Material Influence on the Micropitting and Wear Resistance of Nitrided External and Internal Gears

Due to increasing performance requirements and sometimes tribologically challenging operating conditions in gears (such as those found in wind turbines), alternative heat treatment processes to quenching and tempering or case hardening are becoming more and more important. Nitriding can significantly increase the load-carrying capacity of gears compared to the quenched and tempered initial state. The nitrided surface layer is characterized by a hard compound layer on the surface that is just a few micrometers thick and a diffusion zone that extends deeper into the material. The structure of the compound layer of iron and alloy element nitrides is primarily decisive for the wear behavior of nitrided components. Previous studies on nitrided gears with regard to micropitting and wear focus primarily on a systematic variation of the compound layer on gears made of nitriding steel 31CrMoV9. Results on the influence of the base material on the micropitting and wear resistance of nitrided gears are not yet available. In systematic load and speed stage tests, nitrided gears made from a quenched and tempered steel (42CrMo4) and a material with an alternative alloy composition (34CrNiMo6) were examined in addition to the reference (nitriding steel 31CrMoV9). Both external and internal gear pairings were tested, with the alternative materials showing more favorable tribological behavior in some cases. The results are presented and categorized according to the current state of the art. Finally, recommendations for the practical use of nitrided gears are derived.