Enhancement of the fatigue strength of underwater wet welds by grinding and ultrasonic impact treatment

•Grinding and UIT can extend the fatigue life of underwater wet welds.•Residual stress on the surface of as-welded joint is compressive.•Fatigue crack initiates at the weld metal close to the fusion line.

Grinding + underwater ultrasonic impact treatment (UUIT) is proposed to improve the fatigue property of underwater wet welded joints. Using single grinding or UUIT technique cannot improve the fatigue life significantly while the combination of the two can obtain great enhancement. The microstructures in the weld metal contain proeutectoid ferrite, side-plate ferrite and acicular ferrite and the microstructure in coarse grain heat affected zone (CGHAZ) is dominated by martensite. Compressive residual stress decreases the effective stress intensity factor range and extends the fatigue life. The transverse residual stresses on the surface of as-welded joint are compressive due to the fast cooling effect of water and it is less in weld metal (WM) than in CGHAZ, which results in the initiation of fatigue crack at WM.