Galvanized steel pipes are generally coated with a layer of zinc outside the low carbon steel, and the galvanized layer is generally 20 um thick. Zinc has a melting point of 419 ° C and a boiling point of about 908 ° C. In welding, zinc melts into a liquid that floats on the surface of the bath or at the root of the weld. Zinc has a large 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 intermetallic brittle compounds. These brittle phases reduce the plasticity of the weld metal and cause cracks under tensile stress. If weld fillet welds, especially fillet welds of T-joints, are most susceptible to penetration cracks. When the galvanized steel is welded, the zinc layer on the surface and the edge of the groove is oxidized, melted and evaporated under the action of arc heat to volatilize white soot and steam, which easily causes weld pores.
ZnO formed by oxidation has a high melting point of about 1800 ° C or higher. If the parameters are too small during the soldering process, ZnO slag will be caused. Since Zn becomes a deoxidizer. The FeO-MnO or FeO-MnO-SiO2 low melting point oxide slag is produced. Secondly, due to the evaporation of zinc, a large amount of white soot is volatilized, which has irritating and harmful effects on the human body. Therefore, the galvanized layer at the weld must be polished and disposed of.
