Galvanized steel pipe has the dual advantages of corrosion resistance and long service life, and relatively low costs, so now its usage rate is getting higher and higher, but some users do not pay attention when welding galvanized pipe, and caused some unnecessary problems, so today we are going to talk about the welding of galvanized pipes.
1. The Premise is to Polish
The galvanized layer at the welding zone must be polished off, otherwise air bubbles, trachoma, false welding, etc will be found. It will also make the weld bead brittle and lead to lower rigidity.
2. Welding Characteristics of Galvanized Steel
1) Galvanized steel is generally coated with a layer of zinc on surface 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 liquid and floats on the surface of the molten pool or at the root of the weld. Zinc has a large solid solubility in iron, zinc liquid will erode the weld metal deeply along the grain boundary, and zinc with a low melting point will cause "liquid metal embrittlement". At the same time, zinc and iron can form intermetallic brittle compounds. These brittle structures 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.
2) When galvanized steel is welded, the zinc layer on the groove surface and the edge will be oxidized, melted, evaporated and white smoke and steam will be volatilized under the action of arc heat, which will easily cause weld pores. ZnO formed due to oxidation has a high melting point, above about 1800°C. If the current is too small during the welding process, it will cause ZnO slag inclusion, and at the same time, Zn will become 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
1) 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 welding zone galvanized layer should be carefully handled. For enough penetration, the groove size should be appropriate, generally 60~65°, with a certain gap of generally 1.5~2.5mm; in order to reduce the penetration of zinc into the weld, the galvanized layer in the groove should be removed.
2) In actual work, beveling is adopted, no blunt edge process is 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, considering the ease of operation, E6013 & ER70S-6 are good choices for these welding jobs.
3) Welding method: When welding the first layer of weld seam in multi-layer welding, try to melt the zinc layer and make it vaporize, evaporate and escape from the weld seam, so as to greatly reduce the liquid zinc remaining in the weld seam. When welding the fillet weld, also try to melt the zinc layer on the first 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, when the zinc layer melt, return to the original position and continue to weld forward. For horizontal welding and vertical welding, if short slag electrodes such as E7016 are used, the undercut tendency will be very small; if the forward and backward transport technology is used, defect-free welding quality can be obtained.