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Temper-Bead Weld by Underwater Laser Beam Welding

In repair welding for nuclear reactor vessel, low alloy steels are affected by heat input during welding process. The conventional repair welding for wall steel constructions requires post weld heat treatment (PWHT) to achieve the desired microstructure properties. However, post weld heat treatment is very difficult for some structures in operating plants. In such case, temper-bead welding technique is available as a repair welding method. Temper-bead welding employs a multi-pass deposition of welding metal. Each layer of beads provides heat for thermal treatment of the previous weld bead or layer, which lowers hardness of the heat affected zone (HAZ) and improves mechanical properties like the toughness. Toshiba has developed underwater laser cladding and laser seal welding techniques for reactor components repair welding. In this report, some experimental results of laser based underwater temper-bead welding are presented.© 2009 ASME

Laser desensitization treatment for inside surface of SUS304 stainless steel pipe welds

The purpose of this study was to develop a technology for preventing the occurrence of Intergranular Stress Corrosion Cracking (IGSCC) by irradiating a high power YAG laser beam onto the sensitized Heat Affected Zone (HAZ) surface of SUS304 stainless steel. By irradiating a laser beam of the appropriate power density, a laser de-sensitization heat treatment (LDT) process was realized that formed both a molten layer of approximately 0.2 mm depth and a solution heat treated layer. The results of a Creviced Bent Beam (CBB) test to evaluate IGSCC showed that no cracks had appeared on the surface of the LDT parts. Also, after LDT was applied at a width of 40 mm in the vicinity of welding joints in the inside surface of pipes (thickness: 8 mm), approximately 250 MPa of tensile stress was measured as a residual stress on this LDT- processed surface. On the other hand, the tensile stress on the outside surface of these pipes decreased to the compression stress.

Development of Multifunction Laser Welding Head as Maintenance Technologies Against Stress Corrosion Cracking for Nuclear Power Reactors

Multifunction laser welding head has been developed. The head is able to perform not only underwater laser welding as repair, but also laser peening as preventive maintenance and laser ultrasonic testing as inspection. By using the effect of color aberration with optics, laser beam was focused to the ideal spot size on each process. Underwater laser welding was carried out onto EDM slit with the developed head and sealing ability with deposited weld metal was confirmed. As preventive maintenance, laser peening was also performed on the material surface with the developed head, and stress improvement ability was confirmed. For inspection with the developed head, a new method of visualizing weld defects in water by laser-ultrasonics has developed. Furthermore, developing synthetic aperture focus technique for visualized inspection surfaces 2-dementionally, the inspection result like penetrant testing despite underwater environment was achieved. Therefore, practicality of the developed head on each process was confirmed.Copyright

Laser de-sensitization treatment for inside surface of SUS304 stainless steel pipe welds in nuclear power plants

A technology to prevent the occurrence of Intergranular Stress Corrosion Cracking (IGSCC) by irradiating a high power Nd:YAG laser beam was developed. Laser Desensitization Treatment (LDT) process was realized by irradiating a laser beam onto the sensitized Heat Affected Zone (HAZ) surface of SUS304 stainless steel. LDT was formed by both a molten layer of approximately 0.2mm depth and a solution heat treated layer. The results of a Creviced Bent Beam (CBB) test showed that no cracks had appeared on the surface of LDT. After LDT was applied in the vicinity of welding joints on the inside surface of pipes, tensile residual stress was measured there. On the other hand, the tensile stress of outside surface of the pipes was decreased. From these results, LDT processing on the inside surface of a pipe can be expected to prevent the occurrence of IGSCC owing to the effect of both metallurgical improvement and decrease of the residual stress on the outside surface of the pipe. We developed the LDT processing system and successfully applied on the pipes of some actual nuclear power plants.A technology to prevent the occurrence of Intergranular Stress Corrosion Cracking (IGSCC) by irradiating a high power Nd:YAG laser beam was developed. Laser Desensitization Treatment (LDT) process was realized by irradiating a laser beam onto the sensitized Heat Affected Zone (HAZ) surface of SUS304 stainless steel. LDT was formed by both a molten layer of approximately 0.2mm depth and a solution heat treated layer. The results of a Creviced Bent Beam (CBB) test showed that no cracks had appeared on the surface of LDT. After LDT was applied in the vicinity of welding joints on the inside surface of pipes, tensile residual stress was measured there. On the other hand, the tensile stress of outside surface of the pipes was decreased. From these results, LDT processing on the inside surface of a pipe can be expected to prevent the occurrence of IGSCC owing to the effect of both metallurgical improvement and decrease of the residual stress on the outside surface of the pipe. We developed the LDT processing sy...

Laser technology for maintenance of nuclear pipe

A remote maintenance technology to process cooling pipes internally for the ITER (International Thermonuclear Experimental Reactor) has been developed. The fundamental welding properties of austenite stainless steel obtained by using high power YAG laser indicates the tendency of porosity increase as the welding depth increases. It is also found out that the occurrence of porosity was prevented when N2 gas was used as a shielding gas and that the material strength of weld joint is kept unchanged. Furthermore, a processing equipment that can access the cooling pipes internally has been developed. The main body of this processing equipment can be inserted into a cooling pipe and consists of an focusing optics, clamping and driving mechanism, and an optical fiber. Operational experiments of this processing equipment is carried out and the basic verification experiments such as welding and cutting tests were conducted. As a result, a good quality of pipe welding joint was obtained without any defects.