Effect of Shield Gas on the Quality of Tube to Tubesheet Joint Welded by All-Position TIG Welding

Based on the structure of AP1000 steam generator, the effects of various shield gases on the welding shape and throat dimension of tube to tubesheet joint welded by all-position TIG welding were studied. The morphology and forming mechanism of the cracks appeared during welding under 100%Ar gas and higher heat input were also studied. The results show that under the same welding parameters, the weld pass was the narrowest at the atmosphere of 100%Ar and the widest under 100%He gas. The weld surface was the brightest under 95%Ar+5%H2 gas. The dimension of welding throat can meet with the technical requirements when welded under the atmosphere of 100%He, 95%Ar+5%H2 and 75%He+25%Ar. The optimum quality of tube to tubesheet joint was obtained when using TIG welding at the atmosphere of 95%Ar+5%H2. When improving heat input and lowering weld speed, the dimension of welding throat with 100%Ar could meet the technical requirements, but the probability of thermal cracks also increased. Those cracks were high temperature cracks.