The goal of this project is to determine the relationship between preheat temperature and phase balance of austenite and ferrite in the heat affected zone (HAZ) of a dissimilar thickness weld made of Sandvik SAF 2205 duplex stainless steel (DSS). Various preheat temperatures were applied (no preheat, 63°C, 98 °C, 141 °C, 181 °C, and 214 °C) to a 2-inch thick plate that was welded to a 0.072-inch thin sheet, with a 0.125-inch standoff, using manual gas tungsten arc welding (GTAW). Welding variables (voltage, amperage, and travel speed) were recorded and thermocouples were placed along the backside of the weld, underneath a purging tube of pure argon. For each preheat temperature, a sample was sectioned between the thermocouples and underwent metallography. Phase analysis and microhardness testing were performed at five locations across the weld: thick plate, thick plate HAZ, weldment, thin sheet HAZ, and thin sheet. The percentage of austenite and ferrite was determined using IQ Material software. Microhardness testing was performed in accordance with ASTM E384-17. Welding parameters and thermocouple data were used to calculate heat input and the resulting cooling rates. Results indicated the weldment and HAZ zones did not sufficiently improve their phase balance as preheat temperature increased. Inconsistencies in the welding parameters resulted in a variance of heat input between preheat temperatures, which influenced cooling rates. Since the cooling rate directly affects the phase balance, a relationship between preheat temperature and phase balance could not be determined.