Effects of ultrasonic impact on surface characterization of S355 steel welded joint

The effects of ultrasonic impact treatment (UIT) on the residual stress, the microhardness, the nano-hardness, the microstructure, the electrochemical corrosion resistance, the corrosion morphology, and the chemical composition of the S355 steel welded joints were investigated. In addition, the mechanism of improving the electrochemical performance of the S355 steel welded joints by the UIT was studied. The results showed that the UIT could significantly improve the electrochemical corrosion resistance of the S355 steel welded joints. The corrosion potential and impedance were increased, and the corrosion current density was decreased with the increasing of the UIT time. According to the corrosion morphology, it was observed that after the UIT, the number of corrosion pits in the welds and heat-affected zones of S355 steel welded joints significantly decreased, along with reductions in the depth and size of the pits. After the UIT, the grain size of the S355 steel welded joints was refined, and both the microhardness and the nano-hardness of the welded joint were increased, which indicated that substantial plastic deformation occurred on the S355 steel welded joint. Moreover, the UIT could significantly reduce and transform residual tensile stress into residual compressive stress in areas adjacent to the welds, and the residual compressive stress could be increased with the increasing of the UIT time. It can be concluded that the introduced compressive residual stress, the grain refinement, and the generated plastic deformation were the main factors in improving the electrochemical corrosion resistance.