Welding methods for galvanized steel pipes
Precautions during the welding process
Galvanized steel is typically a low-carbon steel with a zinc coating, and the zinc layer is generally about 20 μm thick. The melting point of zinc is around 419°C, and its boiling point is approximately 908°C. During welding, zinc melts into a liquid and floats on the surface of the molten pool or accumulates at the root of the weld. Zinc has a relatively high solid solubility in iron, and the zinc liquid can penetrate along the grain boundaries and corrode the weld metal, causing "liquid metal embrittlement" due to the low melting point of zinc. Additionally, zinc and iron can form brittle intermetallic compounds, which reduce the plasticity of the weld metal and cause cracks under tensile stress. If welding fillet welds, especially those on T-joints, are most prone to through-cracks. When welding galvanized steel, the zinc layer on the surface and edges of the groove, under the effect of arc heat, oxidizes, melts, evaporates, and emits white smoke and vapor, which can easily cause porosity in the weld. The ZnO formed due to oxidation has a high melting point of about 1800°C or more. If the welding parameters are too small during the process, it will cause ZnO inclusions. At the same time, as zinc acts as a deoxidizer, it can produce low-melting-point oxide inclusions such as FeO-MnO or FeO-MnO-SiO2. 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 zinc layer at the welding site must be ground off before welding; otherwise, it will cause bubbles, sand holes, false welds, etc. It will also make the weld brittle and reduce its rigidity.
Welding Process Control
The pre-welding preparations for galvanized steel are the same as those for general low-carbon steel. It is important to carefully handle the groove dimensions and the adjacent galvanized layer. To ensure full penetration, the groove dimensions should be appropriate, typically 60-65 degrees, with a certain gap, usually 1.5-2.5mm. Before welding, the galvanized layer inside the groove can be removed to reduce the penetration of zinc into the weld.
In actual work, a process of concentrating on making bevels without leaving blunt edges was adopted for centralized control. The two-layer welding process was used, which reduced the possibility of incomplete penetration. The welding rods should be selected according to the base material of the galvanized steel pipe. Generally, for low-carbon steel, J422 is more commonly used due to its ease of operation.
Welding Techniques
When performing the first layer of multi-layer welding, try to make the zinc layer melt and vaporize, and let it overflow out of the weld seam. This can significantly reduce the amount of liquid zinc remaining in the weld seam. When welding fillet welds, the same method should be applied in the first layer: first, move the end of the welding rod forward by about 5-7mm. Then, when the zinc layer melts, return to the original position and continue welding forward. For cross welding and vertical welding, if short slag welding rods such as J427 are selected, the tendency for undercut will be very small; if the back-and-forth welding technique is adopted, even better welding quality without defects can be achieved.
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