Effect of solidification rate on microstructure and toughness of Ca-Ti modified high boron high speed steel

In this work, the effect of solidification rate on the microstructure and impact toughness of high boron high speed steel is systematically investigated. The theoretical and experimental results indicate that the as-cast microstructure of high boron high speed steel consists of α-Fe matrix and eutectic borocarbide. Without Ca-Ti addition, borocarbide presents a continuous and reticular structure. After Ca-Ti modification, borocarbide possesses an isolated, fine structure. Besides, cooling rate obviously affects the extent of spheroidization. In modified high boron high speed steel, calcium possesses surface activity, which restrains the growth of eutectic borocarbide during solidification. Titanium exists in the form of dispersedly distributed TiC particles which act as heterogeneous nuclei for eutectic borocarbide. The impact toughness of high boron high speed steel is remarkably improved by Ca-Ti modification, which can be further improved by cooling rate. The effect of cooling rate on improving of the toughness can provide a scientific basis for the foundry process designing.