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Spatially Resolved X-Ray Diffraction (SRXRD) has been used to identify a previously unobserved low temperature ferrite (δ)/austenite(γ) phase transformation in the heat affected zone (HAZ) of 2205 Duplex Stainless Steel (DSS) welds. In this “ferrite dip” transformation, the ferrite transforms to austenite in the weld HAZ at peak temperatures on the order of 750oC. The austenite then re-transforms to ferrite during cooling, resulting in a ferrite volume fraction equivalent to that in the base metal. Time Resolved X-Ray Diffraction (TRXRD) and laser dilatometry measurements during Gleeble® thermal simulations are performed here in order to confirm the existence of this low temperature phase transformation. Experimental results, combined with thermodynamic and kinetic modeling of interstitial diffusion, have been used to investigate the mechanisms driving the “ferrite dip” transformation. These results indicate that the formation and decomposition of secondary austenite during heating and cooling, respectively, is most likely responsible for this observed behavior. Introduction The ferrite(δ)/austenite(γ) transformation in duplex stainless steels (DSS) during GTA welding has recently been monitored using an in-situ Spatially Resolved X-Ray Diffraction (SRXRD) technique.[1] In this study, x-ray diffraction is used to monitor the phases present at discrete locations in the weld Heat Affected Zone (HAZ). Based on these observations, the phase transformations occurring at locations across the length and width of the weld HAZ are identified. An analysis of the resulting x-ray diffraction patterns also provides a quantitative description of these phase transformations. Figure 1 shows the map of the measured ferrite volume fractions at discrete locations surrounding the weld pool.[1] The isotherms shown on the plot have been calculated using a well-tested three-dimensional thermo-fluids mathematical model.[2] Using this map, the δ/γ phase transformations occurring in the HAZ during the weld heating and cooling cycles can be predicted as a function of both position and time. When coupled with the calculated thermal cycles, unique and previously unattainable kinetic information can be obtained, and a previously unobserved phase transformation is detected. Evidence for this transformation is shown in Figure 2, where the ferrite volume fractions measured at a location 9.5 mm from the weld centerline and corresponding thermal cycle are plotted. This unexpected low temperature transformation sequence is characterized by a decrease in the ferrite volume Liquid δ/γ Solvus Resolidified Zone