Galvanized steel pipes have the dual advantages of corrosion resistance and long service life, and their price is relatively low, so their usage is increasing. However, some users are careless when welding galvanized steel pipes, leading to unnecessary problems. So, what issues should be paid attention to when welding galvanized steel pipes?

1. Grinding is a prerequisite for welding galvanized steel pipes. 

The galvanized layer of the steel pipe at the weld joint must be ground off; otherwise, bubbles, pinholes, and false welds will occur. It will also make the weld brittle and reduce its rigidity.

2. Characteristics of galvanized steel pipes. 

Galvanized steel is generally low-carbon steel coated with a layer of zinc, typically 20µm thick. Zinc has a melting point of 419°C and a boiling point of 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 high solid solubility in iron, and the liquid zinc penetrates deep along the grain boundaries, corroding the weld metal. Low-melting-point zinc causes "liquid metal embrittlement." Meanwhile, zinc and iron can form brittle intermetallic compounds. These brittle phases reduce the plasticity of the weld metal, leading to cracking under tensile stress. Fillet welds on galvanized steel pipes, especially those at the corners of T-joints, are most prone to penetrating cracks. During welding of galvanized steel pipes, the zinc layer on the bevel surface and edges oxidizes, melts, evaporates, and even volatilizes white fumes and vapors under the heat of the electric arc, easily causing weld porosity. The ZnO formed by oxidation has a high melting point, approximately above 1800°C. If welding parameters are too low, ZnO inclusions will occur. Simultaneously, Zn acts as a deoxidizer, producing low-melting-point oxide inclusions such as FeO-MnO or FeO-MnO-SiO2. Furthermore, the evaporation of zinc releases a large amount of white fumes, which are irritating and harmful to humans; therefore, the galvanized layer at the weld joint must be ground off.

3. Welding Process Control of Galvanized Steel Pipes
The pre-welding preparation for galvanized steel pipes is the same as that for ordinary low-carbon steel. It is important to carefully handle the bevel size of the galvanized steel pipe and the surrounding galvanized layer. For complete penetration, the bevel size of the galvanized steel pipe should be appropriate, generally 60~65°, with a certain gap, generally 1.5~2.5mm. To reduce zinc penetration into the weld, the galvanized layer inside the bevel of the galvanized steel pipe can be removed before welding. In practice, a centralized beveling process is adopted for galvanized steel pipes, eliminating blunt edges, and a two-layer welding process to reduce 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 commonly used for low-carbon steel due to ease of operation.

4. Welding Techniques for Galvanized Steel Pipes
When welding the first layer of a multi-layer weld, try to melt the zinc layer and allow it to vaporize and evaporate out of the weld, which can greatly reduce the amount of liquid zinc remaining in the weld. When welding fillet welds, the first layer of zinc should be melted, vaporized, and evaporated from the galvanized steel pipe. This is done by first moving the electrode tip forward about 5-7 mm, then returning it to its original position on the galvanized steel pipe after the zinc has melted, and continuing to weld forward. For horizontal and vertical welding, using short-slag electrodes such as J427 will minimize undercut, and by employing a back-and-forth electrode manipulation technique, defect-free weld quality can be achieved.