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Welding characteristics of galvanized steel pipe

Views: 1     Author: Site Editor     Publish Time: 2020-08-10      Origin: Site

    Galvanized steel is widely used in all walks of life. The advantage of using galvanized steel is to use metallic 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 existence of the Zn-Fe galvanic cell, the relatively active galvanized part can be used as a sacrificial anode, delaying the corrosion of steel, and has good corrosion resistance. However, due to the presence of the galvanized layer, cracks, pores, and slag inclusions are prone to occur during welding, and it is difficult to obtain good welding quality.

    Galvanized steel is generally coated with a layer of zinc on the outside of low-carbon steel, and the galvanized layer is generally 20um thick. The melting point of zinc is 419°C and the boiling point is around 908°C. During welding, zinc melts into liquid floating on the surface of the molten pool or at the root of the weld. Zinc has a large solid solubility in iron. The liquid zinc will etch the weld metal deeply along the grain boundary, and the low melting point zinc will form "liquid metal embrittlement". At the same time, zinc and iron can form brittle compounds between metals, such as Fe3Zn10, FeZn10 and so on. These brittle phases reduce the plasticity of the weld metal and cause cracks under tensile stress. If the fillet welds are welded, especially the fillet welds of the T-shaped joints, penetration cracks are most likely to occur. When welding galvanized steel, the zinc layer on the groove surface and the edge will oxidize, melt, evaporate and even volatilize white smoke and steam under the action of the arc heat, which can easily cause weld pores. The ZnO formed by oxidation has a high melting point, above about 1800°C. If the parameters are too small during the welding process, it will cause ZnO slag inclusion. At the same time, Zn becomes a deoxidizer to produce FeO-MnO or FeO-MnO-SiO2. Melting point oxide slag. If the welding specification is inappropriate and the operation method is improper, it is easy to melt the galvanized layer at the edge of the weld and expand the melting area, which may damage the galvanized layer, especially in the case of elongated arc and large swing operations. 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 is irritating and harmful to the human body. Therefore, the selection of welding methods and materials that produce a lower amount of smoke is also a factor that must be considered.

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