On Modeling of Thermo-Viscoplasticity of Case-Hardening Steels Over a Wide Temperature Range

The aim of this work is the development of a thermodynamically consistent fully coupled thermo-viscoplastic material model for metals.
The model is based on a split ofthe free energy into a thermo-elastic, a thermo-plastic and a purely thermal part and covers nonlinear cold-work hardening and thermal softening. Nonlinear temperature dependent effects are accounted for the elastic moduli, the plastic hardening moduli, the thermal expansion, the heat capacity and the heat conductivity. Furthermore strain rate-dependency of the current yield stress is realized using a temperature dependent nonlinear Perzyna-type viscoplastic model.
The model and its parameters are fitted against experimental data for case hardening steel 16MnCr5 (1.7131). Since for some constitutive parameter sets describing thermal softening the model may result in unphysical behaviour, we introduce necessary conditions to check the thermodynamical admissibility of these parameters.
We discuss the consistent linearisation of the proposed model and its implementation in a monolithic fully coupled finite element framework. Finally, we present results for selected boundary value problems. These show the localization and regularisation behaviour of the proposed model.