| Nuclear power is not only the trends of power development, but also the needs of a low-carbon economic development. In order to reduce the effects of exposure to nuclear radiation, inspection and maintenance of internal equipments in nuclear power plant are often carried out in the water. The current underwater maintenance in nuclear reactors is mainly fulfilled by the manual welding. Considering such factors as the health of operators, repair quality and repair efficiency, the trend of nuclear power station underwater welding is remote control automation. Local dry underwater welding is the first choice of nuclear power station during underwater welding repair because it combines the advantages of high-weld-quality in dry underwater welding and easy-to-do in wet welding. But local dry underwater welding method has not solved the problems such as continuous and effective drainage seal, creating sealed environment for local dry welding and welding automation, which lead to less stability of the welding process and which mean joint quality can not fully meet the relevant standards of nuclear power. Besides the study report is rarely in view on underwater welding technology and welding joints properties of nuclear power structural materials.Firstly, this research studies the nuclear power structural material's weldability through the-in-air welding test of nuclear power station with stainless steel material, and while getting to know the regular pattern of different welding methods, welding shielding gas, wire feed speed and arc length to weld processes and the welded joint formation, the basic data is gained for the following tests.Secondly, this study has established the local dry automatic underwater welding test system based on miniature draining water cover, which is mainly composed of underwater welding test cabin, test environmental system, welding monitoring system, automatic welding platform, electronic control system and welding power supplies. Coordinating action of different parts of system can simulate the welding test environment under 15-meter-depth water, and can realize the real-time remote surveillance of whole process on the welding regions. Integrated the welding quality analysis system in the automatic underwater welding test system can monitor the welding process parameters and the stability to forecasts the welding quality.Effective seal of draining water device is a prerequisite for the local dry automatic underwater welding to carry on smoothly. The research has obtained the gas flow stability in drainage system through theoretical calculation and numerical analysis. Using the combination material gasket design can significantly reduce the damage to the sealed parts from welding arc and weld pool, and extend the effective drainage timeBased on the above, remote control automatic underwater welding experiments of 321 and 304 stainless steel have been conducted. With the experiments the key factors and rules influencing the sealing effect have been recorded, and the law of effect has been established of wire feed speed, shielding gas, moving parameters, groove form and depth of water environment to the welding quality, which says along with the increase of wire feed speed the welding stability of process has been enhanced and adding the active gas to shielding gas can improve the welded joint formation and the optimize melt drop state of transition. The test has also obtained underwater welding technological parameters of 321 stainless steel groove butt welding and 304 stainless steel surfacing welding, built-up welding and groove butt welding.Conducting the experimental study on continually sealing performance of different gasket structure, obtained good sealing property by using combination material gasket.Finally, the comprehensive test has been carried out on underwater weld. The results show:the underwater welding technological parameter obtained in the study can achieve a good underwater welding quality, and not-continuous local dry underwater welding process is the main reason which leads to the underwater welding flaw; compared with the parent metal performance, weld strength meets the requirements while toughness is decreased and hardness is increased slightly; 5m and 15m water depth does not have the obvious different effects on the weld mechanical properties. Chemical composition analysis on the built-up welding weld shows that the anti-corrosive performance of underwater stainless steel weld is superior to the parent metal.According to theoretical analysis and experimental confirmation, the time that weld stay at high temperatures is shorter than in the air due to the forced cooling effect on the weld in underwater environment, and thus the underwater welding of austenitic stainless steel will not have the intercrystalline corrosion. Therefore the welding process parameters can be used to improve welding efficiency, which is very important for the remote control of underwater welding in nuclear radiation environment.The weld macro-analysis of cross-section has showed that underwater welding weld with the parent material can be a good fusion with no visible defects. Microstructure analysis showed that the ferrite-austenite phase transformation has been restricted in underwater welding weld. The distribution of ferrite morphology is different to that in dry environment weld.As for the weld under different environments in terms of water depth, under the premise of good draining water effect, the water cooling effect basically has nothing to do with the water depth, and the water depth has no effect on ferrite content and distribution morphology. Energy-dispersive spectrometry(EDS) analysis shows that the Fe, Cr, Ni contents in underwater environment weld increase to some degree and the Cr/Ni is slightly higher than that in dry environment weld.The above results have laid a good foundation for the local dry automatic underwater welding technology research and austenitic stainless steel materials underwater welding technology research.
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