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Weld surface indentation in submerged arc welded (SAW) steel pipes refers to a defect where the weld surface is lower than the surrounding base material. This is a common issue in submerged arc welding, especially in the production of long weld seams such as those used in spiral submerged arc welded steel pipes, where welds can stretch for thousands of meters. Due to the continuous nature of the welding process, the occurrence of weld surface indentations is relatively frequent.
Hazards of Weld Surface Indentation
- Depth of Indentation Over 1.5mm:
If the indentation depth exceeds 1.5mm, it must be treated as a serious weld defect. Multiple weld pits lower than the base metal could degrade the quality of the steel pipe or even make it unsuitable for use. In any case, this defect will increase the cost of repairs during production.
- Impact on 3PE External Anti-Corrosion Layer:
With the widespread application of 3PE (three-layer polyethylene) anti-corrosion technology for steel pipes, the height of the weld seam plays a crucial role in the uniformity of the anti-corrosion coating. In production, steel pipe manufacturers often try to keep the weld reinforcement height as low as possible—generally under 1mm—to minimize anti-corrosion costs. If the indentation depth exceeds 1mm, the weld will be lower than the base metal, resulting in a significant defect that compromises the anti-corrosion effectiveness of the entire pipe.
- Potential Anti-Corrosion Defects:
For submerged arc welded steel pipes that undergo internal powder spraying and external 3PE anti-corrosion treatment, indentations on the weld surface may lead to anti-corrosion issues such as leakage or the formation of bubbles in the 3PE coating. This affects the overall quality of the pipe's protective layer.
Factors Affecting the Formation of Weld Pitting
Indentations or pitting occur when gas generated during welding becomes trapped in the weld pool. If the flux layer and slag resistance to gas escape are high, it becomes difficult for the gas to escape, leading to pressure buildup and eventual formation of pits on the weld surface.
Measures to Prevent Weld Pitting
- Choose the Right Flux: To ensure the internal quality of the weld, flux should be selected based on its low oxidation, low viscosity, and low surface tension. Flux with low bulk density improves gas venting during welding. In high-current welding, the flux particles should be uniform in size, typically ranging from 10 to 60 mesh, to facilitate gas escape. Coarse or overly fine flux should be avoided, and any recovered flux should be free of impurities such as rust and iron oxide. Additionally, flux should be dried according to the specified temperature to ensure its moisture content remains under 0.1%.
- Proper Use of Flux: Keep the flux clean and free from contamination. Any impurities, including rust, dust, and iron oxide, must be removed from recycled flux using magnetic separation. It is essential to follow the recommended drying procedures to maintain the flux's effectiveness.
- Reasonable Selection of Tube Blank and Welding Wire: Choose materials with low levels of carbon (C), oxygen (O), nitrogen (N), and hydrogen (H) to minimize the generation of gas during welding, thus reducing the potential for pitting.
- Optimize Welding Parameters: Select welding parameters that reduce gas generation. Using a smaller welding current helps minimize gas production, reducing the likelihood of pressure pits. Increasing arc voltage increases the arc cavity size, which allows for thinner welding slag layers, facilitating better gas escape. A slower welding speed increases the time the molten pool remains in its liquid state, allowing gas to escape more effectively and reducing pitting.
- Maintain Proper Welding Inclination: Adjust the welding inclination angle to improve slag flow and reduce the thickness of the slag layer, which can help reduce pitting. For submerged arc welding, controlling the eccentric distance and welding wire angle ensures optimal welding conditions, minimizing the chances of weld surface indentation.
By addressing these factors and implementing appropriate measures during the welding process, manufacturers can significantly reduce the occurrence of weld surface indentation, ensuring better quality control and the long-term performance of submerged arc welded steel pipes.
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