Structure and mechanical properties of powder metallurgy high-speed steel during electric current and traditional heat treating: a comparative study

Electric current heat treatment is an innovative technique to improve microstructures and mechanical properties of metallic materials. The microstructures and mechanical properties of a powder metallurgy high-speed steel (PM-HSS) treated by electric current heat treatment and traditional heat treatment are comparatively investigated. Results showed that after austenitizing at 1130 degrees C, the structure of PM-HSS sample composed of ferrite matrix, M6C, M23C6, and MC carbides, transformed into a martensite matrix accompanied by M6C and MC carbides. Compared to the traditional austenitizing at 1130 degrees C for 30 min, the electric current austenitizing at 1130 degrees C for 5 min dissolved more carbides, resulting in a greater solid solution of alloying elements in the matrix. Further traditional triple tempering led to carbide coarsening, whereas electric current triple tempering promoted the carbide dissolution. Notably, the dissolution of more carbides resulted in a higher C content in the martensite matrix of HSS treated by electric current, significantly promoting the formation of nanotwins (5-20 nm in width). The electric current triple tempering sample exhibited a yield strength of 3097 MPa, compressive strength of 5016 MPa, and a fracture strain of 30.0%, outperforming the traditional triple tempering sample by nearly 600 MPa in yield strength. Analysis revealed that this significant strengthening was primarily attributed to nanotwin formation and solid solution strengthening caused by carbide dissolution.