Friction taper plug welding for S355 steel in underwater wet conditions: Welding performance, microstructures and mechanical properties

The present work reports new results regarding developing the friction taper plug welding (FTPW) process in wet conditions for underwater welding and repairing solutions. In this study, the FTPW process, microstructures and mechanical properties of the weld are investigated. The main findings are as follows: defect-free FTP welds can be obtained with a 7000 rpm rotational speed and an axial force of 30-50 kN. In the heat-affected zone (HAZ) and the lower region of the weld metal (WM), a large volume of lath bainite and small amounts of acicular ferrite, polygonal ferrite, and martensite are observed. However, in the upper region of the WM, the microstructure mainly consists of lath martensite. The best results of ultimate tensile strength and V-notch impact energy are 500 MPa and 39.5 J, respectively, on the joint that was welded with a 45 kN axial force. The impact failure is characterised by brittle fracture with a large area of cleavage and notably little quasi-cleavage and ductile.