A NOVEL APPROACH TO MODEL THE NUCLEATION AND OVERALL TRANSFORMATION KINETICS OF BAINITE IN MEDIUM-CARBON STEELS

摘要

The kinetics of isothermal bainite formation has been calculated using a new approach to model bainite nucleation in combination with the assumption of displacive growth. The nucleation rate is assumed to depend on the number density of potential nucleation sites Ni, a factor λ accounting for autocatalytic nucleation, and an activation energy Q*. In line with the theory for athermal martensite nucleation, Ni is assumed to be proportional to the driving pressure, which makes the present bainite nucleation model different from previously proposed models. In agreement with the Koistinen-Marburger model for martensite formation and other displacive bainite models, the average volume of bainitic sub-units is assumed to be constant over the extent of the transformation and the growth of subunits is very fast, and thus the change in fraction is directly related to the nucleation rate of bainite. These growth assumptions in combination with the nucleation model results in an analytical expression for the fraction bainite as a function of time that contains only two adjustable parameters: a (temperature-independent) autocatalytic parameter λ and a rate parameter κ, which has a temperature dependence that is mainly governed by Q*. The calculations are compared with experimental fraction curves measured isothermally with dilatometry for the lean-Si carbon steels Fe-0.46C, Fe-0.53C, Fe-0.66C and Fe-0.78C at a range of temperatures. For each of the steels a very good agreement between the calculations and the experimental data is obtained at all temperatures using a single value for λ and temperature dependent κ values. Evaluation of the κ values shows that Q* decreases linearly with temperature, which is consistent with other bainite nucleation models. By austenitizing steel C60 at different temperatures it is found that λ depends on the austenite grain size: when the austenite grain size is increased, λ becomes larger.