2009 Volume 27 Issue 4 Pages 307-315
The new process called L-SIP(outer surface irradiated Laser Stress Improvement Process) has been developed to improve the tensile residual stress to the compressive stress at the inner surface near pipes' butt welded joint. The characteristic of this process is to produce the plastic strain into the pipe by applying the temperature difference at the pipes' inner and outer surface without using water cooling method. In this paper, we have studied the stress and strain behavior in rapidly heating the pipes' outer surface. The temperature gradient occurs in the pipe thickness when heating the outer surface rapidly. By the thermal expansion difference between the inner and outer surface, the tensile thermal stress generates at the inner surface and the compressive thermal stress generates at the outer surface. Furthermore, the tensile plastic strain will be produced at the inner surface and the compressive plastic strain will be produced at the outer surface. The plane which balances between inner stress and outer stress moves toward inside, because the compressive strain of the outer surface is larger than that of the inner surface when the temperature becomes even in the pipes' thickness. The compressive residual stress occurs on the pipes' inner surface by this plastic deformation. This mechanism can also be applied to a circumstance when the heat penetrates to around 1/2 of the pipes' thickness and the temperature of the inner surface does not rise because of the short time heating. Therefore, water-cooling the inner surface is not necessary in this method.
This mechanism and the effect of the stress improvement for austenitic stainless steel pipe (SUS316TP 4B×Sch160; O.D.=114.3mm thickness=13.5mm) are verified by the axisymmetric thermo-elastic-plastic finite-element method analysis.
