An experimental methodology was developed for identification of mechanical behavior laws, in which the evolution of the stresses and strains in the surface of the samples is determined by X-ray diffraction and electric resistance strain gages, respectively, as a function of the loading in four-point bending tests. The determined stress-strain curve is then characteristic of the near-surface layers of the samples. The method was validated for different steels by comparing the results of samples with homogeneous properties on the cross section with the results of tension tests. The developed methodology was applied to the study of steel samples submitted to surface treatments, presenting gradients of chemical composition, microstructure, and mechanical properties from the surface to the bulk material.