1. The premise must be polished
The galvanized layer at the weld must be polished off, otherwise, air 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 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 weld pool or at the root of the weld. Zinc has a large solid solubility in iron, and the zinc liquid will deeply erode 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 the weld metal and cause cracks under the action of tensile stress. If the fillet weld is welded, especially the fillet weld of the T-joint is most prone to penetration cracks.
When galvanized steel is welded, 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 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, and at the same time. Because Zn becomes a deoxidizer. Produce FeO-MnO or FeO-MnO-SiO2 low melting point oxide slag inclusions. Secondly, due to the evaporation of zinc, a large amount of white smoke and dust are volatilized, which can stimulate and harm the human body. Therefore, the galvanized layer at the weld must be polished off.
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. To weld 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, before welding, the galvanized in the groove can be Solder after the layer is removed.
In actual work, centralized beveling and no blunt edge processes are used 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. Generally, J422 is more common for low-carbon steel due to its ease of operation.
Welding method: When welding the layer weld of multi-layer welding, try to melt the zinc layer and make it vaporize and evaporate to escape the weld, which can greatly reduce the liquid zinc remaining in the weld.
When welding the fillet weld, the zinc layer should be melted as much as possible to make it vaporize and evaporate to escape the weld. The method is to move the end of the electrode forward about 5~7mm. When the zinc layer is melted Return to the original position and continue to weld forward.
In horizontal welding and vertical welding, if a short slag electrode such as J427 is used, the undercut tendency will be very small; if the back and forth transportation technology is used, defect-free welding quality can be obtained.
