Effect of Surface Rolling on Load Bearing Capacity of Pre-alloyed Sintered Steel Gears with Different Densities

Abstract

Surface rolling experiments and surface durability tests were carried out using Cr−Mo and Ni−Mo pre-alloyed sintered steel spur pinion gears with different densities fabricated by powder metallurgy (P/M). The P/M gear specimens were machined from sintered packs made from the single-press single-sinter route and some were surface-rolled using a high-precision CNC form-rolling machine of two roller-dies transverse-type. A fully densified surface layer less than 2% porosity was obtained on the tooth flank approximately 0.5mm thick when P/M pinions with a density of 7.4 Mg/m3 or more were surface-rolled using the amount of stock rolled at 0.15mm. The load bearing capacity of P/M pinions were estimated by gear running tests using a power re-circulating type testing rig. A counter gear was made of standard grade Cr−Mo wrought steel. All test gears were case-carburized under identical conditions and finished by grinding. The representative damages for every P/M pinion were pitting near the pitch point. The experimental results show that the load bearing capacity of surface-rolled pre-alloyed P/M gears with a density 7.40 Mg/m3 or more completely matches that of a case-carburized Cr−Mo wrought steel gear.