Failure analysis of 304H stainless steel convection tube serviced in an ethylene cracking furnace

Abstract The failure of a convection tube used in an ethylene cracking furnace after service for >6 years was analyzed by various characterizing techniques. Cracks were found on the second convection tube located under the straight tube section. Severe bulging deformation was found around the cracking area. Stereoscopic inspection revealed that the main crack has penetrated through the tube wall, while a circumferential crack network was formed on the failed tube surface. Chemical composition analysis confirmed that the failed tube material belong to the 304H stainless steel family. SEM analysis on the fractured surface showed that the cracking mechanism was mainly intergranular brittle fracture. Microstructure observation found that grains within the cracked area experienced severe growing compared with intact area. Besides, more coarsened chromium carbides distributed within grains and along grain boundaries around cracked area, where voids were also occurred at grain boundaries. Based on the above inspection and analysis, the crack of the convection tube were confirmed to be caused by the localized overheating. The root cause for such localized overheating may due to inadequate steam flow and uneven heat distribution within the convection tube during service.