Abstract: To solve the problem that ethylene pyrolysis tubes produce carburization damage in service at high temperature,about 10 tubes of HP40 alloy after 3-7 years service in an ethylene pyrolysis furnace had been choiced and the carburization damage and carbarization mechanism had been investigated by microstructure and micro-content analysis.The results show that the damage of the inner surface was more seriouse than that of the outer surface.During 5 years service life the carburizing depth of the inner surface was larger than that of the outer surface,but the carbon content of the inner surface was less than that seriouse of the outer surface at the same depth.The carburization damage is an alternant process of oxidizing and carburizing.