Integrated modelling of thermal cycles, austenite formation, grain growth and decomposition in the heat affected zone of carbon steel

The microstructure evolution in the heat affected zone (HAZ) of 1005 low carbon steel during gas tungsten arc welding (GTAW) was quantitatively investigated using a combination of several numerical models. In particular, the a ferrite→γ austenite phase transformation during heating was studied using a Johnson-Mehl-Avrami (JMA) analysis, the γ grain growth was calculated using a Monte Carlo simulation, and the γ→α transformation during cooling was examined using an austenite decomposition model. In addition, the phase equilibria of the 1005 steel were calculated using computational thermodynamics software, Thermo-Calc, while the necessary temperature v. time data for all the microstructure models were obtained from a thermofluid model. These models were then used to calculate the extent of austenitisation with time during heating, the γ grain growth, and the volume fractions of various microconstituents of the final microstructure in the HAZ. It was found that a considerable amount of superheat was required for the initiation and completion of the α→γ transformation under the heating rates typical of arc welding. Significant γ grain growth was found to take place in the HAZ, particularly in the vicinity of the fusion zone (FZ) boundary, where the computed maximum γ grain size was about eight times greater than that of the base metal. The predicted final microstructure in the HAZ was predominantly allotriomorphic and Widmanstatten ferrites, which was consistent with the post-weld metallographic measurements. Overall, the computed microstructure evolution in the HAZ using the multiphenomena models was consistent with the available experimental data. The results reported here indicate that it is now possible to develop a quantitative model of complex weld microstructure evolution with the recent advances in transport phenomena and phase transformation models.