PurposeLaser peening without coating (LPwC) is an innovative surface enhancement technology for introducing compressive residual stress in metallic materials. The purpose of this study is to examine the characteristic at the laser‐peened welded zone and the fatigue lives of the welding joints.Design/methodology/approachLPwC conditions for 490 MPa grades of structural steels were selected. By using the conditions, the characteristic at the laser‐peened welded zone, residual stresses, hardness and roughness of welding toes were examined. Moreover, the fatigue lives of the toes of box‐welded joints and butt welded joints pre‐treated by LPwC were compared to the fatigue lives of those that were not pre‐treated by LPwC.FindingsThe main results are: LPwC conditions for 490 MPa grade steels were established; residual stresses, Vickers hardness and roughness at the laser‐peened welded zone were revealed; and LPwC can dramatically extend the fatigue life of welded joint.Originality/valueThe effects of LPwC on structural steels, which are widely used in bridge members, have not been well clarified; the effect of LPwC on welded zones in these structures is particularly unclear. If LPwC can be carried out such that compressive residual stress is imparted on structural steels and the welded zones in the bridge members, the fatigue lives of bridge members will be greatly increased. The paper fills some of these gaps.

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.

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The authors have applied laser peening without coating (LPwC) to metallic materials. Compressive residual stress nearly equal to the yield strength of the materials was imparted on the surface. Accelerating stress corrosion cracking (SCC) tests showed that LPwC had a significant effect to prevent the SCC initiation of sensitized materials of SUS304, Alloy 600 and the weld metal, Alloy 182. Push-pull type fatigue testing demonstrated that LPwC drastically enhanced the fatigue strength of fillet-welded rib-plates of SM490A.

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