Microstructure and Property of the Weld Heat-Affected Zone of T4003 Ferritic Stainless Steel with Different Mo Contents

In the present contribution, Hot-rolled and annealed ferritic stainless steel T4003 with three distinct Mo contents (0%, 0.1%, and 0.2%) served as the research subject. Weldability tests were implemented by means of gas metal arc welding. Coupled with microstructural characterization, mechanical property assessments, and electrochemical corrosion tests, the regulatory mechanism of Mo on the microstructure and properties of the HAZ was systematically elucidated. Results demonstrate that the influence of Mo content on the evolution of the coarse-grained region structure of heat affected zone becomes significant. The addition of 0.1% Mo refines the grains, increasing the fraction of lath martensite to 70–75% while limiting the maximum width of the coarse-grained zone to 0.64 mm. Meantime, the addition promotes the precipitation of (Nb, Ti, Mo) (C, N) composite carbonitrides, enhancing overall performance through synergistic grain refinement and second-phase strengthening. The sample with 0.1% Mo exhibits an average low-temperature impact energy of 16.3 J at −40 °C, with the highest Vickers hardness in the HAZ, favorable strength–plasticity synergy of the welded joint, and optimal corrosion resistance. The coarse-grained zone of the 0.2% Mo sample is dominated by coarse δ-ferrite and features a larger width, and the HAZ shows inferior mechanical properties and corrosion resistance. The precipitated phases in the 0.2% Mo segregate along the grain boundaries and distribute in a chain-like distribution, exacerbating the deterioration of material properties. These findings provide a technical reference for optimizing the composition design of T4003 ferritic stainless steel and ensuring its safe application in railway freight vehicles.