HFMI-induced fatigue strength improvement of S355 steel transverse non-load-carrying attachments with lack of fusion in the weld root

The effects of high-frequency mechanical impact (HFMI) treatment and preloading on the fatigue behaviour of the S355 steel transverse non-load-carrying attachments welded with technological defect (‘lack of fusion’) in the weld root are studied. The S355 steel joints were purposely welded with incomplete penetration (a technological defect of 3x3 mm formed over the entire weld root length). As established, this technological defect does not affect the fatigue life since the crack initiation and propagation occur along the weld toe. HFMI leads to a tenfold increase in fatigue life of the joints with technological defects. Preloading the as-welded joints at maximum alternating stress of 260 MPa for 64000 cycles leads to partial relaxation of tensile residual stresses. HFMI transforms tensile stresses into compressive ones in both non-preloaded and preloaded samples. Compressive stresses of 315 MPa and 100 MPa were respectively registered at distances of 3 mm and 13 mm from the HFMI-formed groove using the hole-drilling method and electron speckle interferometry. Transmission electron microscopy of the weld-heating affected zone reveals that the bimodal microstructure, containing the areas with ultrafine and larger grains, observed after sequentially applied preload and HFMI remains stable during further fatigue cycling and facilitates the enhanced fatigue strength/life.