Effects of Filler Wire Composition and Post-Weld Heat Treatment on the Mechanical Properties of 4Cr13 Steel Welded Joints

4Cr13 martensitic stainless steel is widely used for plastic molds, but welding repair can produce heterogeneous weld/HAZ microstructures and a strength–toughness trade-off. This study evaluates how filler-wire composition and post-weld heat treatment affect the mechanical performance of repaired 4Cr13 steel. TIG welding was performed using high-molybdenum (HM), low-carbon (LC), and base-metal (BM) wires, followed by either post-weld tempering or quenching and tempering. Post-weld quenching and tempering reduced the hardness gradient and improved the overall hardness uniformity. The average weld-metal hardness followed HM > BM > LC, while the BM joint showed the smallest hardness variation because its nominal composition matched that of the base metal. After quenching and tempering, the tensile strengths of the HM, LC, and BM joints reached 1299.5, 1315.6, and 1300.0 MPa, respectively, compared with 1290.8 MPa for the similarly treated base metal. The corresponding impact energies were 58.9, 116.3, and 36.8 J, with LC showing the highest toughness. Fracture observations were consistent with these mechanical-property differences. The results demonstrate that filler-wire composition and post-weld heat-treatment route should be selected jointly according to the required balance of hardness uniformity, strength, ductility, and impact toughness, providing an experimental basis for repair-welding process selection for 4Cr13 steel.