Interactions between austenite formation, carbide precipitation, and austenite re-transformation during the heat treatment of large size forgings: influence of prior deformation and starting microstructure

The heat treatment of large forged steel components presents significant scientific and industrial challenges due to the complex thermal histories and heterogeneous microstructures generated during open-die forging. Multiple reheating stages into the austenitic range are required for material softening, while deformation occurs under highly non-isothermal and non-steady conditions. As a result, conventional isothermal Temperature–Time–Transformation (TTT) diagrams are not sufficient to describe phase evolution, and Continuous-Cooling-Transformation (CCT) diagrams that incorporate the influence of prior deformation are required. Such data are currently lacking in the literature.

This PhD project aims to develop deformation-dependent CCT models for large forged steels, accounting for the interactions between austenite formation, carbide precipitation, and austenite re-transformation during heat treatment. The research will address the strong gradients in grain size, phase distribution, and second-phase particles that exist between the surface and the center of large components. Experimental work will be carried out using a high-resolution dilatometer under both deformed and undeformed conditions, combined with thermodynamic simulations and advanced microscopic characterization. The experimental findings will be integrated into a physics-based numerical model capable of predicting the final microstructure and hardness distribution after quenching and tempering. This model will serve as a key input for optimizing industrial open-die forging and heat treatment simulations.