Residual stresses in laser surface hardening of large areas

Abstract

Abstract Laser surface hardening is a method of surface treatment in which high density energy is employed as a heat source to harden the substrate surface. Owing to the small spot size of the focused laser beam only a small surface area can be hardened in a single laser pass. To accomplish surface hardening for a larger workpiece, therefore, multiple lasing passes must be used. In this study, the residual stress in the multipass laser surface-hardening process was analysed by using a finite element method. The simulation results revealed that a region of residual tensile stress remains on the hardened surface, if the distance between the neighbouring passes is larger than the width of the hardened zone of each pass. The residual tensile stress on the surface disappears, however, when the distance between the passes is so short that the hardened zone of each pass is partially overlapped. In multipass laser surface hardening, the region of residual tensile stress occurs below the hardened zone of the workpiece, while the maximum residual tensile stress occurs along the centre of each lasing path. In comparison with a gaussian distribution of the beam power, the square beam mode produces a wider and thinner hardened zone, which results in a wider and thinner region of residual tensile stress below the surface.