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

Abstract

Specimens prepared from iron, iron-chromium alloys (0 to 4 wt% Cr) and commercial steels (C45, 41Cr4, 15CrNi6 and 24CrMo13) were powder nitrided at 818 K for 0.25 to 32 h. After cooling to room temperature the resulting morphology, lattice distortions and compositional variations were determined by X-ray diffraction analysis, metallographic methods and electron microprobe analysis. In the diffusion zone of iron and iron-chromium (0.54 wt% Cr), α″-(Fe16N2)- and γ′-(Fe4N)-nitrides were observed, whereas specimens of the iron-chromium alloys with a higher chromium content showed a finely dispersed submicroscopical precipitation of CrN in the matrix and precipitates of Cr2N at the grain boundaries. With increasing nitriding times for the iron-chromium alloy with the highest chromium content (3.82 wt% Cr) a discontinuous precipitation starting from the grain boundaries occurred. For the first time recrystallization phenomena in the diffusion zone were observed, indicating that the inward diffusion of nitrogen introduces large lattice distortions. Large distortions were determined from X-ray diffraction line shift and line broadening respectively. The behaviour of macro- and micro-strain as a function of nitriding time was interpreted in terms of the volume changes caused by nitriding and subsequent precipitation. The residual surface stresses were calculated from the macrostrains applying the Voigt-Reuss-mean model. The experimentally determined ratio of the macrostrains in the (100) and the (110) directions was in good agreement with the value predicted from the model. The corresponding ratio of the microstrains was significantly larger than this theoretical prediction, which can be attributed to precipitates growing along (100) planes in the matrix (such as α″-nitride and CrN). In contrast with the macrostrain, a strong relation was found between the microstrain and the chromium content of the specimen.