Effect of calcium modification on solidification, heat treatment microstructure and toughness of high boron high speed steel

In this study, the as-cast and heat treated microstructures, hardness and impact toughness of high boron high speed steel with Fe-2.0 wt% B-0.4 wt% C-6.0 wt% Cr-4.0 wt% Mo-2.0% Al-1.0 wt% Si-1.0 wt% V-0.5 wt% Mn-x wt% Ca (x = 0.00, 0.08, 0.16, 0.24) is systematically investigated. The experimental results reveal that the as-cast microstructure of high boron high speed steel consists of alpha-Fe as matrix and eutectic borocarbide M-2 (B, C) (M=Fe, Cr, Mo, V, Mn). Borocarbide presents a continuous and reticular structure in alloy without calcium modification. After calcium modified, hardness of matrix increases because of solid solution strengthening. Besides, the continuous network of borocarbide is destroyed and possesses an isolated, fine and spherical structure. Calcium in high boron high speed steel has strong surface activity, which exists in the form of solid solution and mainly segregates at the interface between matrix and borocarbide. As a significant discovery, to calcium modified high boron high speed steel, the heat treatment can ulteriorly improve the shape and size of borocarbide. Moreover, the impact toughness of high boron high speed steel is remarkably improved by calcium modification. All of the modified alloys as well as the unmodified one perform the mechanism of complex of brittle and ductile fracture.