Effect of boron addition on the gear fatigue strength of Fe-Ni-Mo-B-C sintered alloys

Abstract

Iron-based sintered alloy materials have a problem with fatigue strength because the number of pores contained in the material is 10% by volume. Therefore, sintered materials are not applicable to gears used under high speed and high load, such as transmission gears for automobiles. The authors have developed a high fatigue strength sintered gear using the liquid-phase sintering method. In this study, Fe-Ni-Mo-B-C sintered and carburized gears with different boron contents were fatigue-tested using a gear testing machine. The sintering densities of each test gear were controlled to be the same in order to evaluate only the effect of boron addition on fatigue strength. The gear fatigue limit (pmax)lim of a boron-free gear (the 0.0BG), 0.1 mass% boron added gear (the 0.1BG) and 0.4 mass% boron added gear (the 0.4BG) were 1750 MPa, 2150 MPa and 2000 MPa, respectively. The addition of boron was effective in improving gear fatigue strength. A comparatively large difference was confirmed in the pore shape between the boron-free material (the 0.0BG) and the boron-added material (the 0.1BG, the 0.4BG). That is, while the pores of the 0.0BG were irregularly shaped, those of the 0.1BG and the 0.4BG were spherical. Since these spherical pores suppressed the generation and propagation of cracks during the fatigue test, the fatigue strength could be improved by the addition of boron. In addition, the 0.4BG had a brittle Fe23B6 network microstructure that was not seen in the 0.1BG. The reason why the fatigue strength of the 0.4BG was lower than that of the 0.1BG was considered to be due to the formation of network-like Fe23B6 in the 0.4BG.

Keywords

• powder metallurgy
• liquid phase sintered alloy
• boron
• pore
• fatigue strength
• gear