Development of High Fatigue Strength Sintered Gears Utilizing Spherical Pores from Liquid-Phase Sintering

Iron-based sintered alloy materials have a fatigue strength issue due to the material containing 10% pores by volume. Thus, these sintered materials aren’t suitable for high-speed, high-load gears, like automobile transmission gears. The authors developed a high-fatigue-strength sintered gear using liquid-phase sintering. In this study, sintered and carburized Fe-Ni-Mo-B-C gears with various boron levels were tested for fatigue using a gear testing machine. To isolate the boron’s impact on fatigue strength, the sintering densities of the test gears were kept consistent. The fatigue limits (pmax)lim were 1750 MPa for the boron-free gear (0.0BG), 2150 MPa for the 0.1 mass% boron gear (0.1BG), and 2000 MPa for the 0.4 mass% boron gear (0.4BG), showing boron’s effectiveness in enhancing fatigue strength. A significant difference in pore shape was observed; the 0.0BG had irregular pores, while the 0.1BG and 0.4BG had spherical ones. These spherical pores prevented crack formation and propagation during testing, thus improving fatigue strength. Additionally, the 0.4BG displayed a brittle Fe23B6 network structure not present in the 0.1BG, likely causing its lower fatigue strength compared to the 0.1BG due to the network-like Fe23B6 formation.