Numerical modelling of hot forming with integrated heat treatment taking into account the influence of variable stress states on the transformation-plastic strain behaviour

Combination of forming and cooling in hot forging processes influences the microstructural transformation from austenite to martensite, perlite, bainite or ferrite. In addition to elastic, plastic and thermal changes in strain, the microstructural transformation leads to additional transformation-related and transformation-plastic strains. Transformation-related strains result from the change of the lattice structure and the associated volume. Transformation plastic strains occur if a mechanical load superimposes the phase transformation. However, transformation-related and transformation-plastic strains influences the development of residual stresses and can lead undesirable distortions in hot forging processes.

The aim of this project is to develop thermo-mechanical-metallurgical coupled material model for the determination of residual stresses and distortions. The project focus is on the consideration of the load-dependent influence of transformation plastic strains and its influence.

After focusing on diffusionless martensitic transformations in the first project phase, the research work in the second project phase will consider the diffusion-controlled ferritic, pearlitic and bainitic microstructural transformations. Therefore, an experimental test methodology is developed to observe the transformation plastic effects during isothermal microstructural transformation. Such an isothermal approach is particularly suitable for the further development of the FE models created in the first project phase. The numerical models is validated by comparison with an experimental hot forging process including deburring and controlled cooling. The elimination of the distortion-stabilising burr is intended to increase the component's susceptibility to distortion. The component cooling is carried out at slow cooling rates (e.g. in a sand bath or in still air) in order to initiate the diffusion-controlled transformations in the demonstrator component in a targeted manner. All processing steps introduce additional residual stresses in the forged component or release existing residual stresses, which can influence the final distortions. For these reasons, particular importance must be attached to the continuous calculation of residual stresses in forged components, taking into account the effect of variable stress states on the transformation-plastic strain behaviour.