Modeling Start Curves of Bainite Formation

It is demonstrated that calculations with a physically based model give an accurate description of the start curve of bainite formation in a wide range of steels. The temperature dependence of the overall kinetics, which determines the characteristic C shape of the start curve, is controlled by both the undercooling below the start temperature (T (h) - T) and an effective activation energy Q (b) . A systematic analysis of the model parameters extracted from the best fits of published time-temperature-transformation (TTT) data reveals a material-independent relationship, which means that the activation energy is in accordance with known details of the dislocation-based nucleation model of bainite. It is shown that the C shape of the start curve can be determined for a given alloying content using an empirical relationship derived for Q (b) and, in combination with the material-independent relationship, the kinetics of bainite can be predicted at all temperature levels.