Underwater welding by a gravity type wet arc welding process was carried out in tap water for welding of structural steel plates using covered electrodes, comparing with in-air welding. The cooling rate was accelerated, and the higher diffusible hydrogen was generated in the underwater welding. Despite these severe conditions, it was confirmed in tension-tension fatigue test under repeated stress with frequency of 10 cps that the optimal zone of weld heat input condition for obtaining the higher fatigue limit of the notched weld metal specimen of the 6 pass underwater welded joint than that of the base metal could exist, relating to improving the notch toughness such as impact value and strain increase. It might be resulted from a tempering effect by the followed pass welding underwater through 2nd to 6th pass, and from a notch effect on both sides of the weld metal with higher tensile strength than that of the base metal during the tension fatigue test under the applied stress of high frequency (l0cps). However at the lower cycle than about 2x105, the fatigue strength of the notched weld metal was lower than that, resulting from the upper applied stress effecting on the 1200 V-notch.
The repair of offshore structures such as oil production platforms, steel piers and underwater pipelines is sometimes conducted by wet welding underwater. The knowledge of notch toughness and fatigue properties of welded joint is very important for safety of welding structure. The welded joints have metallurgical discontinuity such as weld metal, bond and heat-affected zone, etc. It is well known that the fatigue strength of welded joints reduces to less than that of base metal. There are many factors reducing their fatigue strength for example, geometrical effect of reinforcement, residual stresses and microstructural change in the heat-affected zone.