The role of time-temperature path on the isothermal austenite-to-bainite phase transformation of low alloy 51 CrV 4 steel was investigated and the corresponding microstructures were analyzed. The important finding is that an incomplete initial austenitization treatment leaves undissolved carbides in the matrix, such that lower carbon and chromium content in the matrix result, eventually accelerating the phase transformation. Furthermore, the residual carbides constitute additional nucleation sites for the bainite plates, speeding up the process even further. Also, both plastic pre-deformation of the supercooled austenite and application of external elastic stresses during the phase transformation lead to transformation plasticity by enhancing the stress fields, providing a driving force for the growth of bainite plates along a preferred orientation. Overall, the current results constitute the first step toward establishing a database for constructing a realistic microstructure-based model for simulating metal forming operations involving austenite-to-bainite phase transformation.

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A new model is proposed to successfully predict the initiation and evolution of the austenite-to-bainite phase transformation, capturing specifically the time-dependent transformation kinetics. In particular, the isothermal bainitic transformation in 51CrV4 steel is experimentally observed for various constant stress conditions, and significant improvement is obtained in comparison with the existing models. Specifically, both the transformation kinetics and the resultant transformation strains can be simultaneously predicted using the same variant growth approach. Simulation results are in good agreement with the experiments, evidencing the success of the proposed model in describing the transformation phenomena in terms of kinetics and transformation plasticity. Furthermore, the proposed formulation provides a basis for incorporating variant–variant interactions and cementite formation in the residual austenite matrix.

In the present study, the phase transformation behaviour of 51CrV4 steel was investigated both in an actual flange forming process and under idealized isothermal bainitic transformation conditions. The results demonstrated that superimposed external stresses have only a minor effect on the kinetics of the phase transformation, whereas strains resulting from transformation plasticity were significant. Transmission electron microscopy revealed a preferential alignment of the bainite with respect to the external loading direction. This selection of favourably oriented variants in turn causes macroscopic strains. Understanding and minimizing these strains are a key factor in designing functionally graded components with minimum distortion.