Simulation of Coupled White Layer Formation and Internal Stresses Evolution During Pulsed Thermomechanical Surface Treatment of AISI 1045 Annealed Steel

Abstract

In this work, a three-dimensional (3D) finite element model is developed to investigate the coupled thermal, mechanical and the white layer formation phenomena during electromechanical treatment (EMT) of AISI 1045 annealed steel. White layer is frequently mentioned as specific kind of martensitic structure (untempered martensite). However, through SEM study it was shown that the white layer which is formed during the EMT has a nonuniform ultra-disperse structure which has nothing to do with the conventional martensite. The results of numerical based thermal analysis show that alternating current during the EMT leads to repetitive temperature variations in the surface layer. As a consequence, a regular inhomogeneous structure is formed with alternating fragments of the white layer and self-tempered zones with the initial structure. It was found that the white layer has a dominant effect on the residual stress distribution. The calculation results show that longitudinal residual stress at the surface varies depending on the white layer volume fraction. More detailed analysis indicates that the stress state close to biaxial compression is formed in the regions corresponding to white layer fragments. In the areas with a lower white layer volume fraction two principal stresses are compressive, and the last one with the highest absolute value is tensile.