Modeling the phase transformation during press hardening (PressBain)

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The conventional press hardening produces martensitic microstructure through in-tool quenching of austenite. One variant of the press hardening, known as partial press hardening (PPH), involves pre-heating of the tools at certain regions, where the material is quenched to a temperature above Ms, and consequently transformed to bainite or bainite/martensite duplex microstructure, which improves the retained formability and toughness of the workpiece [Abd2015, Hüt2015].

Modeling the phase transformation during PPH is a challenging problem because of two reasons. First, there is a strong coupling between mechanical load, internal stresses, chemical composition and phase transformation kinetics. Due to the displacive nature of the bainitic transformation, a large eigenstrain is expected for the bainitic ferrite, which results in elastic/elastoplastic deformation in the microstructure. Second, Bainite is a hierarchical structure spanning various length scales [Hüt2015]. With supports of the DFG-SPP1713 project, we developed models of the coupled chemical and thermos-mechanical processes under various length scales.

The non-linear elasticity of each phase is described using ab-initio and phase field crystal simulations conducted by the research team of Robert Spatschek of Jülich research center. At the mesoscopic scale, we simulate the growth of bainitic subunits and sheaves by a multi-phase field (MPF) method coupled with crystal plasticity models [Lin2017]. The activation of different slip systems in both BCC bainitic ferrite and FCC austenite and the rotation of local orientation are emphasized in the crystal plasticity model. At the macroscopic scale, the deformation and cooling of the workpiece is modeled by a finite element model (FEM) where the phase evolution at each integration point is described by the homogenized output of the mesoscale MPF models [Sch2016]. FEM development and high temperature mechanical tests of undercooled austenite are supported by the research team of Martin Hunkel of IWT Bremen.

[Lin2017] M. Lin, U. Prahl, A parallelized model for coupled phase field and crystal plasticity simulation, Computer Methods in Materials Science

[Hüt2015] C. Hüter, M. Lin, D. Schicchi, M. Hunkel, U. Prahl, R. Spatschek, A multiscale perspective on the kinetics of solid state transformations with application to bainite formation, AIMS Materials Science 2, 319 (2015).

[Sch2016] D.S. Schicchi, M. Lin, U. Prahl, M. Hunkel, A combined finite element - phase field model approach on the bainitic transformation, Proc. Europ. Conf. Heat Treatment 2016 on CD (2016).

[Abd2015] Abdollahpoor A, Chen X, Pereira MP, Xiao N, Rolfe BF. Journal of Materials Processing Technology 2015.