What are the welding methods for galvanized steel pipes

Galvanized steel pipes offer the dual advantages of corrosion resistance and long service life, and are relatively inexpensive, leading to their increasing popularity. However, some users neglect certain precautions when welding galvanized steel pipes, causing unnecessary problems. So, what issues should be considered when welding galvanized steel pipes?

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

2. Characteristics of galvanized steel pipes. Galvanized steel pipes are generally made by coating low-carbon steel 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, especially T-joint fillet welds, are most prone to penetrating cracks when welding galvanized steel pipes. During welding, 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 porosity in the weld. The ZnO formed by oxidation has a high melting point, approximately 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 without leaving blunt edges is adopted for centralized control, using 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 zinc layer should be melted and vaporized in the first layer, allowing it to escape from the weld. This is done by moving the electrode tip forward about 5-7 mm, allowing the zinc layer to melt, and then returning it to the original position to continue welding. For horizontal and vertical welding, using short-slag electrodes such as J427 will minimize undercut; employing a back-and-forth electrode manipulation technique will further improve weld quality, resulting in a defect-free weld.