Effect of Pre-deformation on Decomposition and Spheroidization of M2C Carbide in High-Speed Steel

Abstract

One key challenge for tailoring microstructure of high-speed steel (HSS) is to obtain small-sized carbides with a homogeneous distribution. Although diffusion annealing followed by post-deformation offers an opportunity to refine carbides, it requires high annealing temperatures and large strains. In this study, we employ a pre-compression step and examine its role in following carbide decomposition and spheroidization. The result shows that pre-deformation may introduce dislocations into M2C carbides and hence triggers a significant enhancement of M2C decomposition into M6C and MC. Structural defects in M2C act as potential nucleation sites of new precipitates and activate an alternative decomposition pathway via nucleation along dislocations or intersections of stacking faults and dislocations. The decomposition enhancement effect exhibits a dependence on strain and temperature, which may originate from a temperature-mediated balance between dislocation densities and element diffusion rates in M2C carbides. Compared with post-deformation, pre-deformation facilitates carbide spheroidization and enables smaller carbides as well as higher tempered hardness. This may offer an alternate pathway for carbide refinement in HSS.