Abstract: The material of the final superheater tube of a power plant is T23 steel, and it is replaced with T91 after 135, 000 h of service.In order to master the welding quality of the welded joint between T23 pipe after long-term service and T91 new pipe and provide data for the long-term operation of power plant and the pipe replacement transformation of similar power plants, the field welding process was simulated to weld T23 and T91 pipes, and the microstructure and properties of the welded joint were studied.The results show that the weld fuses well with both sides of T23 and T91, the heat affected zone at the T23 side is the weak area during room temperature and high temperature tensile tests, and the joint strength does not decrease significantly compared with that of the T23 base metal.During the welding process, the secondary phase in the coarse grained region at the T23 side melts into the base metal, the precipitated phase content decreases, while the precipitated phases in the fine grained region grow and coarsen.The hardness of the cover welding layer is higher than that of the bottom welding layer, and the hardness of the heat affected zone of the both layers is similar.Using the isotherm extrapolation method to extrapolate the remaining life of the welded joint, the remaining creep life is bigger than 50000 h, which meets the service requirements of power plant.