Influence of elevated temperature and reduced pressure on the degradation of iron nitride compound layer formed by plasma nitriding in AISI D2 tool steels

The stability of an iron nitride compound layer prepared by plasma nitriding of AISI D2 tool steel is investigated using in situ high-temperature X-ray diffraction at rising temperature up to 470 degrees C and for an isothermal treatment at 450 degrees C. Both kinds of experiment were carried out in vacuum. For all treatments, the iron nitride compound layer, which consists mainly of epsilon-Fe3N, decomposes and forms an alpha-Fe 'black' layer. The stability of the iron nitride compound layer is discussed in terms of the phase composition, the relative change of the lattice parameter, the nitrogen content and the decomposition kinetic. The decomposition takes place via a phase transition in two steps: (i) the out-diffusion of nitrogen from epsilon-Fe3N above an annealing temperature of 300 degrees C and (ii) the transformation of epsilon-Fe3Ny into alpha-Fe when the nitrogen content (y) declines below 0.66. The second step is accompanied by the formation of Fe3C precipitates at about 450 degrees C.