<p>In complex welding environments, the conventional 80% Ar + 20% CO₂ shielding gas mixture no longer represents the optimal choice. Advances in welding technology have made it essential to not only optimize process parameters but also to select shielding gases better suited to achieve improved weld quality and performance. This study investigated the effects of four shielding gas mixtures, including 72% Ar + 28% CO₂ (M31), 80% Ar + 20% CO₂ (M21), 92% Ar + 8% O₂ (M22-1), and 90% Ar + 10% O₂ (M22-2), on the microstructure and mechanical properties of S355J2W + N weathering steel welded using high-frequency pulse metal active gas (MAG). All welded joints demonstrated good fusion without defects such as cracks or porosity, achieving single-sided welding with double-sided formation. The weld microstructure formed using M31 was the finest and most uniform, mainly comprising grain boundary ferrite (GBF), acicular ferrite (AF), pearlite (P), and granular bainite (BG). Similarly, M31 showed stronger penetration, achieving greater weld depth with relatively lower heat input. The hardness distribution followed a consistent pattern: in the third weld layer, the order was heat-affected zone (HAZ) &gt; weld zone &gt; base metal, whereas in the first weld layer, it was weld zone &gt; HAZ &gt; base metal. The use of M31 shielding gas resulted in the highest weld impact toughness, with average impact energies of 67.75 Akv/J in the weld zone and 91.55 Akv/J in the HAZ. Furthermore, the “T”-shaped joints welded with M31, owing to their increased penetration capability, exhibited reduced heat input and significantly lower residual stresses. The measured transverse residual stress (σ_x) was 118&#xa0;MPa, while the longitudinal residual stress (σ_y) reached 211&#xa0;MPa. In conclusion, M31 was identified as the most suitable shielding gas for welding S355J2W + N weathering steel, providing effective theoretical guidance and technical support for the welding of bogie frames.</p>