Welding quality depends primarily on the welding material, groove shape, welding method, heat treatment, and welder skill.
Factors Affecting Welding
1) Among welding methods, flat welding is relatively easy, followed by vertical and horizontal welding, and overhead welding is relatively difficult. During welding, the pipe should be rotated as much as possible, and flat welding should be performed.
2) For pipes with a wall thickness of less than 6mm, two layers can be welded; for pipes with a wall thickness of more than 6mm, three or more layers should be welded. Clean thoroughly between weld layers to remove spatter and slag. Joints should be staggered. Avoid high currents during the final surface weld to prevent undercutting. Each weld should be completed in one pass, with no pauses in between.
3) To reduce welding stress and deformation, a reasonable construction method and sequence should be adopted.
Heat Treatment Before and After Welding
1) Preheating: This serves to reduce the temperature difference between the weld metal and the base material, thereby increasing the initial temperature of the weld joint, thereby reducing shrinkage stress, preventing cracks, and facilitating degassing and slag removal. Preheating is generally required when welding important structures, thick-walled pipes, pipes with poor plasticity, or pipes with a strong tendency to harden. Pipes should be preheated locally, with a length of at least three times the wall thickness on either side of the weld center. For pipes prone to hardening or prone to extended cracking, the length should be at least 100 mm on each side.
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2) Post-weld heat treatment: This improves the weld metallurgical structure, eliminates residual stress, increases weld toughness, and ensures strength. Generally, pipes should be preheated locally, with a length of at least three times the weld width on either side of the weld center.
1. Heating rate: After heating to 300°C, the heating rate should be ≤ 220 x 25.4 ÷ s °C/h and ≤ 220°C/h, where s represents the wall thickness in mm.
2. Hold time: For carbon steel, the hold time is 2–2.5 minutes per millimeter of wall thickness; for alloy steel, the hold time is 3 minutes per millimeter of wall thickness, and should be ≥ 30 minutes.
3. Cooling rate. The cooling rate after constant temperature should be ≤ 275 x 25.4 ÷ s °C/h, and ≤ 275 °C/h. Natural cooling is permitted below 300 °C.
Post-weld heat treatment should generally be performed promptly after welding. If repairs are required after heat treatment, reheat treatment should be performed after the repairs.
Post time: Jul-29-2025
