Tooth Flank Fracture Risk Evaluation in Carburized Spur Gears with Simulation-Based Prediction of Residual Stresses

Tooth flank fracture (TFF) is a fatigue failure characterized by a primary crack in the region of the active contact area, which is initiated below the surface and most often observed on carburized gears. Tensile residual stresses that may form between the case and core area during carburizing heat treatment can significantly raise the chances of TFF. There are a number of methods available to address the risks of TFF in carburized spur gears. However, primarily because of oversimplifications in the assessment of residual stresses, the accuracy or applicability range of these methods is typically limited. Factors such as carburized depth, carbon depth profile, material quality and gear size are only partially accounted for. With a particular focus on the estimation and assessment of residual stresses brought on by the carburizing heat treatment, a novel method for evaluating the risks of tooth flank fracture in spur gears is presented in this work. A coupled finite-element (FE) model is built using extensive dilatometric investigations on 18CrNiMo7-6 steel samples. The model integrates key mechanisms responsible for strain and stress development during heat treatment in a thermal-mechanical-metallurgical calculation. Through a comprehensive numerical parameter study on cylindrical samples, fundamental aspects of residual stress evaluation in carburized components are explored, identifying main influencing factors and correlations. The simulation model’s applicability is validated across a wide range of gear sizes and heat treatment procedures using available residual stress measurements. Finally, different methods and sets of assumptions are investigated and included in the simulation framework to predict the risks of TFF in carburized gears.