Abstract: During the maintenance period, the crack defect was detected during the magnetic particle inspection of the welded joint of the outlet header of a high temperature superheater in a power plant. The failure analysis of the pipe socket welded joint was carried out by stress distribution simulation, microstructure observation and hardness test. The results show that during the long-term high-temperature service, the coarse-grained area of the heat affected zone of the welded joint of the high temperature superheater outlet header was seriously aging, and the aging grade reached level of 4.5; the high-temperature mechanical properties significantly decreased. At the same time, the improper on-site installation of strong counterpart caused the additional secondary stress concentration of the tube panel. Under the additional secondary stress, the cracks initiated at the coarse-grained area of the heat affected zone on the upper half circle of the welded joint of the pipe socket. With the extension of service time, the substrate structure in the crack was decarbonized, and high temperature creep cracking occurred in the welded joint.