Degradation of Alloy 800H Used in Ethylene Furnace Tube Application by the Combined Effects of Carburization and Thermal Fatigue

An ethylene furnace tube made of the wrought alloy 800H has developed through the thickness crack. The cracking has been correlated with massive precipitation of embrittling carbide phases due to carburization attack and the thermally induced stresses during decoking cycles. Changes in microstructure and state of stress are reflected on the crack propagation mode, which is observed to change from fatigue near the outer surface to intergranular near the inner surface. It is concluded that the tube has been subjected to an environment of low oxygen potential and high carbon activity precluding the formation of protective oxide by the Cr₂O₃-forming alloy 800H. Process modification to reduce the decoking frequency and/or replacing the tube material with one capable of developing more protective oxide such as the Al₂O₃-forming alloy 214 may be considered to combat the problem.