Thermodynamics of carbon in nickel, iron-nickel and iron-chromium-nickel alloys

Abstract

Iron-nickel alloys with 8 and 16 wt pct nickel and iron-chromium-nickel alloys with 8 pct nickel and chromium contents in the range of 2 to 22 pct were equilibrated with iron and nickel in flowing CH4-H2 gas mixtures and in sealed capsules under partial vacuum at temperatures between 700 and 1060°C. Carbon activities in these alloys were established from the carbon concentrations in the nickel by applying Henry’s law to the solubility of carbon in nickel that was determined in the temperature range of 500 to 1000°C. First-order free-energy interaction parameters were used to relate the carbon activities to composition and temperature in the single-phase austenitic Fe-Ni and Fe-Cr-Ni alloys. An expression was also developed to evaluate carbon activities in Fe-Cr-Ni alloys in the region of higher chromium contents (〉4 wt pct) that result in a two-phase austenite plus carbide mixture at these temperatures.