Modeling of Ferrite to Austenite Transformation and Real Time Mapping of Phases during GTA Welding of 1005 Steel

The ferrite to austenite (α → γ) transformation in AISI 1005 steel weldments was studied during gas tungsten arc (GTA) welding experimentally and theoretically. A recently developed, synchrotron-based, spatially resolved X-ray diffraction (SRXRD) technique was used to obtain the real-time map of phases. A three-dimensional heat transfer and fluid flow model was used to calculate the temperature and velocity fields, and thermal cycles. The experimental SRXRD phase map and the computed thermal cycles were then used to understand the kinetics of the α → γ transformation during heating. A new kinetic equation considering the equilibrium γ fraction in the α+γ two-phase region was developed and used to quantitatively describe the kinetics of the α → γ transformation. The kinetic parameters, i.e., the exponent (n) and the pre-exponential constant (k ₀), were calculated from the SRXRD kinetic data and computed thermal cycles using a known value of the activation energy (Q). These kinetic parameters were then used to reconstruct the α/(α+γ) and (α+γ)/γ spatial boundaries during heating, and the results were compared with the experimental SRXRD phase map. The agreement indicates that the kinetic parameters can be used to quantitatively understand the α → γ transformation kinetics in 1005 steel under various thermal cycles.