| Summary: | During welding, the heat affected zone (HAZ) of X80 linepipe steel is subjected to very steep spatial variations in temperature and concentration of Nb bearing particles which results in a strongly graded microstructure. Therefore, models on the length scale of the microstructure, i.e. the so-called mesoscale, are useful to simulate microstructure evolution in the HAZ. Among mesoscale models, phase field modelling (PFM) is selected because it is based on diffusional time steps and it is a robust tool to capture complex morphologies, e.g. bainitic ferrite. A PFM is developed for austenite grain growth in 2D and 3D that is applicable to rapid heat-treatment cycles taking the pinning/dissolution effects of Nb bearing particles into account by using an effective mobility concept. In addition, a PFM is developed for the austenite decomposition to predict the simultaneous formation of polygonal ferrite and bainite. PFM is coupled with a carbon diffusion model and an effective interface mobility is introduced to implicitly account for the solute drag effect of Nb. For simplicity, the formation of carbide-free bainite is considered and a suitable anisotropy approach is proposed for the austenite-bainite interface mobility. The model is first applied to a TRIP steel in which ferrite and bainite form separately, and bainite can be considered carbide-free bainite. Then the model is applied to simulate the microstructural evolution in the HAZ of the X80 linepipe steel accounting for the thermal and microstructural gradients and validated with microstructure observations made in a weld trial. === Applied Science, Faculty of === Materials Engineering, Department of === Graduate
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