Numerical Simulation of hot forging with an integrated heat treatment considering the impact of unsteady stress state on the transformation induced plasticity

• Applicant: Professor Dr.-Ing. Bernd-Arno Behrens
• Subject Area: Primary Shaping and Reshaping Technology, Additive Manufacturing
• Term: from 2012 to 2019
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 212963651

In the course of a subsequent cooling after a hot bulk metal forming process, a structural phase transformation of austenite into secondary phases (e.g. martensite) occurs. In addition to elastic, plastic and thermal expansion strains, transformation induced volumetric changes as well as transformation induced plasticity (TRIP) arises. Transformation volumetric strain results from the change in the lattice structure of austenite to another phase, which leads to an accompanying change in the material volume. Transformation plasticity strains arise from the micro-plasticity phenomena at the phase boundary during the formation of e.g. martensite particles in an austenitic matrix. They have a decisive impact on the resulting residual stress state and can be the reason for undesirable distortion in the hot forged component. The aim of this project is a further development of the established numerical approaches for the prediction of residual stress state and transformation related distortions. Hereby, the load-dependent influence of transformation plasticity (back flow effect of TRIP) has been taken into account. Based on the fundamental experimental investigations, required material data regarding the load-dependent TRIP-behavior for two typical hot forging steels is to be determined. This data will be used to extend the models developed in the first application period, which will subsequently be implemented in a commercial FE system using user-defined subroutines. Finally, the extended material model will be tested and validated on the basis of a demonstrator component in the context of a closed die forging process chain with an integrated heat treatment. The forming process will be furthermore extended by e. g. a deflashing or a trimming stage. Within the scope of this research proposal, the investigations are focused on the diffusion-controlled transformation types. Therefore, the integrated cooling under moderate or slow cooling rates will be carried out in calm air or in sand bath. To implement the deflashing stage, the corresponding tool system will be redesigned and manufactured. As a final point, the validation of the developed numerical methods is performed by the comparison of calculated distortions and residual stresses with the ones measured on real components within experimental metallographic and XRD investigations.

• DFG Programme: Research Grants