Carburization Assessments of Alumina Forming Alloys in Ethylene Pyrolysis Furnace Tubes

Carburization frequently implies carbon is absorbed into a material at elevated temperature while in contact with a carburizing environment. Presence of high gas phase carbon activity (hydrocarbons, coke, gases rich in CO, CO2, CH4, ethane) and low oxygen potential can lead to carburization. Fe-Cr-Ni alloys are more resistant than carbon steel and low-alloy steels due to their higher Cr and Ni content. Carburization can result in the loss of high-temperature creep ductility, loss of ambient temperature toughness and ductility, loss of weldability, and reduced corrosion resistance. Ethylene pyrolysis furnace tubes are common type of equipment susceptible to carburization. Select alloys with adequate resistance to carburization typically includes alloys that form stable oxides on the surface. In this paper in addition to literature review, experience with alumina forming alloys is described. Also, comparison with traditional non alumina forming alloys is included. Finally factors to consider in the material selection are listed and analyzed. Metallurgical analyses including optical microscopy, scanning microscopy, chemical analyses, hardness readings were used to assess and characterize carburization rates.