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 750°C. 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.