Numerical modelling of residual stresses induced by heat treatment and hard turning of case-hardening steel 16MnCr5 for probabilistic prediction of fatigue strength

• Applicants: Professor Dr.-Ing. Thomas Bergs; Professor Dr.-Ing. Christoph Broeckmann
• Subject Area: Metallurgical, Thermal and Thermomechanical Treatment of Materials
• Term: since 2026
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 580796963

Case hardening increases the load bearing capacity of highly stressed components by introducing residual compressive stresses at the surface and increasing strength through the formation of martensite. Although low-pressure carburization reduces distortion, hard machining is often essential. Depending on the process parameters, in addition to the introduction of further residual stresses, strain-induced transformation of retained austenite in the surface layer can occur. This superimposition of the surface layer influences of both process steps makes it difficult to achieve a resilient, fatigue-resistant design of case-hardened components. As part of the proposed research project, a holistic description of the surface integrity is to be achieved through the coupled modelling of both process steps. The model should capture the interactions along the process chain and enable a virtual description of the material structure and its stability and residual stresses. The originality of the project lies in the holistic description of the process chain, which couples the knowledge of production technology and materials science, and the consideration of the retained austenite content in the surface region. This enables a virtual analysis of the entire process chain, including the interactions between microstructure, process control and final component properties. In contrast to isolated considerations of individual process steps, a coupled overall model is created here, which forms the basis for multi-criteria optimization and reliable component design. The work is divided into two phases, each lasting two years. In the first phase, a heat treatment model and a machining model for the material 16MnCr5 will be developed based on experimental investigations. The heat treatment model describes the evolution of the microstructure and the residual stress state during case hardening and tempering. The retained austenite content and its thermal and mechanical stability are also key target variables. The machining model includes the description of the thermo-mechanical loads and the damage model for modelling chip formation. In addition, the thermally and mechanically induced change in the retained austenite and the associated change in the surface layer are modelled. Finally, the coupled model will be validated using extensive experimental investigations. In the second phase of the project, the coupled model is supplemented by the multiscale description of hard turning, allowing for a computationally efficient description of the surface region in three-dimensional geometries. Meanwhile, the machine data available from both phases and measured process variables are used to optimize the surface quality by means of machine learning. Finally, knowledge of the surface integrity is used to predict the fatigue strength using suitable failure hypotheses. Validation is carried out by means of rotating bending tests on selected material states.

• DFG Programme: Research Grants