Relationship between the activity coefficient of carbon and the carbon concentration in Fe-C binary alloys has been determined by means of CO-CO₂ gas mixtures at 1550°C. It was given as log γ′_C=2.33 (2N′_C-N′²_C)-0.181 (N′_C=0-0.207)
The activity of carbon and the interaction parameters ε^(C)_C, e^(C)_C were calculated in the whole range of carbon concentration N′_C=0-0.207.
The values of ε^(C)_C and e^(C)_C were compared with those given by other authors. Fe-C-X ternary alloys were equilibrated with the CO-CO₂ gas mixtures having a constant carbon activity at 1550°C and the effects of chromium, copper or tin on the activity of carbon in high carbon liquid steel were investigated.
The interaction parameters at constant carbon concentration have been determined as follows:
[ε^(Cr)_C]N_C=0.032=-3.9
[ε^(Cu)_C]N_C=0.032=4.8
[ε^(Sn)_C]NC=0.032=9.8
These values were compared with those at infinitely dilute carbon solutions and at graphite saturated solutions.