Galvanized steel pipe, with its dual advantages of corrosion resistance and long service life, coupled with relatively low prices, has seen an increasing rate of utilization. However, neglecting certain aspects during welding can lead to unnecessary complications. What are the key considerations when welding galvanized steel pipes?
Grinding is Essential
The galvanized layer at the welding point must be ground off, as its presence can cause bubbles, blowholes, and false welds. Moreover, it can make the weld joint brittle and reduce its rigidity.
Characteristics of Welding Galvanized Steel
Galvanized steel typically comprises a layer of zinc coating (around 20um thick) over low-carbon steel. Zinc has a melting point of 419°C and a boiling point of approximately 908°C. During welding, zinc melts into a liquid and floats on the surface of the weld pool or gathers at the root of the weld. Zinc has a high solid solubility in iron, allowing the liquid zinc to infiltrate the weld metal along grain boundaries, leading to "liquid metal embrittlement" due to the low melting point of zinc.
Additionally, zinc and iron can form intermetallic brittle compounds, which reduce the plasticity of the weld metal and may cause cracks under tensile stress. Angular welds, especially those in T-joints, are prone to penetration cracks. The zinc layer on the groove surface and edges, under the heat of the arc, undergoes oxidation, melting, evaporation, releasing white fumes and steam, which can contribute to porosity in the weld.
ZnO formed by oxidation has a high melting point (above 1800°C). If welding parameters are set too low, ZnO inclusions may occur. Also, zinc acts as a deoxidizer, leading to the formation of low-melting oxide inclusions such as FeO-MnO or FeO-MnO-SiO2. Lastly, the evaporation of zinc produces copious amounts of white fumes, which can irritate and harm the human body, underscoring the importance of grinding off the galvanized layer at the welding point.
Welding Process Control
Pre-welding preparation for galvanized steel is similar to that for low-carbon steel, with special attention paid to properly preparing the groove dimensions and removing the galvanized layer nearby. For complete penetration, the groove angle should be appropriate, generally 60-65°, with a gap of 1.5-2.5mm. To reduce zinc penetration into the weld, the galvanized layer within the groove should be removed before welding.
Welding technique: When welding the first layer of a multi-layer weld, efforts should be made to melt and subsequently vaporize the zinc layer, allowing it to escape from the weld joint, significantly reducing the retention of liquid zinc. Similarly, when welding fillet welds, the zinc layer should be melted and vaporized during the first layer by initially moving the electrode tip forward by about 5-7mm, returning to the original position once the zinc layer has melted, and continuing with the welding. For horizontal and vertical welding positions, using short-slag electrodes like J427 minimizes undercutting tendencies. Employing a weaving motion during welding can further ensure flawless welding quality.
