Application of bismuth for solidification structure refinement and properties enhancement in As-cast high-speed steels

In order to exhibit good all-round performance the impact toughness enhancement of as-cast high-speed steels is obligatorily needed. In general, different methods are used commercially to achieve cast structure refinement and, as a consequence, their properties are improved. Introduction into the melt of inoculant particles or surface-active additions is among most beneficial. However, the effect of modifying additions in as-cast high-speed steels has been studied insufficiently. In fact, a restricted number of modifiers is used for structure and properties improvement in the as-cast high-speed steels compared to the common cast alloys. In the present work several kinds of alloys including tungsten-molybdenum high-speed steels of M2 and T30 types and low-alloy tungsten-free 1.1C-5Mo-1.7V high-speed steel were melted to investigate the effect of bismuth on their structures and properties. It has been found that additions of bismuth produce a very fine cast structure and affect the shape of the matrix grains and the morphology of the eutectic carbides as well as the redistribution of the main alloying elements of high-speed steels between solid solution and eutectic carbides. The microstructural changes, induced by bismuth during solidification, are explained by the surface activity of bismuth, which segregates to liquid/solid interface, significantly blocking dendrite growth in the direction along certain crystallographic planes. It has been shown that during eutectic solidification, carbides are also exposed to the barrier effect of bismuth being surrounded in the melt by this element. The main metallographic features of the modified cast structure alongside with the purifying effect produced by bismuth are retained after full heat treatment affecting the final mechanical properties of as-cast high-speed steels. As a result, bismuth significantly increases impact toughness and wear resistance of as-cast high-speed steels but decreases their red hardness.