The behaviour of decarburized layers in steel nitriding

Samples of quenched and tempered 40CrMo4 steel, previously surface decarburized at different depths, have been submitted to gaseous nitriding. The surface layers after decarburization and nitriding was examined by optical microscopy (OM), scanning electron microscopy (SEM) and microhardness tests. The distribution of iron nitrides in the diffusion layer was analyzed by X-ray diffraction (XRD). The nitrogen and carbon profiles in the diffusion layers were determined by scanning electron microscope equipped with a wavelength dispersive spectrometer (EPMA-WDS). For all the specimens, the highest hardness values only change slightly with increasing time and temperature of decarburization, while case hardness depths decrease. In all the specimens, the nitriding depth, as determined by the nitrogen profile, is larger than the one determined by the hardness profile.