Depth distribution of martensite in plasma nitrided AISI H13 steel and its correlation to hardness

The depth distributions of hardness, nitrogen, and crystallographic phases in the diffusion zone of plasma nitrided AISI H13 steel were determined by microindentation, electron microprobe, grazing incidence X-ray diffraction, and conversion electron Mössbauer spectroscopy. For the phase analysis, successively new surfaces were exposed by means of controlled mechanical layer removal. In the diffusion zone, the nitrogen concentration decreases monotonously, while the hardness profile exhibits two distinct regions, one where hardness is roughly constant and another where it decreases to bulk values. Thus, in the case investigated here, the common sense of a linear dependence of hardness on the N concentration does not apply in both regions, but only in the second one. This behavior is discussed in terms of the above mentioned physico-chemical properties.