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Welding Characteristics and Welding Technology of Galvanized Steel Pipe

Jun. 29, 2020
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Galvanized Steel Pipe is widely used in various industries. The advantage of using a galvanized steel pipe is to use metal zinc that can form a dense oxide protective layer in the air to protect the internal steel structure. In the case of welding or scratching, due to the presence of the Zn-Fe primary battery, the relatively active galvanized part can be used as a sacrificial anode to delay the corrosion of steel and have good corrosion resistance. However, due to the presence of the galvanized layer, cracks, pores, and slag are easily generated during welding, making it difficult to obtain good welding quality.

Galvanized steel is generally coated with a layer of zinc on the low carbon steel, and the galvanized layer is generally 20um thick. Zinc has a melting point of 419°C and a boiling point of around 908°C. In welding, zinc melts into a liquid and floats on the surface of the molten pool or at the root of the weld. Zinc has a greater solid solubility in iron, zinc liquid will deeply etch the weld metal along the grain boundary, and low melting point zinc forms "liquid metal embrittlement". At the same time, zinc and iron can form brittle compounds between metals, such as Fe3Zn10, FeZn10, etc. These brittle phases reduce the plasticity of the weld metal and cause cracks under tensile stress. If welding fillet welds, especially T-joint fillet welds are most likely to produce penetration cracks. When galvanized steel is welded, the zinc layer on the surface and edge of the groove will be oxidized, melted, evaporated, and even white smoke and steam will be volatilized under the action of arc heat, which is likely to cause weld porosity. The melting point of ZnO formed by oxidation is high, above 1800°C. If the parameter is too small during the welding process, ZnO will contain slag. At the same time, because Zn becomes a deoxidizer, FeO-MnO or FeO-MnO-SiO2 will be low Melting point oxide slag. If the welding specification is not suitable and the operation is improper, it is easy to melt the galvanized layer at the edge of the weld and even expand the melting area, which may damage the galvanized layer, especially in the case of elongated arc and large swing operation. The wide melting area damages the galvanized layer more seriously. At the same time, due to the evaporation of zinc, a large amount of white smoke is volatilized, which has a stimulating and harmful effect on the human body. Therefore, the selection of welding methods and materials that produce a low amount of smoke is also a factor that must be considered.

Hot Dip Galvanized Steel Pipe

Hot Dip Galvanized Steel Pipe

Galvanized Steel Water Pipe has many welding methods. Generally, gas welding, manual arc welding, CO2 gas shielded welding, submerged arc automatic welding, and tungsten arc welding are commonly used in galvanized steel structures.

Gas welding was used to weld galvanized pipes in the past. Because gas welding heat input is not concentrated, it is easy to produce defects and poor mechanical properties of the weld seam. It is currently basically eliminated in the installation industry. Gas welding causes greater damage to the galvanized layer.

CO2 gas shielded welding has good welding performance on galvanized steel. When suitable welding specifications and matching shielding gas and welding materials are used, high-quality welding joints can be obtained. This method is rarely used in engineering practice.

Tungsten arc welding has concentrated arc energy, less damage to the galvanized layer, and it is easier to form a good single-sided welded double-sided formed joint. It is a welding method worth using, but the welding speed, slower, and most expensive.

Manual arc welding is currently the most common welding method used in pipeline installation. When the electrode is selected correctly, such as J421, J422, J423, and other titanium oxide and titanium calcium electrode welding, because these electrodes contain a large amount of rutile and ilmenite, the melting rate of the electrode is relatively large, relatively Increased melting speed. If it can only destroy the galvanized layer such as the front edge of the molten pool without swinging, it will generally not expand the melting area, which can reduce the penetration of zinc liquid into the weld metal; in the case of using the correct operating method and welding material, Welding quality with good mechanical properties and no defects is obtained. Because manual arc welding is cheaper and faster than tungsten argon arc welding, a manual arc welding process is used when a skilled welder is available.

The preparation of galvanized steel before welding is the same as that of general low-carbon steel. It should be noted that the groove size and the nearby galvanized layer should be carefully handled. In order to weld through, the groove size should be appropriate, generally 60~65°, with a certain gap, generally 1.5~2.5mm; in order to reduce the penetration of zinc into the weld, before welding, the galvanizing in the groove can be Solder again after the layer is cleared. In actual work, centralized beveling is adopted, and no blunt edge process is used for centralized control. The two-layer welding process reduces the possibility of under-welding.

The electrode should be selected according to the base material of the galvanized pipe. Generally, low-carbon steel is generally used because of its ease of operation. Welding method: when welding the first layer of multi-layer welding, try to melt the zinc layer and vaporize and evaporate it to escape the weld, which can greatly reduce the liquid zinc remaining in the weld. In the case of fillet welds, the zinc layer is also melted as much as possible in the first layer and vaporized and evaporated to escape the weld. The method is to move the end of the electrode forward about 5~7mm. When the zinc layer is made After melting, it returns to its original position and continues welding. After the welding is completed, the welding seam should be cleaned in time, and the zinc-rich primer should be brushed to take anti-corrosion measures.

Welding quality assurance measures are controlled from five aspects: human, material, machine, method, and ring.

① The human factor is the focus of welding control. Therefore, before welding, a skilled welder with a welder certificate should be selected for necessary technical training and explanation. It shall not be replaced at will to ensure that the welder who welds the pipeline is relatively stable.

② Control of welding consumables: We guarantee that the purchase of welding consumables is from regular channels, with warranty and certificate of conformity, and meets the process requirements; strict control of electrode head recovery to ensure the flow direction and dosage; welding consumables must be strictly baked according to the process, and once Distribute no more than half a day.

③ Welding machine: The welding machine must ensure reliable performance and meet the process requirements; the welding machine must have a qualified current and voltmeter to ensure the correct implementation of the welding process. The welding cable should not be too long. If the welding cable is too long, the welding parameters must be adjusted.

④ Welding process method: to ensure the strict implementation of the special operation method of galvanized pipe, the welding process shall be checked before welding, the welding process parameters and operation techniques shall be controlled, the appearance quality inspection after welding shall be added, and the non-destructive testing after welding shall be added if necessary. Control the level of welding and the number of welding consumables per pass.

⑤ Welding environment control: ensure that the temperature, humidity, and wind speed during welding meet the process requirements.

In conclusion

Hot dip galvanized steel pipe adopts the correct welding process in the construction, strictly inspects and accepts according to the specification, and after welding is completed, the anti-corrosion treatment of the weld seam (zinc-rich paint) should be done in time. The closed system has certain feasibility during the implementation process, which can increase the construction speed and improve the firmness of the pipeline connection. Therefore, as long as the construction conditions permit, and on the premise of taking relevant protective and anti-corrosion measures, hot-dip galvanized pipes can be welded.