Major factor causing improvement in fatigue strength of butt welded steel joints after laser peening without coating

Abstract

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.

Increase of bending fatigue resistance for tungsten inert gas welded SS400 steel plates using friction stir processing

“…Although fatigue-test conditions such as a load ratio and tensile strengths of structural steel plates investigated are different, the significant increase of the number of cycles to failure was not observed in this study, but it was obtained by further refinement in the fine-grained FSP region (not shown here). The laser peening provided increase of Vickers hardness due to work hardening in the region from immediately beneath to 0.2 mm below the surface and large compressive residual stress (about 400 MPa) in the region from immediately beneath to 1.0 mm below the surface [12]. The shot peening also provided large compressive residual stress (about 400 MPa around the surface) in the region from immediately beneath to 0.6 mm below the surface [13].…”

“…On the other hand, residual tensile stress would be produced in the TIG weld beads, and thus the weld part would be a weak point in the structure. Peening techniques such as laser peening [12] were performed to convert tensile stress remaining in the weld beads to compressive stress. Thus, to understand the effect of FSP on the residual tensile stress in the TIG weld beads, residual stress was measured for the TIG-welded steel specimens with and without FSP, and the results are shown in Figure 8, together with those for steel specimens only with FSP for comparison.…”

“…As mentioned in the introduction, to increase the fatigue strength of weld joints, various surface treatment techniques have been studied. The number of cycles to failure of the weld joints with surface treatments such as the laser peening [12] and the shot peening [13] significantly increased with decreasing applied stress amplitude or maximum applied stress. Although fatigue-test conditions such as a load ratio and tensile strengths of structural steel plates investigated are different, the significant increase of the number of cycles to failure was not observed in this study, but it was obtained by further refinement in the fine-grained FSP region (not shown here).…”

“…However, the FSP technique has not yet been applied to weld joints of steel plates for the purpose of optimizing the surface strength and fatigue performance. The fatigue strength of weld joints is much lower than that of the base metals, and thus various surface treatment techniques such as ultrasonic peening [7][8][9][10][11], laser peening [12], shot peening [13][14][15][16][17][18], and hammer peening [19,20], have been studied as means for improving fatigue strength of the weld joints. Although the application of FSW to steels and other high-temperature materials has been limited due to the absence of a suitable tool material, FSW of carbon steels has been reported by several groups [21][22][23][24].…”

Effect of Laser Peening on Improving Fatigue Strength of Welded Rib of High-Strength Steel

“…Then, they also studied changes in the surface residual stress, the depth distribution of residual stress, the hardness distribution, and the surface roughness in four types of structural steel with different strengths and in the welded zone 9) . Moreover, it is clarified that the fatigue life of the welded joints is substantially extended by laser peening 10 11) and the generation of compressive residual stress by laser peening is the major factor improving the fatigue life 12) .…”