The paper experimentally studied the microstructure and phase composition of the welded joint of centrifugally cast pipes made of austenitic heat-resistant alloys based on the Fe-25Cr-35Ni-0.45C system. We analyzed the macro- and microstructure of the welded joint using light and electron microscopy. There was a significant heterogeneity of the structure in various zones of the welded joint, intermetallic phase forms in the heat-affected zone. We revealed complex nature of the change in the alloy microhardness in different zones of the welded joint and studied the redistribution patterns of alloying elements in the weld joint during welding of Fe-25Cr-35Ni-0.45C alloy pipes. We established that the nature of the alloy structure in different zones of the welded joint varies significantly in dispersion degree, anisotropy, eutectic morphology and carbide inclusions sizes, and the volume fraction of eutectic in the weld metal increases compared to the base metal. The paper assumed the ways the revealed formation features of the welded joint structure of the alloy affects its mechanical properties.