Problems Prone to Welding of Zirconium Alloys at High Temperatures

Zirconium is an expensive corrosion-resistant metal material with excellent resistance to corrosion by acids and alkalis. In some media, it even exceeds metals with good corrosion resistance such as niobium and titanium. Zirconium alloys have been gradually used in recent years as structural materials for equipment and pipelines in the chemical industry due to their good corrosion resistance.

The commonly used zirconium alloy grades include Zr702 (UNSR60702), Zr704 (UNSR60704), and Zr705 (UNSR60705). Among them, Zr702 (UNSR60702) is widely used in chemical projects.

zirconium-tubes

Basic characteristics of zirconium alloy

Zirconium alloy has good welding performance, stable chemical properties at room temperature, and outstanding corrosion resistance. However, its high-temperature chemical properties are lively and have a strong affinity for the pollution of oxygen, nitrogen, hydrogen in the ambient gas, and dust and humidity in the operating environment. As the temperature rises, its chemical activity sharply increases, and it forms ZrH2 with hydrogen at 200 ℃; it can form ZrO3 with oxygen at 300 ℃; it reacts with oxygen in the air above 550 ℃ to form a porous brittle oxide film; at 600 ° C, zirconium absorbs nitrogen to form ZrN; it absorbs oxygen and severely embrittles the material at above 700 ℃. As the temperature increases, its absorption capacity and reaction speed increase. Therefore, the high temperature environment and welding seams generated by welding are the key to restrict chemical equipment.

The excellent corrosion resistance of zirconium alloys comes from the oxide film formed on its surface and depends on the integrity and robustness of the oxide film. When zirconium alloy absorbs a certain amount of oxygen, nitrogen, hydrogen and other gas impurities, its mechanical properties and corrosion resistance will drop sharply. Therefore, strengthening the protection of environmental dust, humidity and heat-affected zone surfaces and the back of welds is a key element of quality control during welding.

Problems prone to welding of zirconium alloys

High temperature is the natural enemy of zirconium alloys with great changes in corrosion performance. Zirconium generally reacts easily with the atmosphere at high temperatures. It starts to absorb oxygen at 200 ℃, hydrogen at 300 ℃, and nitrogen at 400 ℃. The higher the temperature, the more intense the reaction. Because zirconium is active against oxygen, nitrogen and hydrogen, it must be protected with a high-purity inert gas or welded in a good vacuum chamber.

zirconium-alloy-welded

During zirconium welding, the weld seam and heat-affected zone are easily polluted by oxygen, hydrogen, nitrogen and other elements in the air, forming hard and brittle compounds, and producing brittle needle-like structure, which increases the hardness and strength of the welded joint , while the plasticity declines, and the corrosion resistance is also greatly reduced. Therefore, zirconium welding should fully protect the molten pool, weld and heat affected zone to completely isolate the air.

The welding of zirconium alloys is generally performed by the welding method of tungsten inert gas shielded arc. Other welding methods include electron beam welding, plasma arc welding and resistance welding. Its welding performance is close to that of titanium metal welding. Due to the small thermal expansion coefficient and elastic modulus of zirconium, the welding deformation and weld residual stress are relatively small. It is recommended that the stress relief time of the weld at 1100 ° F (594 ℃) be 1 hour / inch thickness.

Another major problem of zirconium welding is that the weld is prone to soften too much and cause the weldment to be distorted. When welding zirconium, the welding piece should be properly fixed and double-sided welding should be used as much as possible. Except for titanium, niobium, silver, and vanadium, zirconium cannot be directly welded to other metals. Therefore, choosing a clean operating environment and strengthening the isolation and protection of welds and heat-affected zones are the keys to ensuring the quality of zirconium alloy welding.

Stanford Advanced Materials supplies high-quality zirconium alloys to meet our customers’ R&D and production needs. Please visit https://www.samaterials.com/ for more information.

Argon Arc Welding Technology of Zirconium and Zirconium Alloy

Zirconium and zirconium alloys have excellent corrosion resistance to acid and alkali, and even surpass niobium, titanium and other metals in some media. Therefore, zirconium and zirconium alloys are gradually used as structural materials such as equipment and pipelines in the chemical industry with strong corrosion resistance due to their good corrosion resistance in recent years.

Due to the high-temperature chemical activity, zirconium and zirconium alloys can react with various elements in the air at high temperature, thus damaging their mechanical properties. Therefore, in the process of zirconium and zirconium alloy welding, the key to ensuring the quality of welding is to select a clean operating environment and strengthen the isolation and protection of welding seams and parts in the heat-affected zone.

Basic properties of zirconium and zirconium alloys

Zirconium and zirconium alloy materials mainly include R60702, R60704, and R60705. Zirconium and zirconium alloys have good welding properties and stable chemical properties at room temperature. However, its high-temperature chemical properties are very active, and it has a strong affinity for the pollution of oxygen, nitrogen, hydrogen and dust and humidity in the operating environment.

The excellent corrosion resistance of zirconium and zirconium alloys comes from the oxide film formed on the surface and depends on the integrity and firmness of the oxide film. When zirconium and zirconium alloy absorb a certain amount of oxygen, nitrogen, hydrogen, and other gas impurities, their mechanical properties and corrosion resistance will decrease sharply. Therefore, strengthening the protection of the surface of environmental dust, humidity and heat affected area and the back of the welding seam is the key element of quality control in the welding process.

Zirconium and Zirconium Alloy
Zirconium and Zirconium Alloy

Factors influencing the welding quality of zirconium and zirconium alloy

  • The tendency of weld cracks

Due to the low thermal expansion coefficient of zirconium and zirconium alloy, the volume change caused by thermal deformation and phase change is very small, and the content of sulfur, phosphorus, carbon and other impurities is very low, there is no obvious trend of cracks in the welding process. However, when the welding seam absorbs a certain amount of oxygen, nitrogen and hydrogen gas impurities, the performance of the welding seam and the heat-affected zone will become brittle. If there is stress in the welding seam, cold cracks will occur.

At the same time, hydrogen atoms have the property of diffusing and aggregating to the high-stress parts in the heat-affected zone with lower temperature, which leads to the formation of relatively weak links in these parts, which may lead to the generation of welding delay cracks.

  • Selection of welding materials

The filler wire for zirconium and zirconium alloy welding should be selected according to the principle of matching the base material composition. The surface of welding wire shall not have heavy skin, crack, the oxidation phenomenon and metal or non-metal inclusion defects. Besides, the welding wire should be cleaned and dried before use.

  • Selection of protective gas

Argon arc welding with tungsten electrode of zirconium and zirconium alloy shall adopt high purity argon with 99.999% purity and the impurity content shall meet the requirements of GB/T4842 current standards.

Argon Arc Welding
Argon Arc Welding

Due to the extremely high requirements on the purity of welding protective gas, continuous gas charging is required during the welding process, and the gas cannot be interrupted in the process; otherwise, argon charging needs to be replaced again. Therefore, the direct gas supply method using ordinary argon in a single bottle cannot meet the protection requirements. It is necessary to increase the gas supply capacity of multiple argon bottles in series and satisfy the simultaneous operation of multiple welders through the air separation cylinder.

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