The new process called L-SIP(outer surface irradiated Laser Stress Improvement Process) is developed to improve the tensile residual stress of the inner surface near butt weld joint of pipes. In L-SIP, the pipe outside temperature rises locally at short time by a laser beam moving in the circumferential direction. If the heating time is short enough, the temperature of the pipe inner surface dose not rise, and a temperature difference occurs between the outer surface and the inner surface. By the thermal expansion difference between the outer surface and the inner surface, the thermal stress occurs in the pipe and the new plastic deformation is produced. After cooling, the compressive residual stress is produced near the inner surface of the pipe with the effect of this plastic deformation. In order to verify the stress improvement mechanism of L-SIP, thermo-elastic-plastic FEM analysis is conducted for austenitic stainless steel pipe (SUS304TP 4B×Sch160; O.D. =114.3mm, thickness=13.5mm). It is showed that the residual stress on the pipe inner surface change to the compressive stress like the axial-symmetry model. The experiment is done for butt weld joint of the same pipe as FEM analysis. The welding residual stress is improved from +200MPa (tensile) to less than -100MPa (compression) on the pipe inner surface. Material qualification tests of welding joint are done after L-SIP. It is verified that L-SIP has no bad influence on the welding joint of the stainless steel pipe.
