Irradiated Laser Stress Improvement Process Research Articles

外面を急速加熱したパイプにおける内面残留応力改善機構の数値解析による検討

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.

レーザで外面照射加熱されたステンレス鋼管溶接継手の残留応力改善

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.

レーザ外面照射応力改善法(L-SIP)の開発と適用

Outer Surface Irradiated Laser Stress Improvement Process (L-SIP) was developed as a counter measure for stress corrosion cracking at the tube and nozzle stub weld in the nuclear power plant and the thermal power plant. L-SIP is a technique for improving the inside tensile residual stress to the compressive stress. Rapidly heating by the laser beam irradiation to outer surface causes the temperature difference between outer surface and inner surface, and it can reduce the residual stress. In this paper, the developed system, the verification test results and the practical use situation are described.

外面を急速加熱したパイプに生じる残留応力分布に及ぼす板厚内の温度分布の影響

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 plastic strain generates near the outer surface and the tensile plastic strain generates near the inner surface of pipes. The compression stress occurs near the inner surface of pipes by the plastic deformation.In this paper, the theoretical equation which calculates residual stress distribution from the inherent strain distribution in the thickness of pipes is derived. And, the relation between the distribution of temperature and the residual stress in the thickness is examined for various pipes size. (1) By rapidly heating from the outer surface, the residual stress near the inner surface of the pipe is improved to the compression stress. (2) Pipes size hardly affects the distribution of the residual stress in the stainless steel pipes for piping (JISG3459). (3) The temperature rising area from the outside is smaller, the area of the compression residual stress near the inner surface becomes wider.

移動する矩形ガウス分布熱源により加熱されるパイプの温度分布の検討

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 theoretical equation for the temperature distributions of pipes heated by moving rectangular Gauss distribution heat source on the outer surface is derived. The temperature histories of pipes calculated by theoretical equation agree well with FEM analysis results. According to the theoretical equation, the controlling parameters of temperature distributions and histories are q/2ay, vh, ax/h and ay/h, where q is total heat input, ay is heat source length in the axial direction, ax is Gaussian radius of heat source in the hoop direction, v is moving velocity, and h is thickness of the pipe. The essential variables for L-SIP, which are defined on the basis of the measured temperature histories on the outer surface of the pipe, are Tmax, F0=kτ0/h2, vh, WQ and LQ, where Tmax is maximum temperature on the monitor point of the outer surface, 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, and LQ is the uniform temperature length in the axial direction. It is verified that the essential variables for L-SIP match the controlling parameters by the theoretical equation.

レーザ外面加熱残留応力改善法（L-SIP）の施工管理範囲の検討

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.

原子力機器へのレーザを用いた残留応力低減技術（２）レーザ外面照射応力改善法の開発

原子力機器へのレーザを用いた残留応力低減技術（２）レーザ外面照射応力改善法の開発