Carburization of high chromium alloys

Iron-nickel-chromium based heat resistant alloys are designed to operate at high temperatures in corrosive gaseous environments. Under mixed oxidizing-curburizing conditions, the microstructure of such materials changes progressively during service and the physical and mechanical properties are altered. One instance where such microstructural changes are encountered is in furnace tubes for pyrolysis applications. In the present study, kinetic experiments in the laboratory are combined with microstructural observations on alloys which have undergone long service times to develop an understanding of the fundamental processes that induce property changes in the material. Based on this study, four distinct stages are identified. These include: Initial oxidation, Oxidation in a carburizing environment, Direct carburization and Internal oxidation. Each of these stages is described. Questions are posed with respect to the sudden alteration in process stream chemistry or temperature. For instance, how does a drop in the oxygen partial pressure to levels where a chromium oxide film is unstable affect a preformed film? What beneficial effects are provided by inhibitors such as H₂S especially under conditions where an oxide film cannot form?