Residual stresses in the surface layer of laser-treated steels

Abstract

Although laser treatment of certain metals may enhance the wear performance in general it may result equally well in large residual stresses which affect the wear performance detrimentally. Tensile stresses generated in the surface layer may lead to severe cracking of the material. This paper describes surface stress situations found after laser treatments in samples of different materials (C22, CK60 and pure Fe) with single laser tracks and in samples with multiple overlapping laser tracks. The principal results of the X-ray measurements are the large stress variations inside and outside the laser tracks. The laser treatment gives rise to both tensile and compressive residual stresses. The residual stresses in and around laser tracks have their origin in plastic deformation due to thermal expansion and in volume changes due to phase transformations. The plastic deformation is dictated by the strength of the material and the temperature function forced upon the material by the laser treatment. The residual stress state in multiple laser tracks is of tensile character. Each new laid laser track exerts a tensile force upon the previous laser tracks by which the compressive stresses disappear. Some annealing and/or tempering occurs, leading to a decrease in the absolute value of the stresses.