For the well-established equilibria: (1) Fe3C+CO2 ⇔ 3Fe+2CO + Δ H1 (2) C + CO2 ⇔ 2CO + Δ H2 (3) [C]1 + CO2 ⇔ 2CO (4) [C]2 + CO2 ⇔ 2CO (5) [C] + CO2 ⇔ 2CO
Where [C] = concentration of dissolved carbon in austenite, [C]1 = saturated value of carbon from carbon, [C]2= saturated value of carbon from iron carbide, and K1, K2, K3... are the gaseous equilibrium constants for these reactions. Thermodynamics demands K2 = K4, K2 = K3 and that K5 is a function of the concentration of carbon in austenite. Since iron-carbon alloys graphitize down to some 0.9% combined carbon or less, Fe3C is metastable with respect to carbon from 720° to 1130°C. Since the heat of formation of Fe3C is -15100 calories, it follows from the equation (d/dT) (Log K) = ΔH/RT2 and from the metastability of Fe3C that the carbon solubility line of the iron-carbon diagram lies to the left of the cementite solubility line and has a greater slope.
Proceedings of the Iowa Academy of Science
©1924 Iowa Academy of Science, Inc.
Maxwell, H. L. and Hayes, Anson
"Solubility in Austenite of Carbon from Carbon and of Carbon from Iron-Carbide,"
Proceedings of the Iowa Academy of Science, 31(1), 284-284.
Available at: https://scholarworks.uni.edu/pias/vol31/iss1/73