Abstract: The microstructure and tensile properties of the heat affected zone of a service-degraded HP40Nb hydrogen production convert furnace tube were studied by the welding thermal simulation. The results show that coarse and continuous G phase and chromium-rich M₂₃C₆ carbides existed on austenite grain boundaries in the service-degraded furnace tube. After thermal cycling at peak temperature of 1 100-1 200 ℃, the precipitates still consisted of G phase and M₂₃C₆ carbides, and both decreased in content with increasing peak temperature. When the peak temperature was 1 250 ℃, the precipitates were composed of M₂₃C₆ and NbC phases. The tensile strength and elongation of the heat affected zone samples obtained at lower peak temperatures (950-1 050 ℃) were lower. When the peak temperature was higher than 1 100 ℃, the tensile strength and elongation increased with increasing temperature. When the peak temperature increased to 1 150 ℃ and above, the tensile strength reached the requirements of the unserviced HP40Nb alloy, while the elongation was smaller than the standard requirement.