Simulation study of the residual stress state in a case-hardened planetary gear

Mårtensson Henrik - Scania CV AB (Sweden)

Case-hardening is a widespread heat treatment technique employed to improve the durability of a mechanical component. The increased fatigue strength can be attributed to a higher local hardness and compressive residual stresses in the case layer which helps to suppress surface crack initiation. On the other hand, there can be significant tensile stresses in the soft core, which can potentially cause internal cracks and a reduced fatigue limit. Accurate numerical predictions of the residual stress and microstructural evolution are therefore vital for determining the expected fatigue life for a case-hardened component.

Heat treatment simulation is a multiphysics problem with complex interactions between mechanical, thermal and metallurgical domains. Over the past few decades, case-hardening simulations have developed into a relatively mature field with numerous examples of successful experimental validations in the literature.  However, a vast amount of hard-to-acquire material data is required to obtain high-quality simulation results. There is also a lack of transparency regarding material data in some published works with experimental verification, which makes transferability difficult.

In this study, we investigate how the choice of different low-alloyed steels and variations in chemical composition affect the residual stress state and mechanical properties in a case-hardened gear. Through heat-treatment simulations, combined with experimental comparisons, the study gives insight into the interaction between chemical composition, hardenability and residual stresses – key factors influencing the final fatigue strength of the component.

In addition to using experimental dilatometry data to derive necessary material data, the study shows how input data can be estimated for a specific alloy composition when experimental data is not available. This is accomplished by utilizing existing alloy-based phase transformation models, empirical relations and continuous cooling temperature (cct)-curve simulations. Indications of the relative importance of the modelled physical phenomena are gained through sensitivity analysis, which also highlights the necessary accuracy of the input data for reliable predictions of the microstructure and the residual stress state in a case-hardened component.