Understanding Decarburization of Steel in Galvanized Steel Pipes: Causes and Prevention

Decarburization of steel is a critical issue that affects the quality and durability of galvanized steel pipes. This process occurs when carbon is lost from the surface layer of the steel due to exposure to oxidizing gases during heat treatment. Understanding the causes and prevention methods can help manufacturers maintain the structural integrity of galvanized steel pipes.

Five Key Causes of Decarburization in Galvanized Steel Pipes

1. Oxidizing Gases (O₂, H₂O, CO₂) React with the Steel Surface

The primary agents causing decarburization of steel are oxygen (O₂), water vapor (H₂O), and carbon dioxide (CO₂). When these gases come into contact with heated galvanized steel pipes, oxidation and decarburization occur simultaneously. Since carbon in the steel has a higher affinity for these gases, it is removed from the surface first, weakening the material.

2. Imbalance Between CO₂ and CO in the Furnace Atmosphere

To prevent decarburization of steel, the ratio of CO₂ to CO in the furnace must be carefully controlled. At the equilibrium point, neither oxidation nor decarburization occurs. However:

· If CO₂ levels exceed the equilibrium point, oxidation and decarburization take place.

· If CO levels are higher, decarburization is minimized.

The exact equilibrium value depends on the steel’s carbon content and furnace temperature.

3. Exposure to Air During Heat Treatment

Since decarburization and oxidation happen simultaneously, reducing air exposure during heat treatment can significantly decrease decarburization of steel. Sealed furnace environments or controlled atmospheres help maintain carbon levels in galvanized steel pipes.

4. Using Protective Nitrogen (N₂) Gas to Prevent Decarburization

Injecting neutral nitrogen gas into the furnace is an effective way to prevent decarburization of steel. Nitrogen displaces oxidizing gases and maintains positive pressure inside the furnace, preventing air infiltration. This method helps protect galvanized steel pipes from losing surface carbon.

5. Residual Contaminants on the Steel Surface

Oxide scales, rust, and leftover residues from cold drawing can decompose during heating, releasing oxidizing gases. By maintaining a reducing atmosphere in the furnace, manufacturers can prevent decarburization of steel and ensure the longevity of galvanized steel pipes.

Conclusion

Preventing decarburization of steel in galvanized steel pipes requires controlling furnace atmospheres, minimizing air exposure, and using protective gases like nitrogen. By addressing these factors, manufacturers can enhance the strength and corrosion resistance of their steel products.

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