A new stress improvement method which reduces tensile stress on inner and outer surface near butt weld joint of piping is described. The procedure of this method is as follows. Circular region on both side of the weld line is heated from outside at the same time to make temperature difference between weld line and heating region. The heating region and weld joint do not cool actively after completion of heating. A tensile strain is produced on the inner surface near weld joint to produce plastic deformation by thermal stress when the circular region is heated. The tensile stress after cooling is decreased by this plastic deformation.
The authors show the mechanism and effect of this method by FEM analysis. The influence of parameters of this method, such as heating width, heating location and heating conditions (intensity and time), on the residual stress is studied by FEM analysis. This method is actually applied to the weld joints of 4BSch40 (O.D.=114.3 mm, t=6.0 mm), 4BSch160 (O.D.=114.3 mm, t=13.5 mm) and 20B (O.D.=508.0 mm, t=8.0 mm) stainless steel pipe and the change of residual stress is measured. The measured welding residual stress on inner surface near GTA weld joint is more than +200 MPa. The residual stress of inside surface near weld joint decreases and changes to compression after treated by this method in all joints.
