What issues should be considered when welding galvanized steel pipes in industrial projects

First, grinding is essential. 

The galvanized layer 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.

Second, the welding characteristics of galvanized steel pipes. 

Galvanized steel pipes are generally low-carbon steel coated with a layer of zinc, typically about 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 into the weld metal along the grain boundaries, causing "liquid metal embrittlement" due to its low melting point. Simultaneously, zinc and iron can form brittle intermetallic compounds. These brittle phases reduce the plasticity of the weld metal, leading to cracks under tensile stress. Fillet welds, especially T-joint fillet welds, are most prone to penetrating cracks. During the welding of galvanized steel, 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 the 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. Secondly, the evaporation of zinc releases a large amount of white fumes, which are irritating and harmful to the human body; therefore, the galvanized layer at the weld joint must be ground off.

Thirdly, process control for welding galvanized steel pipes.

The pre-welding preparation for galvanized steel pipes is the same as for ordinary low-carbon steel. It is important to carefully handle the bevel dimensions and the surrounding galvanized layer. To ensure complete penetration, the bevel size must be appropriate, generally 60-65°, with a certain gap, typically 1.5-2.5mm. To reduce zinc penetration into the weld, the galvanized layer within the bevel should be removed before welding.

In practice, a centralized beveling process without leaving blunt edges is used for centralized control, employing a two-layer welding process to reduce the possibility of incomplete penetration. The welding rod should be selected based on the base material of the galvanized steel pipe; generally, J422 is commonly used for low-carbon steel due to ease of operation.

Welding technique: When welding multi-layer welds, strive to melt the zinc layer and allow it to vaporize and escape from the weld. This significantly reduces the amount of liquid zinc remaining in the weld. Similarly, when welding fillet welds, strive to melt and vaporize the zinc layer in each layer. This is done by first moving the electrode tip forward about 5-7mm, allowing the zinc layer to melt, then returning it to the original position and continuing welding. When performing horizontal and vertical welding, if short slag electrodes such as J427 are used, the tendency for undercut will be very small; if a back-and-forth electrode manipulation technique is used, defect-free welding quality can be obtained.