Application of shot peening for welded joints of existing steel bridges

Concerning environmental protection from removal of existing paint containing lead and poly chlorinated biphenyl on existing steel bridges during re-painting, circulating blasted techniques at bridge site using both steel grits and steel shots that can be reused several times have been recently developed and practically used in Japan. The techniques can be upgraded to apply shot peening techniques by controlling treatment that ensures beneficial compressive residual stress. This study investigated an application of shot peening by utilizing circulating blasted techniques using steel rounded shots to welded joints during re-painting of a pilot existing steel bridge. In order to ensure the quality of the shot peening, the surface coverage near welded joints, indicating whether shot media uniformly hit the treatment area, was measured and controlled. After the surface coverage control, to verify the compressive residual stress introduced by the shot peening, the residual stress in the vicinity of weld toes of the pilot bridge was measured by means of X-ray diffraction method. The measurement results indicated that shot peening can introduce compressive residual stress that is the least − 350 MPa at a depth of 100 μm and is kept up to a depth of 400 to 500 μm. In addition, this compressive residual stress was compared with that of plate specimens treated by shot peening under almost the same quality control as the pilot bridge. The compressive residual stress introduced to the specimens agreed well with that of the pilot bridge, and therefore, this result implied that this shot peening could enhance the fatigue strength of welded joints on existing steel bridges.