Relationship between Microstructure and Mechanical Properties of M50 High-Temperature Bearing Steel

ABSTRACT The relationship between microstructure and mechanical properties of M50 high-temperature bearing steel has been systematically investigated, with particular emphasis on the fracture behavior of primary carbides to clarify the underlying fracture mechanism. The microstructure was characterized using scanning electron microscopy, energy-dispersive spectroscopy, electron backscatter diffraction, and transmission electron microscopy. Instrumented indentation testing was adopted to measure the hardness and elastic modulus of constituent phases, whereas fracture toughness was evaluated at multiple length scales using cube-corner indentation and disc-shaped compact tension (DCT) tests. Compared with the martensitic matrix (hardness ∼7.6 GPa), metal carbon (MC) carbides exhibit higher hardness and modulus but lower fracture toughness (1.73 MPa·m1/2) than M2C carbides (2.27 MPa·m1/2). The macroscopic fracture toughness (KIC) measured by DCT specimens is 17.87 MPa·m1/2, which is consistent with the value reported in the literature. Based on fractographic analysis, a fracture mechanism is proposed in which stress concentration around large MC and M2C primary carbides induces microcracks that propagate through the brittle martensitic matrix, ultimately leading to quasi-cleavage fracture. This work provides quantitative mechanical property data to guide the rational design and microstructural optimization of M50 bearing steel.