Effect of mezzo-heterogeneous microstructure and rolling on the rolling contact fatigue strength of sintered high Ni alloy steels

This study aims to improve the fatigue strength of P/M gears and to substitute P/M gears for conventional wrought steel gears with reduced manufacturing costs. The 4600 series sintered alloy steels with mixed elemental powders show the mezzo-heterogeneous microstructure which consists of high-hard Ni rich areas and the surrounding martensitic structures. This mezzo-heterogeneous structure causes high strength to the products. In this paper, we discuss the effects of Ni contents on the contact fatigue strength, and the effect of the pore size is also discussed for further strengthening. Especially, from the evaluation of the area of isolated pores in the densified surface layer after the rolling, it was found that the decrease of pore sizes is very important to improve the contact fatigue strength.