What should be paid attention to when welding galvanized steel pipes?

Welding galvanized steel pipes requires special attention due to the unique characteristics of the zinc coating. Proper preparation and welding techniques are essential to prevent defects such as cracks, porosity, and embrittlement. Below are important factors to keep in mind when welding galvanized steel pipes.

1. Grinding the Galvanized Layer

The first step in welding galvanized steel is to remove the galvanized coating at the welding point. If this is not done, issues like bubbles, sand holes, false welding, and brittleness can occur. The zinc coating can make the weld less rigid and more prone to failure. Grinding ensures a clean surface for a strong, durable weld.

2. Welding Characteristics of Galvanized Steel

Galvanized steel consists of low-carbon steel coated with a thin layer of zinc, typically around 20 micrometers thick. Zinc has a low melting point of 419°C and a boiling point of about 908°C. During welding, the zinc melts and floats on the molten pool or at the weld root. This liquid zinc can deeply erode the weld metal along the grain boundaries, causing "liquid metal embrittlement" and reducing the plasticity of the weld. As a result, cracks may form under tensile stress, especially in fillet welds like T-joints, where penetrating cracks are more likely.

Zinc also oxidizes and evaporates during welding, producing a significant amount of white smoke and steam. The oxide formed, zinc oxide (ZnO), has a high melting point (around 1800°C), which can lead to slag inclusions if welding parameters are too low. Furthermore, the evaporation of zinc creates toxic fumes, so proper ventilation and protective measures should be in place to avoid harm to the welder.

3. Welding Process Control

The preparation for welding galvanized steel is similar to that for low-carbon steel, but special attention should be paid to the groove size and the surrounding galvanized coating. The groove angle should typically be between 60° and 65°, with a gap of about 1.5 to 2.5mm. To minimize zinc penetration into the weld, the galvanized layer in the groove should be removed before welding.

In practice, a centralized groove and a process that eliminates blunt edges can help control the welding process and ensure full penetration. A two-layer welding technique reduces the risk of incomplete penetration. It is also important to select the appropriate welding rod based on the base material of the galvanized steel pipe. J422 welding rods are commonly used for low-carbon steel due to their ease of operation.

4. Welding Technique

For multi-layer welds, the goal is to allow the zinc layer to vaporize and escape the weld. This minimizes the amount of liquid zinc left in the weld, reducing the risk of defects. When welding fillet welds, try to melt the zinc layer early in the process, allowing it to vaporize before continuing. One method is to move the end of the welding rod forward about 5 to 7mm to allow the zinc to escape, then return to the original position and continue welding.

When welding horizontally or vertically, using short-slag electrodes like J427 minimizes the risk of undercutting. Additionally, employing back-and-forth electrode transport techniques can ensure a defect-free, high-quality weld.

Conclusion

Welding galvanized steel pipes requires careful attention to the preparation and welding process due to the unique challenges posed by the zinc coating. By grinding off the galvanized layer, controlling welding parameters, and using the right techniques, you can achieve strong, high-quality welds while avoiding common issues like embrittlement, porosity, and cracks.