Galvanized steel pipe, which has the advantages of two layers of corrosion resistance and long service life, and the price is relatively low, so its utilization rate is getting higher and higher, but some users do not Attention has caused some unnecessary trouble, so what problems should galvanized pipes pay attention to when welding? Follow Tianjin Galvanized Steel Pipe Factory to take a look.
The premise is to polish
It is necessary to polish off the galvanized layer at the welding place, otherwise air bubbles, trachoma and false welding will occur. It also makes the weld seams brittle and reduces rigidity.
Welding characteristics of galvanized steel
Galvanized steel is generally plated 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 about 908 ° C. In welding, zinc melts into a 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 deepen the erosion of weld metal along the grain boundaries. Low melting point zinc constitutes "liquid metal embrittlement". Together, zinc and iron can form intermetallic brittle compounds. These brittle phases reduce the plasticity of the weld metal, and cracks occur under the effect of tensile stress. It is assumed that the fillet weld, especially the fillet weld of the T-joint, has the simplest penetration crack. During the welding of galvanized steel, the zinc layer on the bevel surface and the edges will be oxidized, melted, and transpired under the effect of arc heat, so that white smoke and steam will be announced, which will easily cause weld seams. ZnO formed by oxidation has a high melting point, above about 1800 ° C. If the parameter is too small during the welding process, it will cause ZnO slag inclusion, together. Because Zn becomes a deoxidizer. FeO-MnO or FeO-MnO-SiO2 low melting point oxide slagging occurs. Secondly, because of the transpiration of zinc, a lot of white smoke is announced, which has an impact on the human body and damages the body. Therefore, it is necessary to polish and remove the galvanized layer at the weld.
Welding process control
The pre-weld preparation of galvanized steel is the same as that of ordinary low carbon steel. What needs attention is to carefully handle the groove specifications and the nearby galvanized layer. In order to weld through, the groove specifications must be appropriate, generally 60 ~ 65 °, and a certain gap must be left, generally 1.5 ~ 2.5mm; in order to reduce the penetration of zinc into the weld, the galvanization in the groove can be made before welding. The layers are removed and soldered later.
In practical work, the convergence beveling is selected, and the blunt edge process is not used for the convergence control. The two-layer welding process reduces the possibility of incomplete penetration. The electrode should be selected according to the base material of the galvanized steel pipe. Generally, for low carbon steel, J422 is more common because of its ease of operation.
Welding method: When welding the first layer of a multi-layer weld, try to melt the zinc layer and vaporize and evaporate to escape the weld. When welding fillet welds, also try to melt the zinc layer in the first layer and vaporize and evaporate to escape the weld. The method is to move the end of the electrode forward about 5 ~ 7mm. After melting, return to the original position and continue welding forward. When horizontal welding and vertical welding are used, if short slag electrode such as J427 is used, the undercut tendency will be very small. Assuming that the back and forth movement technology is used, the welding quality can be obtained without defects.