The authors studied the structure, phase composition, and microhardness of a 14 mm thick pipes welded joint made of HP40NbTi alloy using optical and electron microscopy. The profile of microhardness changing along the welded joint is plotted. The phase composition of different zones of the welded joint has been studied qualitatively and quantitatively. The paper reveals the structural features of different zones of a welded joint of centrifugally cast pipes made of austenitic heat-resistant alloy based on the Fe-25Cr-35Ni-0.45 C system. The authors established heterogeneity of the structure in different sections of the welded joint, detected the formation of segregation of individual alloying elements and the formation of an intermetallic phase in the heat affected zone and weld metal zone. The regularities of changes in the structure and microhardness of the alloy in different zones of the welded joint are studied and explained. The paper presents an analysis of the possible influence of the revealed structural and phase changes in the HP40NbTi alloy during welding on the performance of the pipe welded joint under operating conditions.