Influence of Shot Peening on Surface Durability of Plasma Case-Hardened Sintered Powder Metal Gears (Influences of Shot Diameter and Shot Hardness)

Abstract

In order to investigate the influence of shot peening on the surface durability of sintered powder metal gears, the plasma case-hardened sintered powder metal gears shot-peened with different shot diameter and shot hardness were fatigue-tested using a gear testing machine. The hardness, the compressive residual stress and the surface roughness of the gears increased by the shot peening. The pores near the tooth surface of the shot-peened gears were obviously deformed excect for those of the lightest shot-peened gear in this study. The failure mode of the unpeened gear was pitting, and those of the shot-peened gears was mainly spalling. The failure depth agreed almost with the depth where the amplitude A (τyz/HV) of the ratio of orthogonal shear stress to Vickers hardness became peak near the tooth surface. In these experiments the surface durability of the lightest shot-peened gear was highest comparing with those of the other gears. Therefore, it can be proposed that the light shot peening, which does not cause too large surface roughness and the deformed sharp pore, has to be selected in order to improve the surface durability of sintered powder metal gears.