Residual Stresses in Welded Components Following Post-weld Treatment Methods

Weld seam is the weakest point of the structure. Weld defects, weld geometry such as toe radius, and residual stresses, which are typically tensile in the critical area, weaken the fatigue strength of the weld seam. These factors become more important with the welding of high strength steels (HSS) because structures containing welded seams loose the benefit from the strength of the steel. The weld seam fatigue strength can be improved locally with several different methods. In this paper residual stresses of the weld area and their modifications for improving the fatigue strength are discussed.The residual stresses of the toe area are changed from tensile up to compressive stresses, which are known to improve fatigue strength, with several methods. High tensile stresses can be relaxed mechanically or thermally as postweld heat treatment. Mechanical treatments are more common especially in large structures. Typical mechanical methods are burr grinding, TIG re- melting, hammer peening and needle peeing. A new method is laser re-melting method. All these methods also change the geometry of the weld toe by improving it.This paper concentrates to needle peening (high frequency mechanical impact, HFMI), and presents some results from laser re- melting tests. Both surface measurements and depth distributions of stress measured by X-ray diffraction (XRD) method are presented.