A large body of knowledge on the carburization of engineering alloys used in high-temperature petrochemical processing equipment is being developed. The capability includes: the facility to perform thermochemical assessments of carbide formation on complex alloys in diverse conditions and the capability to predict carburization for various exposure conditions. The alloys considered are widely used, commercial and based upon mixtures of Fe-Cr-Ni-Co. The gaseous exposure conditions considered contain CHa-H2-HzS-H20. The thermochemical data models have been prepared to predict phase formation of potential corrosion products. They are based upon extensive analyses of all available thermochemical data for all possible solid and liquid compounds and solutions based upon all combinations of Fe-Cr-Ni-Co-S-C-O-N and for all possible gaseous species containing S-C-O-H-N. As well, the alloying elements A1, Mo, Nb, Ti, V, W, Mn and Si are fully included in the alloy and carbonitride solution models. Comprehensive solution models are used in Gibbs free energy minimization calculations to assess the interactions of multiple species in variable composition solid and liquid phase alloys, sulfides, oxides, carbides, and nitrides. This capability is used to predict the most stable corrosion product formation, which is then used to infer the dominant corrosion mechanism, in complex conditions.
Many alloys are used to fabricate equipment, which handles conditions that cause carburization during the processing of many different chemicals. Examples of these processes, which make such chemicals, are: ethylene, hydrogen, styrene, and synthetic rubber. Part of the limitations of the conditions imposed upon operations of these processes is the expectation of equipment damage by carburization 1-9. If the extent of carburization could be better predicted and better managed, this would allow more economical selection of alloys or less conservative selection of process conditions. These possibilities should allow increased thermal efficiencies, lower operating costs, and higher process reliability. The range of alloys considered in this project are austenitic and ferritic alloys based upon mixtures of Fe-Cr-Ni-Co. The technologies discussed in this paper allow prediction of stable phases over the temperature range of ambient to liquidus temperatures. This project has compiled and analyzed all available thermochemical data for all known solid and liquid compounds and solutions which can form from all possible combinations of Fe-Cr-Ni- Co-S-C-O-N and for all possible gaseous species containing S-C-O-H-N. As well, the carbide and nitride forming alloying elements A1, Mo, Nb, Ti, V and W, together with the alloying elements Mn and Si, are fully included in the alloy and carbonitride solution models, while the oxides and sulfides of these elements are treated as pure stoichiometric compounds. Details of the thermochemical data analyses will be reported separately. Briefly, the ability to calculate the occurrences of stable carbides, carbide compositions and the co-existing alloys can be calculated for wide ranges of compositions representing commercial alloys. This capability has been combined with a previously available database and software system (ASSET) ~' 11-17 to examine carburization of a number of commercial alloys and to develop a testing program, as reported in the balance of this paper.
Effect of Temperature
The usual experience of higher temperatures causing increasing amounts of carburization is illustrated in Figure 1, calculated by ASSET from real carburization data. In addition, the results of thermochemical calculations for the formation of carbides on these alloys for the same c