What issues should we pay attention to when welding galvanized steel pipes

1. The premise must be polished
The galvanized layer at the welding joint must be polished off, otherwise, bubbles, trachoma, false welding, etc. will occur. It will also make the weld brittle and reduce its rigidity.

2. Welding characteristics of galvanized steel
Galvanized steel is generally coated with a layer of zinc on the outside of low-carbon steel, and the zinc layer is generally 20um thick. The melting point of zinc is 419°C and the boiling point is around 908°C. During 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. The zinc liquid will etch the weld metal deeply along the grain boundaries, 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 fillet welds are welded, especially those of T-shaped joints, they will most likely produce penetration cracks. When welding galvanized steel, the zinc layer on the groove surface and edges will oxidize, melt, evaporate, and even volatilize white smoke and steam under the action of arc heat, which can easily cause weld porosity. ZnO formed due to oxidation has a high melting point, about 1800°C or above. If the parameters are too small during the welding process, ZnO slag inclusion will be caused. Because Zn becomes a deoxidizer. FeO-MnO or FeO-MnO-SiO2 low melting point oxide slag inclusions are produced. Secondly, due to the evaporation of zinc, a large amount of white smoke is emitted, which is irritating and harmful to the human body. Therefore, the galvanized layer at the welding joint must be polished away.

3. Welding process control
The preparation before welding of galvanized steel is the same as that of ordinary low-carbon steel. It should be noted that the groove size and nearby galvanized layer must be carefully processed. To achieve welding penetration, the groove size must be appropriate, generally 60~65°, and a certain gap must be left, generally 1.5~2.5mm; to reduce the penetration of zinc into the weld, the galvanized surface in the groove can be plated before welding. Weld after the layer is removed. In actual work, centralized beveling, centralized control without leaving blunt edges, and a two-layer welding process were used to reduce the possibility of incomplete welding. The welding rod should be selected according to the base material of the galvanized steel pipe. Generally, J422 is more commonly used for low-carbon steel due to ease of operation.
Welding techniques: When welding multi-layer welds, try to melt the zinc layer, vaporize, and evaporate it to escape the weld, which can greatly reduce the liquid zinc remaining in the weld. When welding fillet welds, also try to melt the zinc layer in the first layer and make it vaporize and evaporate to escape the weld. The method is to first move the end of the welding rod forward about 5~7mm. When the zinc layer is melted, Return to the original position and continue welding forward. In horizontal and vertical welding, if short slag electrodes such as J427 are used, the undercut tendency will be very small; if the back-and-forth rod transport technology is used, defect-free welding quality can be obtained.