Fe–N and Fe–N–C phase equilibria above 853 K studied by nitriding/nitrocarburising and secondary annealing

Compound-layer microstructures obtained by gaseous nitriding and nitrocarburising treatments of α-Fe and Fe–N alloy and by subsequent treatments were analysed to obtain new information on the metastable Fe–N and Fe–N–C systems above 853 K at 1 atm. In the case of the binary Fe–N system, invariant temperatures and phase boundaries agree well with the literature data, only the γ/γ + γ' and the γ/γ + ∊ boundaries as determined in the present work by means of electron probe microanalysis are located at N contents up to 1 at.% lower than reported in previous works. In the case of the ternary Fe–N–C system, the invariant eutectoid reaction was found to occur in the range 853 K – 863 K, whereas the invariant transition reaction was found to occur in the range 868 K – 873 K. The obtained data were used to draw isothermal sections of the Fe–N–C system for 853 K and 893 K. The determined temperatures for the invariant eutectoid and transition reactions as well as the extent of the ternary ∊-phase field were compared with phase diagrams calculated on the basis of different thermodynamic descriptions of the Fe–N–C system.