Optimization of Microstructure and Pitting Corrosion Resistance in Laser‐Welded UNS S32001 Lean Duplex Stainless Steel Joints by Post Weld Heat Treatment

This study systematically investigates the effect of post‐weld heat treatment (PWHT) at 950–1250°C for 30 min on the microstructure, mechanical properties, and corrosion behavior of laser‐welded UNS S32001 lean duplex stainless steel (LDSS) joints. The as‐welded weld metal exhibits a ferrite‐dominated microstructure with an austenite content of only 20.7%. PWHT at 1050°C promotes balanced elemental diffusion and phase transformation kinetics, yielding a near‐equal ferrite‐austenite ratio (49.6% austenite) with a refined, equiaxed, and interwoven dual‐phase structure. In contrast, PWHT at 950°C retains undesirable Widmanstätten austenite (52.5% austenite), while higher temperatures (1150–1250°C) cause austenite dissolution (44.3% and 32.5%, respectively) and severe grain coarsening. Correspondingly, the specimen treated at 1050°C exhibits the best pitting corrosion resistance, with the highest critical pitting temperature (∼78.4°C vs. 71.1°C for as‐welded), the highest breakdown potential ( E b = 0.678 V SCE ), the lowest corrosion current density ( i corr = 4.175 × 10 −7 A·cm −2 ), and the largest polarization resistance ( R p = 283.9 kΩ·cm 2 ). This superior performance is attributed to the fine, homogeneous, interwoven dual‐phase structure, which enhances passive film stability, reduces localized electrochemical activity, and acts as an effective physical barrier against pit propagation.