steels consist mainly of dislocated martensite. In the boron-free steel, there are more lath boundary retained austenite films. The boron-treated steel shows higher strengths at all tempering temperatures but with lower Charpy V-notch impact energies. Both steels show tempered martensite embrittlement when tempered at 350/sup 0/C for 1 hour. The properties above 500/sup 0/C tempering are significantly different in the two steels. While the boron-free steel shows a continuous increase in toughness when tempered above 500/sup 0/C, the boron-treated steel suffers a second drop in toughness at 600/sup 0/C tempering. Transmission electron microscopy studies show that in the 600/sup 0/C tempered boron-treated steel large, more or less continuous cementite films precipitate at the lath boundaries, which are probably responsible for the embrittlement. The differences in mechanical properties at tempering temperatures above 500/sup 0/C are rationalized in terms of the effect of boron-vacancy interactions on the recovery and recrystallization behavior of these steels. Boron seems to impair room temperature impact toughness at low strength levels but not at high strength levels. By simple nonconventional heat treatments of the present alloys, martensitic steels may be produced with quite good strength-toughness properties which are much superior to those of existing commercial ultra-high strength steels. It has also been shown that the as-quenched martensitic steels need not be brittle and in fact very good combinations of strength and toughness can be obtained with as-quenched martensitic steels. 56 fig., 5 tables, 75 references. (DLC)