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Dissimilar welded joints (DWJ) of SP2215 and T92 tube were prepared by manual-gas tungsten arc welding (M-GTAW) and ERNiCr-3 filler. The microstructure, mechanical properties and tensile fracture mechanism of DWJ before and after heat treatment were studied. The results show that the weld metal (WM) is solidified into columnar dendrites with complete austenite structure, and Nb element is segregated between dendrites to form Nb-rich secondary phase particles, post welded heat treatment (PWHT) has no significant effect on the microstructure of welded seam. Island or peninsula macro-segregation is formed at both T92/WM and SP2215/WM interface, and a certain degree of carbon migration occurs across the T92/WM interface while the carbon migration across the SP2215/WM interface is not obvious, PWHT has no significant effect on macro-segregation and carbon migration; The hardness of coarse grain heat affected zone (CGHAZ) at T92 side is the highest, followed by fine grain heat affected zone (FGHAZ), while inter-critical heat affected zone (ICHAZ) has the lowest hardness, PWHT reduces the hardness of CGHAZ and FGHAZ significantly and modified the microstructure and mechanical properties of T92 HAZ; δ ferrite is formed near the fusion line in the T92 side, which is the lowest hardness area of the DWJ, PWHT has no significant effect on the hardness of δ ferrite. Both the as-welded and heat-treated DWJs are fractured at the welded seam in a ductile manner at room temperature, while fractured at the T92 side in a ductile manner at 650℃.