| Nuclear power is a safe, economical and clean energy recognized in the world.Many countries have developed nuclear energy as major part of their national energy plan. The type and capacity of nuclear power reactor have many kinds, which is under constantly developing and improving.Currently the most reactor type in the world is the pressurized water reactor (PWR). The primary loop of PWR includes reactor pressure vessel, pressurizer,steam generrator,the primary pump, the primary main pipe and so on.The properties of dissimilar metal weld between reactor pressure vessel and primary main pipe will directly affect the safety of nuclear power plant. This paper studied the properties and the weak position of dissimilar metal weld (DMW) of nuclear power plant reactor pressure vessel, and simulated DMW defect shape, distribution, location and ultrasonic testing parameters by ultrasonic simulation testing, which was verified by multi-channel double-crystal longitudinal-wave focusing ultrasound testing technology.The study of dissimilar metal weld properties mainly carried out the chemical composition analysis, microstructure examination,mechanical property testing, scanning electron microscope examination and weld alloy elements diffusion measurement.The study also analyzed the weak position of dissimilar metal weld (DMW) of reactor pressure vessel. Analysis results showed that the weak position of the whole of dissimilar metal weld was on the low alloy metal heat affected zone,which has obvious presence of coarse grain zone and fine grain zone on the side,and has the 17% higher hardness compared with the coarse grain zone. The Charpy V-notch impact tests of the heat affected zone showed a series of fluctuant unstabilized values, indicating a higher tendency of embrittlement and cracking,which was verified by the tensile tests in which all the rapture position was on the heat affected zone of the specimens. Due to the lower strength of the low alloy compared with the austenitic stainless steel, the heat affected zone on low alloy side was the weak point of DWM.By studing the properties of DMW,the CIVA ultrasonic simulation software constructed a simulation crack in the DMW low alloy metal coarse grain zone.The basic characteristics of simulation crack are as follows:30* 10mm rectangular, parallel to the weld fusion lines, away from the inside surface to a depth of 9mm. The ultrasonic simulation testing results showed that using 1.5MHz,70°TRL FS20 probe for detecting simulation crack can get the maximum response amplitude and the best detection rate.According to the results of ultrasonic simulation testing, a choice of four 1.5MHz,70°TRL FS20 double-crystal longitudinal-wave focusing oblique probes, supplemented with four 1.5MHz,45°TRL FS20 double-crystal longitudinal-wave focusing oblique probes and one 2MHz single-crystal longitudinal-wave right probe were used for the validating scanning to the DMW block from the different directions. The scanning depth of 1.5MHz,70°TRL FS20 probe is 0-13mm,starting from the inner surface of the RPV ultrasonic block. The scanning depth of 1.5MHz,45°TRL FS20 probe is 3-23mm,starting from 3mm under the inner surface.The scanning range coverde the whole ultrasonic simulation crack area. A supplement ultrasonic block with defects was selected to scan, and the scanning result showed there were three defects on the supplement ultrasonic block inner surface,which length/height respectively were 20.5/7.4mm,27.5/13.1mm,38.4/15.4mm.No recordable flaws were found in the weld and in heat affected zones on the RPV block two side,indicating there was no simulated cracks,porosities,inclusion and other flaws in the RPV ultrasonic block.This paper also discussed the service environment and deterioration mechanism of the pressure vessel DMW, effect of coarse grain size to the ultrasonic testing, the significance of simulation technology on the ultrasonic testing, and finally the optimization of ultrasonic testing parameter of the pressure vessel DMW weld. |
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