Bending and contact fatigue strength of innovative steels for large gears

Large gears for wind turbine gearboxes require high performances and cost-effective manufacturing processes. Heat distortion in the heat treatment phase and the consequent large grinding stock are responsible for high manufacturing costs due to reduced productivity. A research project aimed at the identification of new materials, manufacturing and heat treatment processes has been performed. Air quenchable alloy steels, combined with a specifically developed case hardening and heat treatment process, have been identified as an interesting solution, both from the point of view of cost effectiveness, thanks to reduced distortions and grinding stock, and for the environmental sustainability. The research project has been completed by the manufacturing of a full-scale gear, on which the whole process has been validated. Nevertheless, in order to judge the applicability of these steels to large gears, data from specific tests on the performances against typical gear failure modes, like bending and contact fatigue, are necessary as well. Single tooth fatigue bending tests and disc-on-disc contact fatigue tests have therefore been performed on two innovative materials, respectively, a high hardenability steel and a bainitic structure steel, and on a reference traditional case hardening steel. The results of these tests, which provide useful data for gear designers, are presented and discussed in this paper.