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.

In the present study, the fracture toughness of hardened and tempered powder metallurgical (PM) high-speed steel ASP 2030 was investigated using notched and unnotched bending specimens and the finite element method. The normal flexural strength of notched and unnotched specimens marquenched by austenitizing at 1150, 1170, and 1185°C, followed by quenching to room temperature is measured after triple tempering at 560°C for 2 h. The finite element method (FEM) analysis is performed to observe the true stress distribution and calculate the critical fracture stress in the specimens under the experimental conditions of the bending test. The microstructural features of the specimens were investigated by X-ray diffraction (XRD) and a field emission scanning electron microscope (FESEM) with an electron backscatter detector (EBSD). No retained austenite was detected in the tempered specimens, and according to the results of the EBSD analysis and XRD tests, the microstructure of the matrix consists of martensitic ferrite laths. It can be observed that with the increase of austenitizing temperature from 1150 to 11850C, the normal flexural strength of the specimens decreases. The decrease in flexural strength of the specimens is due to the increase in the prior austenite grain size and consequently the martensitic ferrite laths after tempering. In addition, as the austenitizing temperature increases, the volume fraction of the undissolved carbides decreases, which causes the size of the undissolved carbides to increase and the flexural strength to decrease. According to FEM, the critical crack length calculated from the critical fracture stress is approximately equal to the average diameter of undissolved carbides.

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.

Bainitic steels, as a third generation of advanced high strength steels, are potential steel grades for automotive applications. Two grades of bainitic steels with low and high silicon content, with three different thermal treatments per grade and therefore different second phase constituents, are examined under quasi-static and high strain rate deformations. Microstructures are studied by advanced characterization techniques, including X-ray diffraction and scanning electron microscope equipped with an electron backscatter diffraction detector. Subsequently, the quasi-static and dynamic mechanical responses of the steels are correlated to the microstructures. A positive effect of the strain rate is observed for all the examined materials: when the strain rate is increased, both the tensile stress and deformation levels increase, thus also the energy absorption capacity. However, it is shown that the higher the fraction of second phase constituents, the lower the effect of strain rate becomes. In addition, the grain size directly correlates to the strain rate effect too. The phenomenological hardening model of Johnson-Cook is used to simulate the quasi-static and dynamic flow behaviors, allowing to quantify the strain rate sensitivity for each material. A comprehensive literature survey on the strain rate sensitivity of various steel grades reveals that steels with higher strength demonstrate a lower strain rate sensitivity factor. This trend can be approximated by a power law function which clearly is followed by the materials under consideration in this study.