Understanding the phase transformation kinetics from delta-ferrite (δ) to austenite (γ) is essential for optimizing post-weld heat treatment (PWHT) protocols in 9Cr steel welds, which are extensively used in high-temperature pressure components such as steam headers, piping, and turbine casings. The stability and dissolution behavior of δ-ferrite directly influence the final microstructure, mechanical properties, and long-term service performance of these steels. In this study, dilatometry was employed to investigate the δ→γ transformation under precisely controlled heating conditions. δ-ferrite–containing Grade P91 steel specimens, produced via weld metal solidification, were subjected to a range of heating rates representative of industrial PWHT practices. Dimensional changes were continuously monitored to capture transformation events with high temporal resolution. The onset and completion temperatures of the δ→γ transformation were determined for each heating rate, and transformation kinetics were quantitatively analyzed. Results show that both heating rate and prior microstructure exert a pronounced influence on transformation behavior. Higher heating rates shift the transformation to higher temperatures and, in some cases, result in incomplete δ-ferrite dissolution, potentially leading to microstructural inhomogeneity. These findings provide critical insights into the transformation mechanisms in Grade 91 steels and highlight the importance of carefully controlling PWHT parameters to achieve a fully homogenized and stable tempered martensitic microstructure, thereby improving mechanical performance and service reliability in welded components.