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

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.