Thermodynamics of Hägg carbide (Fe 5 C 2 ) formation

Gas carburisation experiments on iron samples were conducted in CO‐H 2 ‐H 2 O‐H 2 S atmospheres with different carbon activities at 500 °C. Carbon activities well above a C = 1 (equilibrium iron/graphite) were used in order to form iron carbides such as cementite (Fe 3 C) and Hägg carbide (Fe 5 C 2 ). The Gibbs energy of formation ΔG for Fe 5 C 2 was determined at 500 °C. This result combined with thermodynamic properties at other temperatures taken from literature strongly supports the existence of the three phase equilibrium α‐Fe+Fe 3 C+Fe 5 C 2 at about 350°C in the binary Fe‐C system.