Development of Stress Relief Method for Weld Joint of Pipe using Ice Plug

A stress relief method using ice plug for butt weld joint of piping is described. The method is as follows. Two ice plugs are formed inside piping, and isolate a section which include weld joint. An internal pressure of isolated section increases as ice plugs grow, and it gives tensile strain to pipe wall of the section. After the pressure reaches target level, the pressure is released and the residual stress decreases. The stress relief method is named by authors as Twin Ice Plug (TIP) method. This paper described details of this process, measuring of internal pressure from outside of piping, controlling internal pressure, and the effect of stress relief.The internal pressure of isolated section is estimated by the strain measured by strain gauge on outer surface of the section. A control of a pressure of the section is done by controlling supply rate of refrigerant (LN2) and heating pipe wall. The maximum internal pressure reached in this method is 40 MPa for 4BSch4O (O.D.=114.3 mm, t=6.0 mm) stainless pipe and 22.5 MPa for 6BSch4O (O.D.= 165.2 mm, t=7.1 mm). The residual stress of as GTA weld joint of stainless pipe is measured, its maximum value is more than +300 MPa. The effects of this method are confirmed by measuring residual stress of 4BSch40 6BSch40 joints that are treated by this method. The residual stress of inside surface decreases and changes to compression, and that of outside surface also decreases to under 100 MPa.