Morphology and lattice distortions of nitrided iron and iron-chromium alloys and steels

Fe, Fe-Cr alloys (0-4 wt % Cr), and commercial steels including Cr, Cr-Ni, and Cr-Mo compositions were powder nitrided at 818 K for 0.25 to 32 hr. The resulting morphology, lattice distortions, and compositional inhomogeneity were determined by X-ray diffraction, metallography, and electron microprobe analysis. Alpha double prime-(Fe16N2) and gamma prime-(Fe4N) nitrides were found in the diffusion zones of Fe and the 0.54% Cr alloy; the higher Cr alloys contained finely dispersed submicroscopical precipitates of CrN in the matrix and Cr2N precipitates at the grain boundaries. The macro- and microstrains as functions of nitriding time were related to the volume changes from nitriding and precipitation; residual surface stresses were computed using the Voigt-Reuss-mean model.