FEM simulations of a multi stage forming process on Sandvik maraging steel 1RK91 describing the stress assisted and the strain induced martensite transformation

Sandvik steel 1RK91 combines good corrosion resistance with high strength. The steel has good deformability in austenitic conditions. This material belongs to the group of metastable austenites, so during deformation a strain-induced transformation into martensite takes place. After deformation, transformation continues as a result of internal stresses. Depending on the heat treatment, this stress-assisted transformation is more or less autocatalytic. Both transformations are stress-state and temperature dependent. This article presents a constitutive model for this steel, based on the macroscopic material behaviour measured by inductive measurements. Both the stress-assisted and the strain- induced transformation to martensite are incorporated in this model. Path-dependent work hardening is also taken into account. The model is implemented in the commercial FEM code MARC for doing simulations. In the simulations the tools are treated as rigid bodies, friction is taken into account because it influences the stress state during metal forming. The material properties after a calculation step are mapped to the next step to incorporate the cumulative effect of the transformation and work hardening during the different steps. A multi-stage metal-forming process is simulated. The process consists of different forming steps with intervals between them to simulate the waiting time between the different metal-forming steps. Results of the transformation behaviour are presented together with the shape of the product during and after metal forming. Finally, this article shows the results of the calculation in which the material transforms autocatalytic, as a result of a specific heat treatment.