Key points of heat treatment of galvanized steel pipes

Galvanized steel pipe is a type of welded steel pipe that undergoes surface treatment through either hot-dip or electro-galvanizing processes, using Q235 steel as its base material. The primary advantage of this pipe lies in the protective zinc coating, which forms a dense barrier against oxidation and corrosion, significantly enhancing its durability while maintaining a cost-effective profile. Due to its excellent corrosion resistance and affordability, galvanized steel pipes are widely used in building water supply and drainage systems, municipal pipeline engineering, power cable protection, and scaffolding construction. They serve as a practical and long-lasting solution for transporting water, gas, and oil, combining both strength and versatility.

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Ten Key Considerations for Heat Treatment of Galvanized Steel Pipes

- Pre-Treatment Cleaning
Before heat treatment, ensure that galvanized steel pipes and fixtures are completely free of impurities such as oil, residual salts, and paint to prevent contamination during processing.

- Vacuum Furnace Preparation
Fixtures used for the first time in a vacuum furnace should undergo degassing and purification at a vacuum level that meets or exceeds the workpiece requirements to avoid contamination.

- Heating Support for Deformation-Prone Workpieces
Workpieces prone to deformation should be heated using specialized fixtures. The recommended preheating process includes:

- First preheating at 800°C

Second preheating at 500–550°C and 850°C

The heating rate of the first preheating stage should be carefully controlled to avoid thermal stress.

- Preheating for Complex Workpieces
Preheating is essential for workpieces with complex shapes, sudden cross-sectional changes, or large effective thicknesses to minimize the risk of thermal stress and cracking.

- Suitability of Salt Bath Furnaces
Workpieces such as those with grooves and no through holes, castings, welded components, and machined stainless steel parts are generally not suitable for heating in salt bath furnaces due to the risk of contamination and material degradation.

- Adequate Heat Soaking Time
The heating process must allow for sufficient soaking time based on the effective thickness and conditional thickness of the workpiece (calculated as the actual thickness multiplied by the workpiece shape coefficient).

- Post-Quenching Treatments

Martensitic stainless steel and heat-resistant steel galvanized pipes should undergo cleaning, cryogenic treatment, or tempering only after being quenched and cooled to room temperature.

Heat treatment after welding should be conducted within 4 hours to maintain material integrity.

- Surface Cleaning Methods
Depending on the surface condition of the galvanized steel pipe, various cleaning methods can be employed, such as alkali washing, water-soluble detergents, chlorine solvents, sandblasting, or shot blasting to remove contaminants before and after heat treatment.

- Stress Relief Annealing After Correction

If dimensional correction is performed, stress relief annealing should be conducted at a temperature lower than the original tempering temperature.

For workpieces with complex shapes or strict dimensional requirements, additional correction may be necessary using shaping fixtures combined with tempering.

- Repeat Heat Treatment Limitations

If the mechanical properties of the galvanized steel pipe do not meet the required standards, heat treatment can be repeated.

However, repeated quenching or solid solution treatments should not exceed two cycles.

Supplementary tempering is not considered a repeat treatment.

Before re-quenching martensitic stainless steel or heat-resistant steel in its quenched state or after low-temperature tempering, a preheating, annealing, or high-temperature tempering process should be performed to maintain material stability.