Kinetic modeling of phase transformations occurring in the HAZ of C-Mn steel welds based on direct observations

In situ Spatially Resolved X-Ray Diffraction (SRXRD) experiments were performed in the heat-affected zone (HAZ) of gas tungsten arc (GTA) welds of AISI 1005 C-Mn steel to directly observe welding induced phase transformations. These real-time observations were semi-quantified using diffraction peak profile analysis to construct a phase transformation map revealing ferrite (α) and austenite (γ) phase concentration gradients in the HAZ. Weld thermal cycles were calculated using a three-dimensional heat transfer and fluid flow model and then combined with the SRXRD phase map to provide a complete description of the HAZ under actual welding conditions. Kinetic modelling of the α→γ phase transformation during heating was performed using a Johnson-Mehl-Avrami analysis, modified to take into account non-uniform weld heating and transformation in the α+γ two-phase field. The results provide the most accurate JMA kinetic parameters to date for this alloy, n=1.45 and 1n(ko)=12.2, for an activation energy Q=117.1 kJ/mole. Using this kinetic description of the α→γ phase transformation, time temperature transformation (TTT) and continuous heating transformation (CHT) diagrams for this alloy were constructed to illustrate how the combination of SRXRD experiments and numerical modeling from one weld can be used to predict phase transformations for a variety of welding and heat treating applications.