Modeling of the coupled processes of residual stress formation in a metallic alloy taking into account structure transformation due to pulse thermo-force surface hardening

A framework for finite-element modeling of structure formation and stress-strain state in metal alloys subjected to electromechanical treatment (EMT) is presented. The processes of creating the hardened layers with ultradisperse structure and improved operational characteristics on the surface of metal products are considered within a unified modeling system. The main stages of constructing a set of models of temperature-force actions during EMT leading to structural-phase transformations and the formation of technological residual stress fields are described. The models used are based on the constitutive relations of thermal conductivity during electric heating of metal by alternating current and the theories of plastic flow with isotropic-translational hardening, which take into account the dependence of thermal and mechanical properties of the material on temperature, strain rate and phase composition. An analysis of the calculated results and their comparison with experimental data are carried out. It is shown that the pulsed thermal action of the alternating current contributes to the formation of a regular surface structure with the alternating fragments of the hardened layer sections and self-tempering zones. The calculated patterns of the structural zones are in good agreement with the experimental results of metallographic analysis of the EMT hardened surface of the studied alloys. The residual stress fields have the same periodic nature - the zones of compressive stresses in the hardened sections alternate with the zones of tensile stresses in the interlayers of unhardened metal. The basic mechanisms (force, thermal, and phase) determining the magnitude and sign of residual stresses in the above zones are investigated. An approach to the analytical description of residual stress distribution along the radius of a cylindrical specimen is proposed. It appears to be useful for predicting the results of technological action during hardening of the product surface based on the EMT technique, reconstruction of residual stress diagrams in the case of a limited amount of experimental or calculated data, and approximation of numerical results. The approximation of circumferential stresses in the form of a transformed sinusoid is adopted as a basic function, the argument of which is a transforming function that corrects the transformation of the basic sinusoid. The functions of radial and axial stresses are determined by solving the equilibrium equations and the physical relations of the sample material.