Bending fatigue strength and impact strength of case-carburized gears subjected to case-depth modification by additional heat treatments

Discarding case-carburized gears with excessive case-depth causes financial and environmental losses. To save the losses, we experimentally investigated the applicability of case-depth modification by additional heat treatments (AHTs) to such gears in terms of bending fatigue and impact strength. The AHTs consisted of an intermediate softening treatment (IST) (vacuum normalizing, tempering, induction normalizing, or none) to case-carburized spur gears with an excessive case-depth, followed by induction heating and quenching. All the AHTs reduced the effective case-depth, maintained the surface residual stress, refined the microstructure near tooth root surfaces, and maintained the bending fatigue limit. Moreover, all the AHTs increased the impact limit because of the refinement of the microstructure, whereas omitting an IST reduced this increase. Thus, case-depth modification by AHTs is applicable in terms of bending fatigue and impact strength, and conducting an IST is highly recommended.