<p>The 9Cr-3W-3Co-1Cu-VNbBN steel, designed for long-term service at 630-650°C, has been successfully applied in engineering practice. To address challenges of localized heat treatment during on-site welding—such as rapid heat dissipation and uneven heating—a low-temperature, long-duration post-weld heat treatment (PWHT) was developed at 740 ± 10°C for 10 h, with heating and cooling rates of 25°C/h. Welded joints were prepared using consumables matching the base material’s chemical composition and tempered according to the developed PWHT parameters. Radiographic testing confirmed defect-free welds. Mechanical tests showed yield strengths of 700-800 MPa at room temperature, and the high-temperature mechanical properties compared to the base material in the temperature range of 550-650°C. The impact toughness met design criteria. Microstructural analysis revealed M₂₃C₆ carbides precipitating along prior austenite grain boundaries (PAGBs), enhancing grain boundary stability and creep resistance, while fine Nb- and V-rich carbonitrides provided dispersion strengthening. Excessive M₂₃C₆ accumulation correlated with reduced impact toughness, emphasizing the importance of controlling welding heat input and PWHT parameters. These findings confirm that the PWHT process we designed produces welded joints with balanced strength and toughness suitable for engineering applications.</p>