The until now controversially discussed existence of the two-phase equilibrium α-Fe + ∊-Fe3(N, C)1+x in the ternary system Fe – N – C was proven by the investigation (optical microscopy, X-ray diffraction, Electron Backscatter Diffraction) of compound-layer microstructures generated by gaseous nitrocarburising of pure iron and subsequent secondary annealing experiments under static inert-gas atmosphere. The invariant transition reaction γ-Fe4N + Fe3C → α-Fe + ∊-Fe3(N, C)1+x could be shown to proceed at a (single) temperature (at constant pressure of 1 atm) between 833 K and 843 K.
International audienceMassive cementite layers with a time-dependent thickness were grown on ferrite substrates by nitrocarburising in a dedicated NH3/H2/CO/N2 containing gas atmosphere at 783 K, 823 K and 843 K. Nitrogen diffusion through the cementite layer into the ferrite substrate took place in conjunction with growth of the cementite layer; a significant, i.e. measurable solubility of nitrogen in cementite was not observed. The nitrogen concentration-depth profiles in the substrate, underneath the growing cementite layer, were quantitatively determined using a calibrated microhardness-measurement technique. The nitrogen concentration-depth profiles were simulated on the basis of a model using an implicit finite-difference method. The simulation yielded values for the diffusivity of nitrogen through cementite, including the activation energy
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