Analysis of effects due to stepped quenching during case hardening of gears

Hunkel Martin - Leibniz-Institut für Werkstofforientierte Technologien – IWT (Germany)

The VariQuench process is a stepped quenching process during case hardening. The components are rapidly quenched to a surface temperature around martensite start temperature of the carburized surface. Afterwards, a slow cooling rate is applied. This can significantly improve the properties of case-hardened gears. The cause of the improved performance of components due to the stepped quenching is currently still under debate. An optimised stress state but also more tolerant microstructural properties are discussed. To summarize, optimisation and transfer to different component geometries is actually only possible through an extensive trial-and-error process.

Dilatometer tests and accompanying metallographic investigations were used to investigate self-tempering effects in the case-hardening steel 20MnCr5 (SAE 5120) at two carburized states to determine microstructural effects. During stepped quenching the retained austenite stabilization, the martensite tetragonality together with transition carbides precipitation from martensite were identified as significant microstructural effects. They all depend on the step temperature or the cooling rate afterwards.

The identified self-tempering effects were modelled and transferred to component simulations. The required parameters for the models were determined by fitting to the dilatometer results. The models and its parameters were included in a user subroutine UMAT for the FE-software Abaqus. The microstructure and residual stresses of gears for pulsator tests were calculated using the FE-simulation. A reference heat treatment with direct, non-stepped quenching and two VariQuench variants were simulated. The simulation results obtained are discussed in comparison with experimental results from component tests applying these quenching conditions. The temperature evolution within the gears, the hardness and the residual stresses where determined experimentally after heat treatment.