Effectiveness And Influence Factors Of Ultrasonic Impact Treatment On Weld Residual Stress In Thick Joints

During welding,due to the localized quick heating and cooling induced by the welding heat source,un-uniform strain occurs in the weld zone,resulting in inevitable welding residual stress.As the increase of steel strength grade and thickness of welded structures,the intensity of restraint applied to the weld seam arises,resulting in higher and more complex welding residual stress.It is well known that tensile welding residual stress has a significant effect on the corrosion,fracture resistance and fatigue performance of welded structures.It is very important to measure and migrate the high tensile welding residual stress.In this paper,high-strength welding joints with the thickness ranging from 30 mm to 50 mm and yield strength greater than 345 MPa were prepared.Hole-drilling(HD)method,X-ray diffraction(XRD)method and contour method(CM)were adopted to measure the surface and internal stresses in the joints.The distributions of welding stress in these joints were analyzed.The stresses after post-weld ultrasonic impact treatment(UIT)and post-weld heat treatment(PWHT)were compared and distinguished.The adaptability of layered ultrasonic impact treatment(LUIT)to migrate the welding stress in joints with different strength grades and thicknesses was studied.The effects of restraints and UIT on welding residual stresses in the restrained joint were also discussed.The results of residual stresses after welding and UIT in welding joints with different strength grades and thicknesses show that the high tensile residual stress appears in the weld zone,and the peak value of tensile residual stress occurs in the region near the surfaces of weld zone.UIT causes the reduction of welding residual stress near the surface of weld seam,and introduces a compressive stress layer with the depth of about 2-5mm.The decrease of welding residual stresses at the surface in welded joints with different strength grades is about 800 MPa.The internal residual stresses in weld zone changes little after LUIT.Joint thickness and weld strength level have no effect on the effectiveness of LUIT on welding residual stresses.The longitudinal and transverse stresses after PWHT range from 0 to 100 MPa in the whole joint,the stresses decrease by 400 MPa as compared with the as-welded stresses in the weld zone.There are almost full tensile stresses existing on the top and bottom surfaces across almost the whole plate width due to the restraints exerted by the saddle-shape plates.The depth of the tensile stresses zone induced by the welding of the saddle-shape plates is about 13 mm with the peak tensile residual stresses greater than the yield strength of weld metal.The welding of restraining plates can cause large tensile stress in the welding zone.After UIT,the high tensile stress in the treated region changes into compressive residual stresses.UIT has no effect on the stress outside the area treated by UIT.