Sintered materials are superior in productivity because of their simple process, but they are not applied to highload gears because of their insufficient strength. To improve the fatigue strength of sintered materials, the authors have developed liquid-phase sintering which can achieve high-density without using secondary processing. In this study, the effect of boron addition (0-0.4 mass%) on the rolling contact fatigue strength of Fe-Ni-Mo-B-C sintered and carburized material was evaluated. In addition, in order to evaluate only the boron addition effect excluding the influence of density, the sintered density of each specimen was controlled to be the same. In the test range of this study, the rolling contact fatigue limit (pmax)lim of material with an additional quantity of boron of 0.1 mass% showed the highest value exceeding 1700 MPa. This value was not only significantly higher than the (pmax)lim of the boron-free material (1100 MPa), but also an extremely high value comparable to the (pmax)lim of wrought steel (1900 MPa). The reason why the (pmax)lim of a 0.1B roller was remarkably high was investigated from the viewpoints of both pore structure and material structure. As for the pore structure, the pore shape of the boron-free roller was irregular, whereas the pore shape of the 0.1B roller was spherical. As a result of CAE analysis of the orthogonal shear stress inside the roller during the rolling contact fatigue test, it was found that the maximum value of orthogonal shear stress around the pores of the 0.1B roller was about 35 % lower than that of the boron-free roller. This result suggests that cracks are less likely to occur in the 0.1B roller than in the boron-free roller. In other words, it is thought that the pore shape of 0.1B material affects the improvement of rolling contact fatigue strength.

Sintered materials are superior in productivity because of their simple process, but their mechanical properties are low. To improve the mechanical properties of sintered materials, we focus on liquid-phase sintering. In this study, we selected boron as sintering aids and evaluated on the effect of the addition quantity of boron (0-0.6 mass%) to the mechanical properties of liquid-phase sintered and heat-treated materials. In the test range of this study, the tensile strength of material with an additional quantity of boron of 0.1 mass% showed the highest value, 1386 MPa, which is about 35% higher compared to the tensile strength of material without adding boron. On the other hand, materials with an additional quantity of boron of 0.2 mass% or higher showed higher-density than material with an additional quantity of boron of 0.1 mass%, but both their elongation and tensile strength were significantly decreased. The precipitates changed between the boundary of the quantity of boron added of 0.1 and that of 0.2 mass%. Only Fe 23 B 6 was formed in the material with the boron addition amount of 0.1 mass% or less, and Fe 23 B 6 and Fe 2 B were formed in the material with the boron addition amount of 0.2 mass% or more. By cross-sectional observation of the test specimen after the tensile test, it was confirmed that in the material with the boron addition amount of 0.2 mass% or more cracks occurred and propagated at the Fe 23 B 6 /Fe 2 B interface and finally resulted in fracture. In the material with the boron addition amount of 0.1 mass%, however, such a fracture was not confirmed. The phenomena could be explained the fact that the mechanical properties were sharply changed between the quantity of boron added of 0.1 mass% and 0.2 mass%. The existence of Fe 23 B 6 /Fe 2 B interface would significantly affect the strength of the liquid-phase sintered materials.

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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 (p max ) 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 Fe 23 B 6 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 Fe 23 B 6 in the 0.4BG.