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In this study, sintered and powder-forged rollers and gears with different densities and nickel contents were fatigue-tested using a roller testing machine and a gear testing machine in order to elucidate their fatigue strength. The densities of the sintered and powder-forged rollers and gears were in the range 7.01 g/cm to 7.84 g/cm, and the nickel contents of the metal powders were chosen as 0.5 % and 3.0 %. These experimental results were compared with the results for ingot steel ones. The hardness near the surface of the test specimens with a nickel content of 3.0 % was lower than that of the other ones. The pores in the sintered rollers became smaller or disappeared upon hot forging. The failure modes of the rollers and gears were mainly spalling due to subsurface cracking and pitting due to surface cracking, respectively. The fatigue strength of the sintered rollers and gears was the lowest in this experimental range. The fatigue strength of the powder-forged rollers and gears was roughly equivalent to that of the ingot steel ones, respectively. The fatigue strength of the test specimens increased as their density increased. It was clear that the fatigue strength of the sintered and powder-forged rollers and gears was proportional to the hardness at the failure depth for nickel contents of 0.5 % and 3.0 %. In other words, the fatigue strength of the rollers and gears with a nickel content of 3.0 % was similar to those of the others because of the toughness effect of nickel under the same material density.

Influence of density and nickel content on fatigue strength of powder-forged gears - Comparison with ingot steel and sintered steel -