Residual stresses in girth butt welded pipes and treatments to modify these

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A number of Type 304 stainless steel pipes are used in the primary cooling systems of nuclear plants. Intergranular stress corrosion cracks (IGSCC) were found at some welded joints in these piping systems due to very high tensile residual stress, sensitization of the material due to welding, and corrosive environment, all occurring simultaneously. Investigations have shown that at least one of the above factors must be eliminated to prevent IGSCC. This report describes experimental results on the temperature variations during pipe welding by conventional techniques and by the heat sink welding (HSW) technique. The mechanism of residual stress generation due to welding is also discussed. The pipe used in these experiments was 4B Sch80 Type 304 stainless steel. It was found that the temperature distribution through the thickness of the pipes was almost uniform for the conventional welding technique, but had a very sharp gradient for HSW. In the pipe axial direction, the temperatures varied sharply for both welding techniques. This implies that the sensitization of metal due to HSW is lighter than that of conventional welding and that the residual stress on the inside surface of the heat sink welded pipe is compressive. The induction heating stress improvement (IHSI) method has been investigated analytically and experimentally. In the IHSI method, a pipe is heated with an induction coil while cold water is pumped through it. This causes a temperature gradient throughout the pipe wall which generates high thermal stresses. This, in turn, generates compressive stresses on the inner surface of the pipe. This method is designed to eliminate tensile residual stresses near the weld heat affected zone on the inner surface. Temperature analysis and subsequent thermoelastic-plastic analysis show that tensile weld residual stresses at a joint were changed into compressive stresses on the inner surface of a pipe. It was confirmed experimentally that these stresses suppressed fatigue crack propagation in the heat affected zone (HAZ) of a welded pipe. Therefore, the IHSI method is effective not only in preventing crack initiation but also in suppressing crack propagation. As for the relaxation of residual stresses, no significant relaxation was measured when external loads were applied at as much as 80% of the yield strength in the experiments.