Methodical Investigation of Distortion Biasing Parameters During Case Hardening of Spur Wheels

Highly stressed gears are typically heat treated to satisfy operational demands. Case hardening is a common heat treatment associated with power train components. During its different process steps the heat treatment deploys potentially carriers of distortion coming from other predecessor production steps like machining or forging. Distortion is introduced during the quenching process derived from thermal and transformation stresses. Therefore the step of heat treatment typically introduces complex size and shape changes in the process chain. The method Distortion Engineering was applied to the process chain of small module spur wheels concerning the case hardening in detail. The identification of significant influencing parameters on distortion of these gears is the first step to the acknowledgment of distortion mechanism. Object of investigations were spur wheels of small module. These mainly were case hardened utilizing low pressure carburization with subsequent high pressure gas quenching in a heat treatment furnace of industrial proportion. In strategic focus were the three contributing heat treatment steps of heating, carburization and quenching and their influence on the distortion of the gear wheels. Additionally a variation of the case hardening process was performed using gas carburization and oil quenching as well as gas carbo-nitriding for case hardening of the gears. The identified main influencing factors on size and shape changes of the base body and the toothing will be presented and discussed.