Modeling and real time mapping of phases during GTA welding of 1005 steel

Evolution of the microstructure in AISI 1005 steel weldments was studied during gas tungsten arc (GTA) welding experimentally and theoretically. The experimental work involved real-time mapping of phases in the heat-affected zone (HAZ) using a synchrotron-based spatially resolved X-ray diffraction (SRXRD) technique and post weld microstructural characterization of the fusion zone (FZ). A three-dimensional heat transfer and fluid flow model was used to calculate the temperature and velocity fields thermal cycles, and the geometry of the FZ and the HAZ. The experimental SRXRD phase map and the computed thermal cycles were used to determine the kinetic parameters in the Johnson-Mehl-Avrami (JMA) equation for the ferrite to austenite transformation during heating in the HAZ. Apart from providing a quantitative expression for the kinetics of this transformation, the results are consistent with a decreasing nucleation rate of austenite from a ferrite matrix with time. In the FZ, the volume fractions of microconstituents were calculated using an existing phase transformation model and the computed thermal cycles. Good agreement was found between the calculated and experimental volume fractions of allotriomorphic and Widmanstatten ferrites in the FZ. The results indicate significant promise for understanding microstructure evolution during GTA welding of AISI 1005 steel by a combination of real time phase mapping and modeling.