Rolling contact fatigue properties of selected PM-materials for gear-box applications

Rolling contact fatigue tests with a sliding of - 24 per cent were performed with the mixed alloys Fe+1.5 per cent Cu, carbonitrided, Fe+1.5 per cent Cu + 0.5 per cent C, as-sintered as well as quenched and tempered, diffusion alloyed powder Fe4.0 per cent Ni-1.5 per cent Cu-0.5 per cent Mo, as-sintered, plasmanitrided, carbonitrided, case hardened and additionally mixed with 0.5 per cent C and prealloyed powders Fe-4.0 per cent Ni-0.5 per cent Mo and Fe-1.5 per cent Mo with addition of Cu and C in different heat treatments. The densities investigated were 6.8, 7.1, 7.4 and 7.8 g/cm3. The tribological environment was the gear-box oil SAE 80 at 80 deg C. With superposition of sliding the highest rolling contact fatigue strength was achieved with powder forged specimens (density of 7.8 g/cm3). An optimum for the sintered specimens in the range of 6.8 to 7.4 g/cm3 was found at around 7.1 g/cm3 for the most sintered steels independent of the heat treatment. This is due to effects based on open and closed porosity in interaction with the tribological environment. In case of variable loading the fatigue strength under constant loading can be exceeded by factors of 1.42 to 1.76.