The authors studied the microstructure and phase composition of a pipe welded joint made of HP40NbTi alloy based on the Fe-25Cr-35Ni system of an ethylene pyrolysis furnace using light and scanning electron microscopy and energy-dispersive spectroscopy. The short-term mechanical properties at room and elevated temperatures, as well as the long-term strength of the base metal of the pipe and the welded joint are determined. The paper shows that the structural heterogeneity of the weld and the formation of the G-phase in the structure of the HP40NbTi alloy in the thermal influence zone have the following impact: the strength and plastic characteristics of the welded joint are not affected at room temperature; the ductility is slightly reduced at elevated temperatures; the long-term strength is significantly reduced at operating temperatures of pyrolysis plants. The paper investigates the nature of the fracture of the base metal and the welded joint. The results reveal the role of the G-phase in the formation of cracks during the operation of the w elded joint made of the HP40NbTi alloy.