Thermodynamic equilibrium calculations were conducted to understand the carburizing environment of ethylene pyrolysis in the 9Petrochemical industries. Calculated equilibrium, P02, was 21 estimated to be in the range of 10 to 10 1 atm for the operating condition of actual ethylene pyrolysis plants. In this environment, chromium oxide was stable for up to 1030~1040°C. However, Cr203 converted to chromium carbides such as Cr7C3 and Cr2C3 above 1030-1040°C. Based on the calculations, laboratory carburization tests were carried out. Chromium was an effective alloying element, and more than 25mass%Cr was necessary for austenitic alloys to provide protection against carburization environments at 1000°C. Chromium as a Cr203 protective oxide scale on the metal surface prevented carbon ingress for alloys. At 1100 and 1150°C, however, Cr203 scale did not provide protection, and silicon oxide, which formed underneath the Cr203 scale, was effective in reducing the carbon ingress. This behavior agreed well with the thermodynamic stability of the oxide scales. Alloy of Fe-25%Cr-38%Ni-l.8%Si-l.5%Mo had excellent carburization resistance similar to Fe-32%Cr-43%Ni -1.7%Si cast alloy in the laboratory tests.
Keywords: high temperature, carburization, ethylene pyrolysis, hydrocarbon, oxide, carbide, thermodynamic equilibrium