Zirconium alloy welding performance is better, at room temperature chemical performance is more stable, corrosion resistance is outstanding, but its high temperature chemical performance is very active, oxygen, nitrogen, hydrogen and operating environment in the dust, humidity and other pollution have a strong affinity. With the increase of temperature, the chemical activity of ZrH2 increases sharply and forms ZrH2 with hydrogen at 200℃. ZrO3 can be formed with oxygen at 300℃. At above 550℃, it reacts with oxygen in the air to form porous brittle oxide film. At 600℃, zirconium absorbs nitrogen to form ZrN. Above 700℃, the material is severely embrittlement due to oxygen absorption. With the increase of temperature, the absorption capacity and reaction rate are enhanced accordingly. Therefore, the high temperature environment and welding seam are the key to restrict the chemical equipment.
The excellent corrosion resistance of zirconium alloy comes from the oxide film formed on its surface, and depends on the integrity and firmness of the oxide film. When zirconium alloy absorbs a certain amount of oxygen, nitrogen, hydrogen and other gaseous impurities, its mechanical properties and corrosion resistance will decline sharply. The reaction generates a porous brittle oxide film. Therefore, strengthening the protection on the surface of the area affected by environmental dust, humidity and heat and the back of the weld is the key factor for quality control in the welding process.
Because zirconium alloy has low thermal expansion coefficient, small thermal deformation and volume change during phase transition, and low sulfur, phosphorus, carbon and other impurities, there is no obvious trend to form cracks in the welding process. However, when a certain amount of oxygen, nitrogen and hydrogen gas impurities are absorbed by the welding seam during the welding process, the properties of the welding seam and the heat-affected zone will become brittle. If the stress exists in the welding seam at the corresponding link, cold welding cracks will occur. At the same time, hydrogen atoms have the ability of diffusing and accumulating to the high-stress parts in the heat-affected zone of lower temperature, which promotes the formation of relatively weak links at these parts, and thus may lead to the generation of delayed cracks in welding.
Zirconium alloy gTAW adopts high purity argon with a purity of no less than 99.999%, and the impurity content conforms to the current GB/T4842 standard. Because of the extremely high purity requirement of welding protective gas, the welding process should be continuously inflated without interruption, otherwise argon should be replaced. Therefore, the direct supply of single cylinder of ordinary argon cannot meet the protection requirements, so it is necessary to increase the supply capacity of multiple cylinders in series to meet the needs of multiple welders working at the same time through the cylinder.
