Heat treatment of LSAW steel pipe after welding

Heat treatment of Longitudinal Submerged Arc Welded (LSAW) steel pipes after welding is a modern technique used to eliminate residual stress. This process involves preheating the material to the heat treatment temperature before welding, maintaining this temperature during the welding process, and then allowing the pipe to cool slowly using thermal insulation materials. This method effectively reduces welding residual stress and improves the metal's resistance to stress corrosion.

Internal Weld Nondestructive Testing (NDT)

For large steel pipes used in water supply projects, especially those with a wall thickness of 30mm used as pipe bridges, both internal pressure and the weight of the water must be considered. Therefore, the welding quality requirements are particularly stringent. For large-diameter pipes with a 30mm thickness used in pipe bridges, both longitudinal and girth welds require 100% X-ray film examination and 100% ultrasonic flaw detection. For buried pipes with a 24mm thickness, longitudinal seam welds require 20% X-ray film examination and 50% ultrasonic flaw inspection.

Benefits of Post-Weld Heat Treatment

Post-weld heat treatment (PWHT) significantly reduces welding residual stress. The higher the heat treatment temperature, the more effectively it reduces residual stress. For large-diameter pipes, PWHT enhances stress corrosion resistance; the higher the heat treatment temperature, the more significant the improvement in stress corrosion resistance.

Advanced Anti-Corrosion Techniques

The diffusion method for metal carbide coating technology involves placing the workpiece in a special medium, allowing a metal carbide layer, ranging from a few microns to tens of microns, to form on the workpiece surface. This technique improves the pipe's anti-corrosion properties.

For buried large-diameter steel pipes, anti-corrosion measures include using galvanized wire hangers and applying a cement mortar lining with a compressive strength of not less than 30N/mm².

Conclusion

Heat treatment after welding is crucial for LSAW steel pipes, particularly in high-stress applications like water supply projects and pipe bridges. This process reduces residual stress, improves stress corrosion resistance, and enhances overall durability. Additionally, advanced NDT methods and anti-corrosion techniques ensure the highest quality and longevity of the pipes.

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