CARBURIZATION RESISTANCE OF Fe-25 mass percent Cr-38 percent Ni-1.8 percent Si- 1.5 percent Mo ALLOY IN LABORATORY CH_4-CO_2-H_2 GAS ENVIRONMENTS AT 1000 approx 1150 deg C

摘要

Thermodynamic equilibrium calculations were conducted to understand the carburizing environment of ethylene pyrolysis in the oetrochemical industries. Calculated equilibrium, P_(O_2), was 21 estimated to be in the range of 10~(-21) to 10~(-19) atm for the operating condition of actual ethylene pyrolysis plants. In this environment, chromium oxide was stable for up to 1030 approx 1040 deg C. However, Cr_2O_3 converted to chromium carbides such as Cr_7C_3 and Cr_2C_3 above 1030-1040 deg C. Based on the calculations, laboratory carburization tests were carried out. Chromium was an effective alloying element, and more than 25mass percent Cr was necessary for austenitic alloys to provide protection against carburization environments at 1000 deg C. Chromium as a Cr_2O_3 protective oxide scale on the metal surface prevented carbon ingress for alloys. At 1100 and 1150 deg C, however, Cr_2O_3 scale did not provide protection, and silicon oxide, which formed underneath the Cr_2O_3 scale, was effective in reducing the carbon ingress. This behavior agreed well with the thermodynamic stability of the oxide scales. Alloy of Fe-25 percent Cr-38 percent Ni-l.8 percent Si-l.5 percent Mo had excellent carburization resistance similar to Fe-32 percent Cr-43 percent Ni -1.7 percent Si cast alloy in the laboratory tests.