The galvanized layer at the welding site must be polished, otherwise air bubbles, trachoma and false welding will occur. It will also make the weld brittle and reduce the rigidity.
Welding characteristics of galvanized steel
Galvanized steel is usually coated with zinc outside the low carbon steel, and the galvanized layer is usually 20 um thick. The melting point of zinc is 419 degree C and the boiling point is about 908 degree C. In welding, zinc melts into liquid and floats on the surface of the molten pool or at the root of the weld. Zinc has a high solid solubility in iron. The liquid zinc will deeply etch the weld metal along the grain boundary, and the low melting point zinc will form "liquid metal embrittlement". At the same time, zinc and iron can form intermetallic brittle compounds. These brittle phases reduce the plasticity of weld metal and cause cracks under tensile stress. If the fillet weld, especially the fillet weld of T-joint, is most likely to produce penetrating cracks.
When galvanized steel is welded, the zinc layer on the groove surface and the edge produces oxidation, melting, evaporation and even evaporation of white fume and vapor under the action of arc heat, which easily causes weld pore. The melting point of zinc oxide formed by oxidation is higher than 1800 C. If the welding parameters are too small, it will cause zinc oxide slag inclusion, and at the same time. Because zinc becomes a deoxidizer. FeO-MnO or FeO-MnO-SiO_2 low melting point oxide slag is produced. Secondly, due to the evaporation of zinc, a large amount of white smoke is volatilized, which has stimulating and harmful effects on human body. Therefore, it is necessary to polish the galvanized layer at the welding site.
Welding process control
The preparation of galvanized steel before welding is the same as that of common low carbon steel. It should be noted that the groove size and the galvanized layer nearby should be carefully handled. In order to penetrate, the groove size should be appropriate, generally 60-65 degrees, leaving a certain gap, generally 1.5-2.5 mm. In order to reduce the penetration of zinc into the weld, before welding, the galvanized layer in the groove can be removed and then welded.
In practical work, centralized grooving without blunt edge is adopted to centralize control, and two-layer welding process reduces the possibility of non-penetration. Welding rod should be selected according to the base material of galvanized rigid pipe. J422 is a common choice for low carbon steel due to its easy operation.
Welding manners: When welding the first layer of multi-layer weld, try to make the zinc layer melt and vaporize and evaporate out of the weld, which can greatly reduce the liquid zinc left in the weld. When fillet weld is welded, the zinc layer is melted in the first layer and vaporized and evaporated to escape from the weld. The method is to move the end of the electrode forward about 5-7 mm, and then to continue to weld when the zinc layer is melted back to its original position. If short slag electrode such as J427 is used in cross welding and vertical welding, the undercut tendency will be very small; if the front and back transport rod technology is used, the defect-free welding quality can be obtained.
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