Direct observations of austenite, bainite, and martensite formation during arc welding of 1045 steel using time-resolved X-ray diffraction

In-situ time-resolved X-ray diffraction (TRXRD) experiments were performed during stationary gas tungsten arc (GTA) welding of AISI 1045 C-Mn steel. These real-time synchrotron-based experiments tracked phase transformations in the heat-affected zone of the weld under rapid heating and cooling conditions. The diffraction patterns were recorded at 100 ms intervals, and were later analyzed using diffraction peak profile analysis to determine the relative fraction of ferrite (α) and austenite (γ) phases in each diffraction pattern. Lattice parameters and diffraction peak widths were also measured throughout the heating and cooling cycle of the weld, providing additional information about the phases that were formed. The experimental results were coupled with temperatures calculated by a thermo-fluids weld model, allowing the transformation kinetics of the α→γ phase transformation to be evaluated. During heating, complete austenitization was observed in the heat-affected zone of the weld, and the kinetics of the α→γ phase transformation were modeled using a Johnson-Mehl-Avrami (JMA) approach. The results from the 1045 steel weld were compared to those of a 1005 low-carbon steel from a previous study. Differences in austenitization rates of the two steels were attributed to differences in the base metal microstructures, particularly the relative amounts of pearlite and the extent of the allotriomorphic ferrite phase. During weld cooling, the austenite transformed to a mixture of bainite and martensite. In situ diffraction was able to distinguish between these two nonequilib rium phases based on differences in their lattice parameters, diffraction peak widths, and their transformation rates, resulting in the first real-time X-ray diffraction observations of bainite and martensite formation made during welding.