This study compares the fracture toughness of high-speed steel produced by powder metallurgy and subjected to different heat treatments to obtain either martensitic or bainitic/martensitic microstructures. The heat-treatment process involved austenitization at 1150 °C, followed by either martempering or austempering at 235 °C, and final tempering. Microstructural analysis was performed using electron backscatter diffraction (EBSD), field-emission scanning electron microscopy (FESEM), and X-ray diffraction (XRD). Fracture toughness was evaluated using circumferential notched tensile (CNT) specimens. The results showed that austempered CNT samples exhibited significantly higher fracture toughness compared to martempered ones, indicating improved resistance to crack propagation. Microstructural characterization revealed distinct differences: the austempered samples featured bainitic laths, retained austenite blocks, and martensite plates, whereas the martempered sample contained martensite plates and austenite islands. However, small differences in prior austenite grain size, lath thickness, and dislocation density were insufficient to fully account for the enhanced toughness in the austempered sample. Further analysis indicated that the increased fraction of high-angle grain boundaries and higher kernel average misorientation (KAM) in the austempered sample acted as effective barriers to crack propagation. Additionally, a greater volume fraction of nano-sized carbides contributed to a more pronounced strengthening effect, further enhancing fracture toughness.

This study presents the fracture toughness improvement of the powder metallurgical high speed steel ASP2030 by replacing the martensitic matrix with a bainitic one. For this purpose, the fracture toughness of the steel is compared in both the austempered and marquenched states. The heat treatment of the samples was carried out by austenitizing in the range of 1150-1185C followed by either austempering at 235C or marquenching processes. Triple tempering at 560 C for 2h was also performed on all specimens. Fracture toughness measurements were performed on circumferentially notched tensile specimens. The microstructural features of the specimens were investigated using X-ray diffraction, optical and electron microscopy, and electron backscatter analysis. The results showed that the fracture toughness is sensitive to small changes in austenitizing temperature at both heat treatment conditions. By increasing the austenitizing temperature from 1150 to 1170C, the fracture toughness increases and then decreases with a further increase in temperature to 1185C. The results suggest that the changes in fracture toughness are due to the simultaneous effects of the volume fraction of undissolved carbides and the lattice microstrain values at different austenitizing temperatures. It can be concluded that the fracture toughness increases when the martensitic matrix is replaced by a bainitic structure. This improvement is related to finer bainitic laths width and also to the larger spacing between the particles of the undissolved carbides.