2009 Volume 27 Issue 4 Pages 343-351
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 the butt welded joints of pipes in the compression stress. The temperature gradient occurs in the thickness of pipes in heating the outer surface rapidly by laser beam. By the thermal expansion difference between the inner surface and the outer surface, the compression stress occurs near the inner surface of pipes.
In this paper, the essential variables for L-SIP is studied by experimental and FEM analysis. The range of the essential variables for L-SIP, which are defined by thermo-elastic FEM analysis, are Tmax=550–650°C, LQ/√rh ≥3, WQ/√rh ≥1.7, and, 0.04≤F0≤0.10 where Tmax is maximum temperature on the monitor point of the outer surface, F0 is k×τ0/h2, k is thermal diffusivity coefficient, τ0 is the temperature rise time from 100°C to maximum temperature on the monitor point of the outer surface, WQ is τ0×v,υ is moving velocity, LQ is the uniform temperature length in the axial direction, h is thickness of the pipe, and r is average radius of the pipe. It is showed by thermo-elastic-plastic FEM analysis that the residual stresses near the inner surface of pipes are improved in 4 different size pipes under the same essential variables. L-SIP is actually applied to welding joints of 4B×Sch160 and 2B×Sch80 SUS304 type stainless steel pipes within the defined range of the essential variables. The measured welding residual stresses on the inner surface near the welding joints are tensile. The residual stresses on the inner surface change to compression in all joints by L-SIP.
