This study is concerned with experimental curves of: hardness, residual stress and concentration of nitrogen at the precipitation layer at alloy steels after gas nitriding. The purpose of this treatment is that through experimental curves to identify the relationship and causality between hardness, on one side, and on the other, residual stress respectively concentration of nitrogen. Comparing the shape and slope of these curves we can conclude that, concentration of diffused nitrogen on the steel lattice, respectively ferrite, is a cause of increased hardness at the precipitation layer. More to the point, as a result of nitrogen diffusion, residual stress emerge which cause increase of hardness. The intensity and slope of the curves of hardness depends on the intensity and slope of the nitrogen concentration respectively residual stress.