Hardening crack in gear. Photo courtesy GEARTECH © 2014.
Definition: Hardening cracks develop in gears during or after the heat treatment process.
Identification: Hardening cracks occur at component edges and other stress concentration locations. They are relatively deep and tend to propagate towards the center of mass.
Discussion: Hardening cracks are caused by volume changes due to phase changes in steel microstructure and to thermal gradients in the component. Cracks tend to occur at locations of stress concentration such as holes and keyways.
Prevention: Make gear blank symmetric and of uniform section. Minimize stress concentration features. Chamfer all edges of teeth. Optimize quenching process. Temper gear immediately after quenching.