Oxidation Control and Descaling Techniques in the Heat Treatment of Galvanized Round Pipe

In the production of galvanized round pipe, oxidation during heat treatment is one of the most common and challenging issues affecting product quality, surface finish, and overall yield. Understanding how oxidation occurs, its influencing factors, and effective control methods is critical for improving manufacturing efficiency and extending furnace life.

1. Understanding Oxidation in Galvanized Round Pipe Heating

When steel pipe billets are heated in a furnace, they are exposed to oxidizing gases such as CO₂, H₂O, and O₂. These gases react with the steel surface, forming iron oxide scale. On average, each heating cycle can cause a burn loss of about 0.5% to 3%, reducing the final yield of galvanized round pipe.

This oxide layer, also known as mill scale, typically consists of three distinct layers:

· Fe₂O₃ (Hematite) – the outermost layer, making up about 10%

· Fe₃O₄ (Magnetite) – the middle layer, around 50%

· FeO (Wüstite) – the inner layer, about 40%

The melting point of these combined oxides ranges between 1300°C and 1350°C. Since the thermal conductivity of oxide scale is much lower than that of metal, excessive oxidation can reduce heating efficiency and cause uneven temperature distribution in the steel billet.

2. Key Factors Affecting Oxidation

Several parameters directly influence the oxidation rate of steel during heating. Among them, furnace atmosphere, heating temperature, and time at high temperature are the most significant. Their relative impact can be approximated as 6:3:1, respectively.

1. Heating Temperature:

Oxidation accelerates sharply once the billet temperature exceeds 800°C. Below this temperature, oxidation remains relatively moderate.

2. High-Temperature Retention Time:

The longer the billet stays in the high-temperature zone, the more severe the oxidation loss becomes. Controlled heating schedules are therefore essential to minimize scale formation.

3. Furnace Atmosphere Composition:

A strongly oxidizing furnace atmosphere increases both the thickness of the oxide layer and the overall metal loss. Adjusting gas ratios to create a mildly reducing atmosphere can help suppress oxidation.

3. High-Pressure Water Descaling in Heat Treatment

During the hot rolling and heat treatment processes of galvanized round pipe, high-pressure water descaling plays a vital role in removing oxide scale and ensuring superior surface quality. The descaling process involves directing high-pressure water jets at the hot pipe surface, effectively stripping off the oxide layers formed during heating.

Advantages of High-Pressure Descaling

1. Improved Surface Quality:

Similar to hot rolling operations, descaling significantly enhances the smoothness and appearance of the pipe surface, providing a better base for galvanizing.

2. Enhanced Quenching Efficiency:

After scale removal, the pipe surface achieves more uniform cooling during quenching. This reduces deformation, enhances heat exchange, and ensures consistent mechanical properties.

3. Reduced Equipment Wear:

Oxide scales increase friction and wear on straightening rollers. Descaling minimizes contact abrasion, prolonging the service life of mechanical components.

4. Better Non-Destructive Testing (NDT) Results:

Oxide residues can interfere with ultrasonic or eddy current inspection, sometimes making flaw detection impossible. Clean surfaces from descaling ensure accurate and reliable testing outcomes.

4. Optimizing Furnace Conditions for Galvanized Round Pipe

To control oxidation and achieve high-quality galvanized round pipe production, the following strategies are recommended:

· Maintain Proper Furnace Atmosphere: Adjust oxygen potential using CO/CO₂ balance control.

· Use Stepwise Heating: Gradually increase temperature to prevent rapid oxidation in the early heating phase.

· Regular Furnace Maintenance: Remove accumulated scale from the furnace floor to prevent refractory corrosion.

· Integrate Automated Descaling Systems: Ensure consistent removal of scale before galvanizing or quenching stages.

Conclusion

Effective oxidation control and high-pressure water descaling are key to improving the surface quality, dimensional precision, and service life of galvanized round pipe. By optimizing furnace temperature, time, and atmosphere, manufacturers can significantly reduce oxidation losses and enhance overall production efficiency.

In modern steel pipe production lines, combining advanced heat treatment control with automated descaling systems ensures that galvanized round pipes meet the highest standards of performance, corrosion resistance, and visual quality demanded in today’s global market.