Microstructural Transformation and Hardness Response of TIG‐Welded UNS S31803 Duplex Stainless Steel Subjected to Post‐Weld Heat Treatment

This study investigates the effect of post‐weld heat treatment (PWHT) on the microstructure and hardness of TIG‐welded UNS S31803 duplex stainless steel, with the aim of establishing processing guidelines for industrial applications. PWHT was performed at temperatures ranging from 900 to 1200°C for 1 h, followed by water quenching. An additional aging treatment at 850°C for 1 h was intentionally applied to evaluate phase stability within the critical σ‐phase precipitation range. SEM–EDS analyses revealed temperature‐dependent microstructural transformations, including the reformation of austenite (γ) from ferrite (α), as well as the precipitation and dissolution of the σ‐phase and Cr‐rich M 23 C 6 carbides. The hardness exhibited a non‐monotonic trend, reaching a maximum value of 345 HV at 1000°C in the FZ. This increase is attributed to precipitation strengthening, microstructural refinement, and pronounced elemental partitioning (Niγ/α ≈ 2.19), which enhance phase boundary density and dislocation pinning. At higher temperatures, the hardness decreased to approximately 250 HV at 1100°C and further declined at 1200°C due to grain growth, precipitate dissolution, and recovery phenomena, accompanied by reduced elemental partitioning (Niγ/α ≈ 1.44). These findings highlight the critical role of PWHT in controlling microstructural evolution and phase stability.