Microstructure and hardening behavior of Al-modified Fe-1.5Â wt%B-0.4Â wt%C high-speed steel during heat treatment

The microstructure and age-hardening behavior of a new kind of Al-modified low carbon high boron high-speed steel (BHSS) were investigated. The results show that the as-cast microstructure of BHSS mainly consists of M2B borides and dendrite martensite. Al addition can promote the formation of pearlite and ferrite. After heat treatment, proper Al addition can not only remarkably increase the bulk hardness of BHSS during destabilized condition, but also promote the subsequent precipitation-hardening effect. The tempered BHSS with 0.6 wt%Al exhibits the best precipitation-hardening behavior at 520 °C tempering, which is attributed to the presence of some dispersive M23(B,C)6 and M6(B,C) secondary precipitations. The proper Al can promote the formation of M7(B,C)3 and inhibit the coalescence of secondary precipitations. TEM analysis indicates that there exists an orientation relationship of (112)M23(B,C)6//(112)M6(B,C) in the steels.