1. The premise must be polished
The galvanized layer at the weld must be polished off, otherwise, bubbles, trachoma, false welding, etc. will occur. It will also make the weld brittle and reduce rigidity.
2. The welding characteristics of galvanized steel
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. Zinc has a melting point of 419°C and a boiling point of around 908°C. During welding, zinc melts into a liquid that floats on the surface of the molten pool or at the root of the weld. Zinc has a large solid solubility in iron, and zinc liquid will erode the weld metal deeply along the grain boundary, and zinc with a low melting point will form "liquid metal embrittlement". At the same time, zinc and iron can form intermetallic brittle compounds, and these brittle phases reduce the plasticity of the weld metal and cause cracks under the action of tensile stress.
If the fillet welds are welded, especially the fillet welds of T-joints, penetration cracks are most likely to occur. When galvanized steel is welded, the zinc layer on the groove surface and the edge will be oxidized, melted, and evaporated and white smoke and steam will be volatilized under the action of arc heat, which will easily cause weld pores. The ZnO formed due to 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, and at the same time. Since Zn becomes a deoxidizer. Produce FeO-MnO or FeO-MnO-SiO2 low melting point oxide slag. Secondly, 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 galvanized layer at the welding point must be polished and disposed of.
3. Welding process control
The pre-welding preparation of galvanized steel is the same as that of ordinary low-carbon steel. It should be noted that the groove size and the nearby galvanized layer should be carefully handled. For penetration, the groove size should be appropriate, generally, 60~65° and a certain gap should be left, generally 1.5~2.5mm; to reduce the penetration of zinc into the weld, the galvanized groove in the groove can Solder after the layer is removed.
In actual work, centralized beveling, no blunt edge process is adopted for centralized control, and the two-layer welding process reduces the possibility of incomplete penetration. The welding rod should be selected according to the base material of the galvanized steel pipe. For general low-carbon steel, it is more common to choose J422 due to the consideration of ease of operation.
Welding method: When welding multi-layer welded seams, try to melt the zinc layer and make it vaporize, evaporate and escape the weld seam, which can greatly reduce the liquid zinc remaining in the weld seam. When welding fillet welds, also try to melt the zinc layer in the layer and make it vaporize and evaporate to escape the weld. The method is to move the end of the electrode forward about 5~7mm. Get back to the original position and continue to weld forward. For horizontal welding and vertical welding, if short slag electrodes such as J427 are used, the tendency of undercutting will be small; if the back-and-forth forward and backward transport technology is used, defect-free welding quality can be obtained.