Thermodynamics of the Fe-N and Fe-N-C Systems: The Fe-N and Fe-N-C Phase Diagrams Revisited

Abstract Several thermodynamic descriptions of the Fe-N and Fe-N-C systems were proposed before now. The results of these descriptions significantly deviate from more recently obtained experimental data. The present work provides a revised thermodynamic description of these systems. The new description for the Fe-N system agrees distinctly better with the experimental data especially for the equilibrium of γ′-Fe 4 N 1− x and ε-Fe 3 N 1+ z . The new thermodynamic description for the Fe-N-C system considering the Fe-rich part of the system with less than 33 at. pct N and less than 25 at. pct C excellently agrees with the new experimental data for both the temperatures of the invariant reactions and the phase boundaries. This in particular concerns the temperature range of typical technical nitriding and nitrocarburizing treatments [723 K to 923 K, (450 °C to 650 °C)], within which three invariant reactions occur in the ternary system.