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 wasexamined 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). Thenitrogen and carbon profiles in the diffusion layers were determined by scanning electron microscopeequipped with a wavelength dispersive spectrometer (EPMA-WDS).For all the specimens, the highest hardness values only change slightly with increasing time andtemperature of decarburization, while case hardness depths decrease.In all the specimens, the nitriding depth, as determined by the nitrogen profile, is larger than the onedetermined by the hardness profile