Computer prediction of the iron-carbon phase diagram : the [alpha]/[gamma] equilibrium and the solubilities of graphite in ferrite and austenite.
Thesis DisciplineMechanical Engineering
Degree GrantorUniversity of Canterbury
Degree NameDoctor of Philosophy
The LFG. MD and Moon first-order quasi-chemical equations for modelling the activity of carbon in binary Fe-C austenite and ferrite were examined and shown to be mathematically equivalent. Divergence in the activity of carbon calculated using the LFG and MD equations at certain values of the carbon-carbon pairwise interaction energy was demonstrated to be the result of computer arithmetic precision truncation. The Moon equation was determined to be fully convergent. The numerical precision requirements and other characteristics of this equation were established for its subsequent use. The experimental procedures for determining the activity of carbon in iron by gas carburisation with mixtures of CO-CO2, CH4-H2 and CO2-H2 were examined. It was concluded that the CO-CO2 gas mixture is unsuitable for this application. Suitable CH4-H2 equilibration data for the activity of carbon in austenite and ferrite were selected for subsequent analysis. An improved method for determining the carbon-carbon interaction energies and relative partial enthalpies and excess non-configurational entropies of solution of carbon in austenite and ferrite by regressing the experimental activity data with the Moon equation was established. The selected experimental activity data were then regressed to determine these values. Finally, the a/ equilibrium and solubilities of graphite in ferrite and austenite were calculated.