Rolling contact fatigue of sintered steels under Hertzian pressure and sliding

In most investigations the rolling contact fatigue properties of sintered steels are determined without superposition of an additional sliding which is present on the flanks of gears during operation. In such investigations an improvement of the fatigue strength is principally related to an increase in density. However, the realistic and most severe condition between sliding gear teeth in contact is a sliding of -24 percent (relative velocity between two sliding surfaces). The rolling contact fatigue tests with sliding were performed with the mixed alloys Fe+1.5 percent Cu, carbonitrided, Fe+1.5 percent Cu+0.5 percent C, as-sintered as well as quenched and tempered, in the densities of 6.8, 7.1 and 7.4 g/cbcm, and the diffusion alloyed powder Fe-4.0 percent Ni-1.5 percent Cu-0.5 percent Mo, as-sintered, carbonitrided, case hardened, plasmanitrided (densities 7.1 and 7.4 g/cbcm) and additionally mixed with 0.5 percent C (densities 7.0 and 7.2 g/cbcm). The tribological environment was th e gear-box oil SAE 80 at 80 degree Celsius. With superposition of sliding an optimum rolling fatigue strength at about 7.2 g/cbcm is observed for the investigated PM' steels in the as- sintered as well as differently beat-treated states in the density range of 6.8 to 7.4 g/cbcm. This is due to effects based on open and closed porosity in interaction with tribological environment.