Residual stresses of girth butt welded pipes and its improvement

This report describes experimental results on temperature variations during pipe welding by conventional techniques and the heat sink welding (HSW) technique. Also, the mechanism of residual stress generation due to welding is discussed. The pipe used in these experiments was 4B Sch80 Type 304 stainless steel. It turns out that the temperature distribution through out the thickness of the pipes was almost uniform for conventional welding, 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 stresses on the inside surface of the heat sink welded pipe is compressive. The induction heating stress improvement (IHSI) method, bas 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 through out 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. (orig./GL)