Role of Overlapping Passes on Fatigue Performance of Microalloyed 17MnCuNi7 Steel Treated by Continuous CO₂ Laser Beam

Abstract

The widely used surface treatment by continuous CO₂ laser beam requires superposition of several laser passes, which can lead to a softened zone and to the creation of tensile residual stresses in overlapping passes. In this study, the effect of overlapping passes, microhardness, and metallurgical transformations on the fatigue performance of 17MnCuNi7 steel has been investigated by three series of bending fatigue tests corresponding to three types of continuous CO₂ laser beam treatments. It has been concluded that suitable selection of material and control of the laser treatment parameters provides for the elimination of tensile residual stresses in softened zones, and can lead to homogeneous hardening of the surface. The fatigue limit of 17MnCuNi7 steel depends mainly on the residual stress in the layers affected by laser treatment, despite any hardness loss caused by overlapping passes. The improvement made by this treatment varies from 10 to 20% compared with the fatigue limit in the tempered state.