Carburization Resistance of Austenitic Alloys in CH₄-CO₂-H₂ Gas Mixtures at Elevated Temperatures

Thermodynamic calculations were conducted to understand the carburizing environment of ethylene industries. The equilibrium $PO2$ was calculated to be from 10^–21 atm to 10^–19 atm for the operating conditions of the actual furnace tubes from 900°C to 1,100°C. In this environment, protective chromium oxide (Cr₂O₃) scale was considered stable for up to 1,030°C to 1,040°C, but it becomes unstable and relative to chromium carbides such as Cr₇C₃ and Cr₃C₂ at above 1,030°C to 1,040°C. Laboratory carburization tests of four commercial alloys simulating the actual pyrolysis environment were conducted at temperatures from 1,000°C to 1,150°C. At 1,000°C, chromium was an effective alloying element, and more than 25 mass%Cr was necessary for alloys to protect against the carburization environment. Chromium in the alloy formed a Cr₂O₃ protective oxide scale to prevent carburization. At 1,100°C and 1,150°C, however, Cr₂O₃ scale did not provide complete protection, but silicon dioxide (SiO₂) scale, which formed underneath the Cr₂O₃ scale, reduced the carburization of the alloys. The depth of the internal carburization zone of each alloy was calculated by the Wagner’s internal oxidation model at 1,100°C and was compared to the experiment. The mole fraction of carbon at the external surface of each alloy was obtained from the isothermal phase stability diagram at 1,100°C, and the depth of the internal carburization zone of each alloy was calculated. The calculation generally provided smaller values than the experiment, but semi-qualitative interpretation explained the different carburization depths for the tested alloys. Alloy resistance to carburization can be improved by the formation of a uniform protective/stable oxide scale, and/or by increasing alloying elements such as Ni, which can decrease the carbon content in the γ matrix at the external surface.