Modeling Start Curves of Bainite Formation

Abstract

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 À 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 dislocationbased 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.

Effects of Q&P Processing Conditions on Austenite Carbon Enrichment Studied by In Situ High-Energy X-ray Diffraction Experiments

“…All the details about the sample preparation can be found in these prior papers. The Ms value of the studied steel, 295 • C, was measured during the in situ investigations, and was consistent with the value calculated with the model developed by Van Bohemen (298 • C) [25,26].…”

“…For the investigated alloy, the predicted retained austenite fraction as a function of the QT after the long partitioning time is represented in Figure 2. The calculation relied on a Carbon Constraint Equilibrium (CCE) assumption and the empirical Van Bohemen et al [25,26] equations for martensitic transformations (initial and final). The CCE is an interface condition between martensite and austenite assuming that only the chemical potential of carbon must be equilibrated in both phases, and that the interface is fixed as a consequence [4][5][6].…”

A physically based approach to model the incomplete bainitic transformation in high-Si steels

“…In the case of high-Si steels, when the carbon concentration of the austenite increases, j will decrease due to a higher effective activation energy Q b and a smaller undercooling (T h -T) [9]. Furthermore, the above equations show that the kinetics are inversely proportional to the austenite grain diameter d c [8,12] because the austenite grain boundaries provide the nucleation sites initially present.…”

“…[8] the constants d and Z, respectively the effective thickness of the austenite grain boundary and a geometrical factor, are chosen as d = 1 nm and Z = 6. The parameter K 1 is a material constant [9], and C =d(DG m )/dT is the derivative of the maximum driving force DG m with respect to T. The autocatalytic parameter k can be determined on the basis of isothermal fraction-time curves [3,8]. It has been found that k increases with increasing austenite grain size d c [8] and some studies suggest that k might also be dependent on the chemical composition [3,17].…”

“…[8] the number density of the potential nucleation sites N i is assumed to be dependent on the temperature, which is differ-ent compared to previous displacive models. Experimental data of lean-Si steels was used to validate the model [8], and in a later paper it was shown that the model can give satisfactory predictions of the bainite start curves [9].…”