Kazuhiko Kamo

Development of Automatic GTAW Technology using Visual Sensor in Narrow Gap all Position

In recent manufacturing scene, one of the serious problems is the age of the highly skilled welders. Application of highly full automatic welding system without welder’s skill is needed and the system must keep the quality of the product and the production efficiency. In this study, we developed the full automatic welding system with the visual sensor and adopted this system for narrow gap all position GTAW of piping circumferential joint instead of welder’s highly skill. It was verified that welding was automatically performed using the functions of this system as follows: adaptive control of positions of electrode, adaptive control of positions of filler wire, adaptive control of welding condition (current condition), monitoring electrode consumption.

Development of Outer Surface Irradiated Laser Stress Improvement Process (L-SIP)

Improvement of residual stress is effective in a countermeasure to deal with the stress corrosion cracks in pipe welds. A irradiated laser stress improvement process (L-SIP) will be introduced as a method to improve residual stress inside steel pipes. This work method is to improve inner surface residual stress from tensile stress to compressive stress by irradiating laser beam around the welds of steel pipe and utilizing the temperature differences between inner and outer surface.Copyright

Application of L-SIP to Pressurizer Nozzles

It has long been known that the most effective in a countermeasure for stress corrosion cracking in pipe and nozzle welds is by reducing the residual stress in the portion of the weld exposed to the corrosive environment. An irradiated laser stress improvement process (L-SIP) was introduced as a method to improve residual stress inside steel pipes and nozzles. L-SIP has been applied to the pressurizer nozzles in actual plant, Tsuruga unit 2 Japan, for the first time in the world. The nozzles to which this process was applied are the surge nozzle (September 2007), safety nozzles, relief nozzle and spray line nozzle (April 2010). L-SIP can be applied without inner surface cooling because the high power laser beam can generate the sufficient temperature difference without such cooling. Where necessary to achieve optimum temperarure difference, water cooling may also be applied at the inner surface. At Tsuruga unit 2, L-SIP was successfully applied to the spray line nozzle in air-cooling mode, and the surge nozzles, 3 safety nozzles and relief nozzle in water-cooling mode.Copyright

Application of Outer Surface Irradiated Laser Stress Improvement Process (L-SIP) to Pressurizer as Residual Stress Improvement Method for Alloy 600 PWSCC Mitigation

Improvement of residual stress is effective in a countermeasure to deal with the stress corrosion cracks in pipe welds. A irradiated laser stress improvement process (L-SIP) will be introduced as a method to improve residual stress inside steel pipes. This work method is to improve inner surface residual stress from tensile stress to compressive stress by irradiating laser beam around the welds of steel pipe and utilizing the temperature differences between inner and outer surface. Recently this method is applied to PWR pressurizer surge nozzle on TRUGA unit 2.Copyright

Repair Welding of Irradiated Reactor Vessel Steel by Low Heat Input GTAW and LBW

Weldability in neutron-irradiated low alloy steel for a reactor (pressure) vessel has been studied by a temper-bead welding technique using low-heat-input GTAW and LBW(YAG). The base and weld metals of low alloy steel with clad or without clad were irradiated up to 1,4 X 10 24 n/m 2 (>1 MeV) at 290 °C, which approximately corresponds to the maximum neutron fluence of a 60-year-operation plants vessel. The mechanical property tests, such as tensile, impact, side bend and hardness, were carried out after the repair-welding. The weld cracking did not occur in base and weld metals of irradiated low alloy steels during the repair-welding by GTAW and LBW. Small porosities were formed in the first and second layers of the repair weld metal. Only a few porosities were found in weld metal compared with in base metal. From the results of mechanical properties, the weld metal of low alloy steel could be welded up to a He concentration of 12.9 appm, while the base metal could be done up to only 1.7 appm He. On the other hand, weld cracks occurred in stainless steel and Ni base alloy clad on low alloy steel caused by He bubble formation Based on these results, the rule of repair-welding procedure selection was proposed for the reactor (pressure) vessel.

Temper-bead repair welding of neutron-irradiated reactor (pressure) vessels by low-heat-input TIG and YAG laser welding

In recent years, we have seen examples of damage to the structures in the reactors at nuclear power plants in Japan. It is essential that repair technology be established for these, and it can be assumed that repairs using welding where there is damage arising in the initial stages for equipment and structural materials and parts that are difficult to replace. Nuclear reactors (pressure) and structural materials for the reactors are irradiated by neutrons. When the materials used are irradiated by neutrons, aggregations of irradiation defects are formed, and material changes from the generation of solutes / segregation of elemental impurities and the generation of helium (He), because of transmutation of nickel (Ni) and boron (B). Therefore, it is known that distinctive defects easily arise because of welding. Starting in the latter half of the 1980’s, a large number of welding studies were carried out on irradiated materials targeting austenite stainless steel. From these it was found that in stainless steel welds, intercrystalline cracks arise, because of the formation and growth of transmuted He bubbles at the grain boundaries in the parts affected by the welding heat, because of the effects of heat and stress in the heating and cooling processes during welding. 4 In addition, it was shown that the weldability of irradiated materials can be arranged according to the He concentration in the material and the welding heat input. Occurrences of similar cracks in the parts affected by the welding heat have been found even in ferrite stainless steel (HT-9) and Ni based alloy (600 alloy) that has been irradiated with neutrons. 7 These results suggest that welding at as low a heat input as possible is necessary for preventing weld cracks in the irradiated material. As a result of carrying out YAG laser welding, which is one low heat input welding method, it was discovered that cracks did not arise if the input heat was 0.042 MJ/m or less for austenite stainless steel 304L containing approximately 9 appm He. On the other hand, in the welding of low alloy steel that has been irradiated, materials and non-irradiated materials have been joined using electron beam welding, laser welding, arc stud welding, projection welding and the like as reconstruction techniques for surveillance samples. However, there have been no studies on the occurrence of defects in the weld parts, and there has been no research focusing on He amounts. In addition, there has not been any research concerning methods for repair welding when damage arises in nuclear reactor (pressure) vessels. Based on the background above, the Japan Power Engineering and Inspection Corporation received a consignment starting in FY 1997, and further, its successor, the Japan Nuclear Energy Safety Organization, carried out operations for verifying repair weld techniques targeting internal reactor structures and nuclear reactor (pressure) vessels that had been irradiated by neutrons, as grant operation for the Ministry of Economy, Trade and Industry from October 2003 to March 2005. In this report, we discuss the results of the operations described above for nuclear reactor (pressure) vessels and consider the welding conditions that can be repaired safely. Here, the targets of the research were low alloy steel or parts with low alloy steel with stainless steel or Ni based alloy cladding.