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 alpha-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 gamma/gamma + gamma' and the gamma/gamma + epsilon 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 epsilon-phase field were compared with phase diagrams calculated on the basis of different thermodynamic descriptions of the Fe-N-C system.