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.

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Recently, materials with low transformation temperature have been investigated for improvement of reliability in welded joint. Due to the introduction of compressive residual stress by using the material, fatigue strength or resistance to cold cracking are improved. In this report the authors examined the effects of phase-transformational behavior on residual stress and their distribution using some basic models and some types of the behavior. Expansions and temperature range of phase-transformation detected in the previous report using the laser speckle strain measurement are also used in this report. Results show that reduction of residual stress from tensile to compressive occurs only with severe restraint condition and low transformation temperature, especially low Mf temperature. In the previous report, the authors suggested the possibility of superplasticity during phase-transformation. Experimental results of residual stress distribution indicated the necessity of consideration for superplasticity by using the low transformation temperature materials.