This study aims cost reduction of high load gear manufacturing, and attempts to substitute P/M gears for conventional wrought steel gears. In order to improve the fatigue strength of the P/M gears, we focused attention on the high performance 4600 sintered alloy steels with heterogeneous microstructure which were made from the mixed elemental powder in comparison with homogeneous microstructure from the prealloyed powders. The fatigue strength was measured by a two cylinder type rolling contact fatigue testing machine, and the effect of the different microstructure on the fatigue strength was mainly investigated. As the result, the endurance limit at N=1.0×107 of the mixed elemental powder compacts showed 1.45 GPa, which was slightly higher than that of the prealloyed powder compacts (1.32 GPa). Also, their values are about 75 % of the endurance limit of wrought steels (SCM415 case-hardened).
