The effects of carburising and quenching process on the formation of residual stresses in automotive gears

Carburisation and quenching of automotive gears result in improved wear properties by promoting martensite transformation and formation of a case hardened surface layer. The martensite transformation causes a volumetric expansion which puts the surface into a ‘compressive’ residual stress state which promotes fatigue resistance. The relationship between the martensite transformation on the resultant residual stress distribution is complex because the martensite transformation kinetics are influenced by the carbon content which varies through the thickness depending on the carbon diffusivity, carburisation times and temperature. In addition, the time–temperature histories are expected to vary in different locations in the gear during the quenching process and are dependent on both geometry of the part and the heat transfer characteristics. A thermo-mechanical finite element (FE) model has been developed to model the carburisation and quenching process in order to predict the inhomogeneous martensitic transformation, the case-hardened layer as well as the associated volumetric expansions, therefore enabling the prediction of the final strain distribution and the resultant stress distribution. The results show that the combined effects of an applied load or torque on the gear and the residual stresses formed by martensitic transformation can be evaluated. Therefore, the carburisation times and the quenching process can be optimised not only to improve wear by case hardening but also to promote compressive residual stresses at critical regions in the tooth of the gear to improve fatigue performance and extend the service life of the gears.