Changing Properties of Metal Surface Layers during Thermo-Chemical Treatment

In this article a two-stage technology for constituting surface layers is proposed. First, the material is subjected to initial hardening by burnishing and next to a nitriding process. The surface layer after such processing should have considerably better strength properties compared to those created through a conventional nitriding process. A description of the state of the surface layer after these stages of the proposed process is obtained by defining suitable energy states for the materials. These states are expressed by residual strain and stress function and also by the functions of external work during the burnishing process and thermal energy given in the nitriding process. Methods of identifying the above functions through experimental research on the residual stress state and selected parameters of the surface layer, and also through numerical analysis of the coupled processes, are also proposed. The main part of the work consists in the mathematical description of complex deformation phenomena occurring in the material during the proposed process. In the given description, coupled stress fields appear as a result of the additivity of elastic, plastic, thermal, diffusional, and phase strains.