Seiichiro Kimura

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.

High-temperature mechanical properties of laser welds in Co-base superalloy and its improvement by laser surface melting

Summary The high-power CO2 laser is extensively used for practical applications such as cutting and welding of metals. Laser welding of superalloys requiring high-quality and high-efficiency welding, however, is seldom employed in heavy industry. Conventional laser welding of metals with filler wire produces the columnar structure which meets at the weld bead centre and leads to deterioration of creep properties. To improve the creep rupture behaviour of joints within this context, this paper describes the development of a laser surface melting technique involving the weld bead surface being remelted by laser. In the experiments described, Haynes 188 Co-base superalloy with a thickness of less than 3 mm is used as the base metal. The spot size of the laser beam used to irradiate the first layer of the weld bead surface is around 3 mm. The depth of the surface melting layer is varied by the travelling speed. The results suggest that both the elongation at fracture and creep rupture life after laser surface...

Plasma behaviors in laser cutting

Behaviors of the plasma, the emission intensity, distribution, source and species were evaluated using a high-speed video camera, a color CCD camera, and a spectrum multi-channel analyzer. It was found that a strong blue radiation from iron atoms of workpiece, nitrogen molecules or ions of a gas jet, when the cut surface quality was not acceptable. As cutting speed increased, plasma formation region where a laser beam was focused was more extended. It was found that flow speeds of molten material at bottom in which the plasma was induced became slower than those of top and middle points. Also, scattering angles of molten material droplets at a bottom exit of a kerf became larger and, at the same time, reattachment of molten material at the bottom was observed. Moreover, it was found that additional branches of the flows of molten material with smaller and darker droplets were formed intermittently, and that parts of the branches of the flowing droplets were being attached as dross to the bottom edges. From these results, it was confirmed that the plasma formation (region) and sizes and angles of striations (cut surface quality) were strongly correlated. Possibilities of the in-process evaluation of the cut surface quality by monitoring the plasma sizes and/or brightness in laser beam cutting were discussed.

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...

Development of in-pipe access welding and cutting tool using YAG laser

In the International Thermonuclear Experimental Reactor (ITER), the blanket is categorized into the schedule maintenance component and has to be replaced by remote handling technology due to activation by the 14-MeV neutrons during DT operations. The blanket is segmented into a number of modules to facilitate remote operation and requires the welding and cutting of cooling pipes connected to each module from the inside of the pipe due to space constraints. A prototype tool fabricated for branch pipe welding/cutting demonstrates the required mobility for traveling the cooling pipe with a diameter of 102.3 mm and a bent radius of 400 mm, and for accessing to the branch pipe with a diameter of 54.5 mm. The welding and cutting performance has been also tested, including the dependency of laser power, processing speed, and gaps on weldability. In addition, a composite optical fiber composed of a number of thin fibers arranged around a core fiber is also tested for direct viewing of the edge preparation before welding and for monitoring during welding/cutting. This paper describes the test results of the prototype tool performance and of in-pipe access welding and cutting operations.

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.

Development of Laser Beam Delivery Control System.

A laser beam delivery control system has been designed to align a laser beam axis and monitor its beam properties such as beam position, beam diameter, power, peak power and 3-D profile. A laser machining system for long term operation will make this system necessary because those beam qualities will be changed. The developed system is composed of two beam sensors, two angle control bending mirrors with actuators, and a host controller. A rotating wire method was applied for this sensor. In this system, the accuracy of the beam diameter was ±0.5 mm compared with the measurement from a burn pattern of acrylic resin. A far-field laser beam position of 17 m away from an oscillator could have been measured and controlled automatically within an accuracy of ±1 mm under on-line conditions. Thus this developed system is very promising for evaluating beam qualities.