Ten Key points of Heat Treatment of Galvanized Steel Pipes

Jane

Galvanized steel pipe is a welded steel pipe with a surface treated by hot-dip or electro-galvanizing process, with Q235 steel as the base. Its core advantage lies in the dense protective film formed by the zinc layer, which can effectively resist oxidation and corrosion, significantly extend the service life of the pipe, while maintaining a low cost. It is widely used in scenes such as building water supply and drainage, municipal pipeline engineering, power cable protection and scaffolding construction. It is an economical choice for transporting media such as water, gas, and oil, and has both durability and versatility.

Ten things to note when heat treating galvanized pipes:

1. Before heat treatment, galvanized steel pipes and fixtures must be free of impurities such as oil, residual salt, and paint.
2. The fixtures used in the vacuum furnace for the first time should be degassed and purified in advance at a vacuum degree not lower than the workpiece requirement.
3. During the heat treatment process, easily deformed workpieces should be heated on special fixtures. The preheating method includes: the first preheating to 800℃, the second preheating to 500-550℃ and 850℃, and the heating rate of the first preheating should be limited.
4. Preheating should be performed for workpieces with complex shapes, drastic cross-sectional changes, or large effective thickness.
5. Workpieces with grooves and no through holes, castings, welded parts, and processed stainless steel workpieces are generally not suitable for heating in salt bath furnaces.
6. According to the effective thickness and conditional thickness of the workpiece (actual thickness multiplied by the workpiece shape coefficient), galvanized steel pipes need to have sufficient insulation time when heated.
7. Galvanized steel pipes of martensitic stainless steel and heat-resistant steel can only be cleaned, cryogenically treated or tempered after quenching and cooling to room temperature. The time interval for heat treatment after welding shall not exceed 4 hours.
8. According to the requirements of the galvanized steel pipe and its surface state, alkali washing, water-soluble detergent, chlorine solvent sandblasting, shot blasting and other methods can be used for cleaning.
9. After correction, stress relief annealing should be carried out at a temperature lower than the original tempering temperature. For workpieces with complex shapes or strict dimensional requirements, further correction is required after correction using a shaping fixture combined with tempering.
10. When the mechanical properties of the galvanized steel pipe do not meet the standards, heat treatment can be carried out, but the number of repeated quenching or solid solution is usually not more than two times. Supplementary tempering of galvanized pipes is not considered as repeated treatment. Before repeated quenching of martensitic stainless steel and heat-resistant steel workpieces in the quenched state or after low-temperature tempering, preheating, annealing or high-temperature tempering should be carried out.

What are the precautions for welding galvanized steel pipes?

1. Human factors are the key to gas galvanized pipe welding control. Due to the lack of necessary post-welding control methods, it is easy to cut corners and affect quality; At the same time, the welding characteristics of galvanized steel pipes make it difficult to ensure welding quality. Therefore, before the project starts, a skilled welder who holds the corresponding boiler pressure vessel or equivalent welder certificate should be selected to provide necessary technical training and instructions, and the on-site welder assessment and approval should be carried out according to the boiler situation.
2. Welding material control: It is necessary to ensure that the purchased welding materials come from formal channels, have quality assurance and certificates, and meet the process requirements; the acceptance, sorting, and distribution procedures of welding materials must be formal and complete, and the recovery control of welding rod heads must be strictly controlled to ensure the flow direction and quantity: welding materials must be baked strictly according to the process, and the amount of welding materials used must not exceed half a day.
3. Welding machine: The welding machine is a welding tool, which must ensure reliable performance and meet the process requirements; the welding machine must have qualified current and voltage meters to ensure the correct implementation of the welding process. The welding cable should not be too long, and the welding parameters should be adjusted if it is too long.
4. Welding process method: Ensure strict implementation of the special operation method for galvanized steel pipes, perform bevel inspection before welding according to the welding process, control welding process parameters and operation methods, check appearance quality after welding, and add non-destructive testing after welding if necessary. Control the welding level and the amount of welding consumables for each pass.
5. Welding environment control: Ensure that the temperature, humidity and wind speed during welding meet the process requirements. Welding is not allowed without any conditions.

Read more: How to Distinguish the Materials of Seamless Pipes and Galvanized Steel pipes?