Production and application of domestic low-tin Zr-4 cladding tube electron beam welding Yibin Nuclear Fuel Element Factory (644000) Evaporation, this vacuum electron beam welding process is suitable for domestic low-tin Zr-4 alloy. More than 10,000 domestic low-tin Zr-4 cladding nuclear fuel rods were welded by this process.
0 Preface Nuclear fuel rods are an important part of nuclear power plant reactor core fuel assemblies (hereinafter referred to as fuel rods). The fuel rod cladding tube is the first barrier for the safe operation of the reactor. In order to improve its corrosion resistance in the reactor, its material is currently changed from the standard Zr-4 alloy (tin content of 1.2% to 1.7%) to low Tin Zr4 alloy (tin content is 1.2% ~ 1.5%) For the welding of PWR fuel rods, China generally uses vacuum electron beam 1. To ensure the welding performance of fuel rods, corrosion, metallography, X-ray transillumination should be carried out , Helium leak detection, appearance and other inspections, only after all the inspections are fully qualified and have high reliability, can they enter mass production. Among them, the corrosion test is the most important inspection item for the quality of fuel rod girth welding. The corrosion of fuel rod 1 by vacuum electron beam welding 1.1 the form of fuel rod welding joint The form of fuel rod welding joint is as follows. The fuel rod cladding tube adopts domestic Zr-4 alloy, while the fuel rod end plug adopts the international standard Zr-4 alloy. According to reports, the welding performance of fuel rods is mainly related to the alloy composition and structure of the cladding tube, and years of practice have proved this.
1.2 Characteristics of domestic low-tin Zr-4 alloy cladding tube Compared with foreign imported tubes, the domestic low-tin Zr-4 alloy cladding tube has the following characteristics: the tube blank is purchased from abroad, and the milk is quenched in the middle; A special cold milk system is adopted. The finished tube adopts an annealing process of 550X3h. The crystallization is sufficient. The grains are relatively coarse and equiaxed. The deformed structure in the metallography is less elongation and strength increase at the same time. White and brown products.
China has been using imported low-tin Zr-4 alloy cladding tubes for several years. The quality of welds after welding is good, especially white and brown products rarely appear after corrosion tests. In recent years, domestic production of low-tin Zr-4 alloy cladding tubes has begun in China. In this paper, the fuel rod vacuum electron beam welding of domestic low-tin Zr-4 alloy cladding tubes was studied.
Washing, surface treatment process of inner surface spraying, etc.
Due to the above-mentioned different characteristics, the domestic low-tin Zr-4 alloy cladding tube adopts the electron beam welding process that is similar to the imported Zr-4 tube. A lot of experimental research has been carried out.
1.3 The test equipment adopts basically the same two electron beam welding machines CA330M imported from Russia (referred to as 03 welding machine, the same below) and CA330M1 (referred to as 06 welding machine, the same below).
2 Test results 2.1 In the statistical test of the unqualified samples after corrosion, different welding parameters were used, and a total of 250 samples were welded. Shows the percentage of various types of unqualified samples in the total sample (250).
In the usual method of mass production, the acceleration voltage and welding speed are used as fixed values, and the welding current (abbreviated beam current, the same below) and the number of welding cycles are changed. These parameters are differently matched, and another 250 samples were welded. Table 1 shows the distribution of these samples.
2.2 The influence of welding parameters on corrosion resistance The welding parameters of electron beam welding include acceleration voltage, welding current, number of welding turns, welding speed, etc. The influence of each welding parameter on ring seam forming and welding quality is different. In the experiment, reduce the energy input density of the electron beam to understand the influence of welding parameters on the welding performance of the ring seam. Take the experimental input and test results in Table 1 beam current 1 / mA welding number of samples number of metallographic investment inspection (qualified number) Cavitation input number (qualified number) average penetration ~ 1.66 (excessive penetration) -1.02 (excessive penetration) -0.96 (excessive penetration) -0.83 (all qualified) -0.89 (all qualified) , The unqualified sample is a repair welding sample, -0.60 (small penetration) is not fully penetrated, and the penetration is too small. Note: one sample is used for corrosion test twice. Fixed parameters: acceleration voltage is 750.5kV ; The welding speed is 26.5 rpm / min when defocusing, the welding speed is 21.5 rpm / min when focusing (these two parameters characterize the welding speed), the vacuum degree of the welding chamber is better than 9.31X103Pa. 2.3 Two typical sample fuel rod vacuum electrons The beam welding adopts the convergence and defocusing procedure. The focusing parameters and defocusing parameters must be well matched, and their selection is very erosive. In Table 2, the focus welding and defocus welding of sample P03151 are improperly matched, while the parameter matching of focus welding and defocus welding of sample 06FR003602TC is reasonable.
2.4 Scanning electron microscopy analysis of the internal surface of the domestic low-tin Zr-4 tube The surface morphology and composition of the internal surface of the domestic low-tin Zr-4 tube were analyzed with an electron microscope. The results are shown in Table 3. The inner surface of the tube head is uneven, there are mosaic particles, the main component is C, and some particles contain anti-corrosion equivalent to circumcision if you choose inappropriate defocus welding. However, the shape of the molten zone bookmark3 Table 3 surface composition electron microscopy scan results (wt%) The composition tube head tube has very small particles in the middle of the tube, containing trace elements of C, O, Si, Al and so on.
3 Analysis and discussion The focus energy density used in actual production is 100500MW / m2. This test also uses the focus energy density in this range. The welding test shows that when the focus energy density selected for electron beam welding is less than 1000MW / m2, the depth of the electron beam energy penetrates the alloy at the shallower surface of the fuel rod. The alloy melting is similar to other fusion welding. This method is called melt forming (in contrast, when the energy density of the electron beam focal point exceeds 1000MW / m2, the molten alloy evaporates strongly, the molten pool sinks to form a cavity, and the molten metal is Repelled behind the direction of the electron beam.
As the electron beam moves forward, the molten metal crystallizes to form a weld, which is called deep forming. At home and abroad, we have studied the evaporation phenomenon of the surface of the fuel rods welded by electron beams. In the past, it was believed that no matter whether it is melt forming or in-depth forming, the evaporation of alloying elements generally does not exceed 30%. There are significant evaporation phenomena with other alloy elements. The content of Sn alloy elements is less than 0.5% after evaporation. In fact, when vacuum electron beam welding, it seals the welding point of the fuel rod in the vacuum chamber, the vacuum degree is generally 1.33 X102Pa or more, and because the focus energy density used in vacuum electron beam welding is 100 rods, the temperature of the fusion zone of the ring seam reaches 2X103C or more, so the evaporation of alloy elements such as Sn, Fe, and Cr becomes easier (because of the vacuum The evaporation temperature of each element is lower than that under normal pressure). It is pointed out that for the Zr4 alloy, the Sn element content is less than 0.5%. Since the effect of harmful impurities cannot be offset, the corrosion resistance becomes extremely poor. In other words, the Sn content is 0.5%. At a turning point, it can be concluded that it is precisely because of the severe evaporation of Sn that its content is less than 0.5%, which causes the corrosion resistance of Zi-4 alloy to deteriorate. The results in Table 2 also prove this conclusion.
Obviously, in addition to the vacuum degree, the evaporation of alloy elements is also related to the temperature of the melting zone, that is, the energy input density, which is proportional to the welding current when other relevant parameters are fixed, and it is also proportional to The residence time of the molten pool at high temperature is closely related to the cooling conditions. Table 1 shows that when the beam current is greater than 6.0mA, the weld penetration is large, indicating that the input energy of the electron beam is large, which causes Sn, Fe, Cr to evaporate greatly, and the chance of corrosion failure is large, and the beam current is 5.5 ~ 5.8mA, the penetration depth is more suitable. After the sample is corroded, except for the repair welding sample, it will be transformed into the surface of the thermal fuel rod to produce 1 evaporation phenomenon. Then call it. It all passes. Repair welding is to perform welding once again on the basis of the original welding, which aggravates the evaporation of the ring stitch gold component, so the chance of corrosion failure is large. It shows that the repair welding sample has a higher chance of unqualified corrosion.
In electron beam welding, on the one hand, the molten pool metal forms crystal nuclei and grows continuously; on the other hand, the gas in the molten pool continuously floats out. If the floating speed is greater than the crystallization speed, no pores will be generated. Obviously, if the sand blasting process on the inner surface of the cladding tube is unstable, the contaminated low-melting material vaporizes, the molten pool boiling is intensified, and the floating speed is faster than the crystallization speed. Although it is not easy to produce welding defects such as porosity, it accelerates the evaporation of alloy elements.
Although these surface contamination particles will not affect the corrosion resistance of the cladding tube base material, it will affect the corrosion resistance of the fuel rod electron beam ring seam. It shows that the internal surface treatment process of the domestic low tin Zi-4 tube is unstable, plus the use of general tap water to rinse, so the surface is attached with elements such as C, O, Si, Mg, K, etc. These impurities are vaporized during welding, causing The boiling of the welding pool is intensified (when welding the domestic Zi-4 cladding tube ring seam, the spattering phenomenon is very serious), thereby intensifying the evaporation of alloy elements.
The fuel rod electron beam welding adopts a defocused welding procedure. The original purpose is to use the defocused beam to modify the surface of the weld. It is found through experiments that it also has the effect of changing the composition of the alloy on the surface of the weld. This is because the end plug is a standard Zi-4 defocus welding. The beam focus is enlarged, the energy density is very small, the weld is widened, and the end plug material melted by the focus welding is redistributed throughout the fusion zone to improve the welding. The content of alloy elements on the surface of the seam ensures the corrosion resistance of the weld.
The defocusing parameters and the focusing defocusing parameters must be well matched. Their selection is very important. If not selected properly, defocusing welding is equivalent to repair welding, which increases the evaporation of alloy elements on the surface of the weld. The focus of sample P03151 in Table 3 Welding and defocus welding aggravate the evaporation of alloy elements on the surface of the weld, and try to match the parameters of the focus welding and defocus welding of -02TC. Only deeper focus welding can melt the end plug material. Only wide and shallow dispersion Only the coking weld can redistribute the end plug material melted by the focus welding in the entire melting zone, increase the content of alloy elements on the surface of the weld, so as to compensate for the evaporation of alloy elements on the surface of the weld, and ensure that the weld has good corrosion resistance. performance.
4 Optimized process parameters and mass production In order to ensure the reliability of fuel rod welding joint performance, the optimized process parameters are as follows: small energy input density, welding current 5.5 ~ 5.8mA, and the penetration depth of the welding sample is controlled by 0.9mm ; (3) Because of the inflation hole in the center of the upper end plug, the evaporation of the alloy elements in the weld is slightly more intense, so the welding of the upper and lower girth of the fuel rod should be treated differently. When welding the upper girth (with the hole), the beam current For 5.5 ~ 5.7, the poly-defocus welding procedure is used, that is, the surface weld is widened with a low-energy beam, and the end plug material (standard Zi-4 alloy) is used to increase the alloy element content of the weld surface.
It can be seen that only one of the 250 welding samples has a corrosion current of less than 6.0mA and does not repair welding. The number is 4-60-60. After analysis by electronic probe and gold equal test, the The sample is an abnormal sample. It is concluded that the end plug was not cleaned before being pressed into the cladding tube, and the non-metal solid substance with a melting point was mixed in the area corresponding to the weld in the contact gap between the end plug and the guide tube.
There is no abnormality in the online inspection of the corrosion resistance of more than 10,000 fuel rods with the furnace sample, which proves that this welding process is completely feasible.
Calculating the risk of mass production using this welding process, the probability of occurrence of unqualified samples is 1.54X104, which is slightly lower than the damage rate of reactor fuel rods that are commonly used in the world.
5 Conclusions In the electron beam welding process at a higher vacuum degree, there is an evaporation of alloy elements, especially the process of the internal surface treatment of the low-tin Zi-4 cladding tube is unstable. When there are some particles with low melting points on the surface, the weld will be increased. The evaporation of alloying elements in the zone resulted in (gray) white products after cavitation, which reduced the corrosion resistance of Zi4 alloy.
Appropriately reduce the energy input density during welding, that is, reduce the welding current, control the penetration depth in an appropriate range, focus and defocus without welding repair should be reasonable, so as to reduce the evaporation of alloy elements in electron beam girth welding. Suitable for girth seam welding of domestic Zi-4 alloy cladding tube, this welding process can improve the corrosion resistance of Zi-4 alloy after welding
The fuel rods produced in mass production according to this welding process are fully qualified for corrosion resistance.
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