Lecture
Press-hardened components in the automotive industry have experienced a growing demand during the last decades. Commonly, a martensitic microstructure is obtained through cooling of the forming tools (conventional press hardening), however, a variant of this process employing pre-heated tools (about 350-550 °C) produces a bainitic microstructure with improved ductility (partial press hardening). The latter process involves a displacive transformation of undercooled austenite into ferrite, accompanied by the diffusion of carbon and the precipitation of carbides. Finite element (FE) simulations of this process commonly employ the Johnson-Mehl-Avrami (JMA) equation with experimentally fitted parameters to describe the transformation kinetics. However, due to the displacive nature of bainitic transformation, there exist complex influences of mechanical load, internal stress and pre-deformation of austenite which greatly limit the application of the JMA model.
In the presented work we couple a phase field (PF) model in the microstructure scale and a finite element model in the macroscopic scale to simulate the chemical and mechanical processes, respectively. The stress, strain and temperature of each element in the FE model are incrementally passed to an individual phase field domain as boundary conditions, while the phase fraction of this PF domain is returned to the FE model to describe the transformation induced plasticity. The phase transformation, solid diffusion and micromechanical equilibrium are calculated using the software package OPENPHASE. Thermomechanical experiments in sheet specimens of a manganese-boron steel, 22MnB5, are used in this work as base for comparison with numerical results, where the bainitic transformation kinetics under external stress is studied.
