Abstract
Thermodynamic calculations were conducted to understand the carburizing environment of ethylene pyrolysis furnace tubes in the petrochemical 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.
