Microstructure and mechanical properties of ethylene pyrolysis furnace tube weld joints after service

The microstructure and mechanical properties of weld joints in ethylene pyrolysis furnace (Cr25Ni35NbM alloy and Cr35Ni45NbM alloy tube) after service were investigated in this work. The microstructure of the new weld joint and base metal of pyrolysis furnace tube vary widely, and the high temperature creep rupture time of the weld joint is only reached 50% of the base metal (1100°C, 16MPa). After high-temperature service, the microstructure of weld metal and base metal deteriorated and the high temperature creep strength decreased. Therefore, the microstructure of weld metal and base metal is tend to uniform, and the creep strength of weld joint and base metal reached the same level. After high-temperature aging, the primary carbides coarsen and secondary carbides precipitation results in a uniform microstructure and creep property of weld metal and base metal. The high temperature creep rupture time of weld joint reached 90% of base metal (1100°C, 16MPa). As the dispersion strengthening effect of the secondary carbide disappears, the creep strength of the weld joint no longer increases, but begins to decrease. Both high temperature service and elevated temperature aging result in the homogenization of the weld joint and base metal, and the high-temperature creep strength also reaches the same level.