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Modelling of bainitic microstructure evolution

The steel manufacturing process of press hardening is used to produce steels with excellent properties, utilized, for example, in the automotive industry. The underlying concept is the simultaneous cooling and pressing of the blank alloy steel after the heating process. The deliberate use of temperature and stresses during cooling initiates the formation of advantageous microstructures and results in high strength steels.
Bainite is a non-equilibrium phase in carbon steel, which consists of ferrite, austenite and carbides. The formation starts with pre-strained austenite at a high temperature. Lowering and subsequently holding the temperature constant leads to the formation of bainite while the material is under stress due to the applied load of the forming process. These stresses significantly affect the transformation, and therefore it is important to understand their role. Additionally, the transformation of bainite during press hardening is influenced by carbon concentration and diffusion. During the transformation from austenite to bainite, carbon can stay inside the bainitic phases or diffuse to the surrounding austenite, depending on total concentration and temperature. Rapid cooling after the formation of bainite retains the non-equilibrium structure.
Aim of this research project is the modeling of bainite formation on a microscale using the phase field model. The influences of carbon diffusion and applied stresses on the transformation are investigated.

Modelling of bainitic microstructure evolution phase field