Laser peening without coating (LPwC) can introduce compressive residual stress to the surface and, therefore, is effective in enhancing the fatigue strength. This study used butt welded structural steel joints to investigate changes in the residual stress and the hardness near the toe of the welded zone and to examine the major factor causing the improved fatigue strength due to LPwC. It is concluded that the generation of compressive residual stress by LPwC is the major factor improving the fatigue strength, because the reduction in compressive residual stress due to stress relief annealing decreased the fatigue strength to the same level as that of butt welded joints without LPwC.

PurposeThe purpose of this paper is to investigate the behavior of compressive residual stress induced by laser peening under external loading on an age‐hardened high‐strength aluminum alloy A2024‐T3, a low‐carbon austenitic stainless steel SUS316L (Type 316L) and a nickel‐based alloy NCF600 (Alloy 600).Design/methodology/approachThe surface residual stress was measured intermittently by X‐ray diffraction during cyclic uniaxial loading.FindingsThe compressive residual stress due to laser peening significantly decreased during the first few cycles at stress ratio of 0.1 with the maximum loading stress exceeding the 0.2 per cent yield stress. No remarkable decrease was observed afterward until the end of the loading cycles.Originality/valueUnder symmetric loading at the stress ratio of −1 to A2024‐T3, a major decrease took place in the compression side of the first loading cycle. The surface residual stresses remained in compression within all the extent of the present experiments, even if the maximum loading stress exceeded the yield stress of the materials.

Laser peening can introduce compressive residual stress to the surface and, therefore, is effective in enhancing the fatigue strength. This study targets 780 MPa grade high-strength steel (HT780) in order to clarify whether laser peening generates compressive residual stress on the surface of HT780, and whether such stress would account for prolonged fatigue life in the welded zones of HT780. As a result, large and deep compressive residual stress was generated on the base metal surface and at the boxing toe of HT780 under the peening conditions employed for 490 MPa grade steel. The smaller the applied stress range, the greater was the improvement of the fatigue life of the high-strength steel boxing toe by laser peening.

Laser peening which introduces compressive residual stress on surface is effective in extending fatigue lives of welded components, because tensile residual stress after welding is one of the most important factors reducing the fatigue lives. Steels for structures are widely used for bridges, buildings, etc., however the effects of laser peening on steels for structures are not well established. In this study, the residual stress, Vickers hardness and fatigue strength of the toe of butt welded joints pretreated by laser peening were examined and compared to those without laser peening. These results were further compared to those of annealed specimens in order to clarify the main factor of improving the fatigue strength. Main results are summarized as follows. 1) Large and deep compressive residual stress was generated around the toe of the butt welded joints by laser peening. 2) Vickers hardness also increased by laser peening but the incremental level was small. 3) Fatigue strength at 10 7 cycles of the butt welded joints became much higher by laser peening. 4) It can be considered that the main factor of improving fatigue strength is the generation of compressive residual stress.

Recently, several cracks caused by stress corrosion cracking (SCC) have been found on welds of shroud supports in Boiling Water Reactor (BWR) plants. The major cause of SCC in a weld joint is considered due to welding residual stress generated in the fabrication processes of the components. For continuous safety operations, it is necessary to estimate the structural integrity of such shroud supports with cracks based on the distribution of residual stresses induced by welding. In order to know and to validate the numerical method of residual stresses induced by welding of large scale and complex shaped components, a BWR shroud support mock-up with a hemispherical base of reactor pressure vessel (RPV) was fabricated by Japan Nuclear Energy Safety Organization (JNES) as a national project. The mock-up has a 32° section of actual BWR shroud supports with approximately the same configurations, materials and welding conditions of an actual component. During welding in the fabrication process of the mock-up, temperature was measured and after completion of the mock-up fabrication, surface residual stress distributions for each weld were also measured by the sectioning method. In addition, through-thickness residual stress distributions were investigated. Residual stress for each weld was calculated by using axisymmetric models considering temperature dependent elastic-plastic material properties. Though the actual structure of shroud supports is essentially complex, we simplified axisymmetric models in the center of the cross section. The analysis results show a similar profile and good agreement with the measured results on the surface of all the welds and through the welds at the upper and lower joints of the shroud support leg.