Galvanized steel pipe welding technology

Galvanized steel pipes are made from low-carbon steel coated with a layer of zinc, typically around 20 µm thick. The zinc layer serves as corrosion protection but introduces unique challenges during welding due to its melting point (419°C) and boiling point (approximately 908°C). Here's a detailed overview of the technology and methods for welding galvanized steel pipes.

Challenges in Welding Galvanized Steel Pipes

- Zinc Layer Interference

During welding, the zinc coating melts and forms a liquid layer that can float on the weld surface or seep into the weld root.

Zinc has high solubility in iron, leading to liquid metal embrittlement as it infiltrates grain boundaries.

Low-melting zinc may form brittle intermetallic compounds with iron, reducing the weld's plasticity and increasing susceptibility to cracking, especially in fillet welds like T-joints.

- Porosity and Slag Formation

The heat of welding oxidizes the zinc layer, forming ZnO, which has a high melting point (over 1800°C). Insufficient welding parameters can result in ZnO slag inclusions.

Zinc acts as a deoxidizer during welding, leading to the formation of low-melting-point oxides (e.g., FeO-MnO or FeO-MnO-SiO₂), which can compromise weld quality.

- Health and Safety Risks

Zinc vaporizes during welding, releasing white smoke and fumes that are harmful if inhaled.

Proper ventilation and protective equipment are necessary to mitigate these risks.

Welding Process Control

Preparation

- Surface Treatment:
Before welding, remove the zinc coating from the groove area to reduce contamination and porosity. Grinding the zinc layer near the weld site is essential.

- Groove Design:

Ensure an appropriate groove angle of 60–65°.

Leave a root gap of 1.5–2.5 mm for proper penetration.

Centralized groove preparation and controlled blunt edges help minimize incomplete fusion or penetration.

Selection of Welding Materials

Choose the welding rod based on the base material of the pipe. For low-carbon steel substrates, J422 electrodes are commonly used due to their ease of operation.

Welding Techniques

- Initial Pass:

For multi-layer welding, the first pass should aim to melt and vaporize the zinc coating, allowing it to escape as fumes.

For fillet welds, remove the electrode from the start point by 5–7 mm to let the zinc layer vaporize before proceeding with welding.

- Horizontal and Vertical Welding:

Use techniques like short-arc welding (e.g., J427) to minimize slag and undercutting.

For improved quality, employ back-and-forth oscillation or strip techniques to achieve defect-free welds.

- Ventilation and Safety:

Ensure adequate ventilation or fume extraction during welding to protect workers from harmful zinc fumes.

Post-Weld Treatment

Conduct thorough inspections for porosity, slag inclusions, or cracks.

Apply proper post-weld treatments to restore corrosion resistance, such as galvanization repair using zinc-rich paints or sprays.

Conclusion

Welding galvanized steel pipes requires careful preparation, proper technique, and safety measures to address challenges posed by the zinc coating. By removing the zinc layer from critical areas, optimizing welding parameters, and employing the right electrodes, high-quality welds can be achieved while maintaining safety and structural integrity.