Welding Of Square Tubes

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When welding square and rectangular tubes, due to their hollow cross-section and sharp edges, some unique challenges do arise. The main problems and precautions are as follows:

1. Burn-through or collapse at the corner: Heat tends to accumulate at the 90-degree angle, causing the pipe wall to be burned through.
2. Difficult control of deformation: Weld shrinkage can lead to overall bending, twisting or local depression.
3. Poor weld formation: At the corner transition, the angle of the welding gun is difficult to control, resulting in uneven welds, undercutting or incomplete penetration.
4. Internal weld bulges and oxidation: Internal parts of the pipe may form weld bulges, and the internal welds are prone to oxidation, affecting strength and appearance.

1. Pre-weld preparation: Thoroughly remove oil, rust, and moisture from the welding area. For thick-walled pipes or critical structures, it is recommended to machine the bevels.
2. Control heat input: Use a smaller current, a faster welding speed, and employ the segmented jump welding method to disperse heat, reduce deformation, and minimize the risk of burn-through.
3. Pay attention to welding sequence: For long weld seams, adopt a symmetrical welding sequence from the middle to both ends. When assembling multiple weld seams, also plan the symmetrical welding sequence.
4. Use appropriate techniques: For thin-walled square and rectangular pipes, prioritize gas shielded welding (such as MIG/MAG welding), as it has concentrated heat and minimal deformation. During welding, the welding gun should maintain an appropriate angle relative to the welding plane, and a smooth transition is required at the corners.
5. Post-weld treatment: Clean up the slag and spatter promptly. For components with appearance or precision requirements, correction (such as mechanical correction or flame correction) may be necessary.

The key to welding square and rectangular tubes lies in strictly controlling the heat input and planning a reasonable welding sequence. This is the most effective way to avoid deformation and defects. For important load-bearing structures, it is essential to refer to relevant design specifications and conduct process evaluations.

When welding square and rectangular tubes, the welding environment and the selection of welding rods are the key factors to ensure quality. The specific requirements are as follows:

Square tube is more sensitive to the environment due to its hollow structure.

1. Wind resistance and moisture protection: Windbreaks must be avoided. When the wind speed exceeds 2m/s, a wind shelter must be set up. Welding is strictly prohibited when the air humidity is too high (such as over 90%) or there is condensation on the tube wall. Otherwise, the weld seam is prone to produce pores.
2. Environmental temperature: When the environmental temperature is below 0℃ (for low-carbon steel) or 5℃ (for low-alloy steel), the welding area within 100mm should be heated to above 15℃ (preheating) to prevent cold cracks. Outdoor work in rainy or snowy weather is strictly prohibited.
3. Safety and ventilation: Ensure good ventilation in the working area. Especially when welding in a confined space, a forced ventilation device should be used to discharge welding smoke and any accumulated protective gas (such as Ar, CO₂), to prevent asphyxiation or affect the protection effect.

The choice depends on the material, wall thickness and usage requirements of the square and rectangular tubes.

1. Matching based on the base material:
◦ Square and rectangular tubes made of low-carbon steel such as Q235: The most commonly used welding rods are the E43 series (such as E4303, E4315).
◦ Q345 and other low-alloy high-strength steel square and rectangular tubes: E50 series welding rods (such as E5015, E5016) must be selected to ensure the strength of the joints.
◦ Stainless steel square and rectangular tubes: It is necessary to select matching stainless steel welding rods (such as A102, A132, etc.).

Core principle: The strength grade and chemical composition specified on the welding rod manual should match or be slightly higher than those of the base material.

2. Select based on the type of flux (key distinction):

◦ Alkaline electrodes (such as E4315, E5015): The flux contains a large amount of carbonate and fluoride. The weld metal has good toughness and strong crack resistance (especially for thick-walled or critical structures), but the process performance is slightly poorer. It is sensitive to oil rust and requires strict drying (heated at 350-400°C for 1-2 hours) and direct current reverse connection. Suitable for important load-bearing structures, thick-walled pipes, steel materials with poor weldability or in low-temperature environments.

◦ Acidic welding rods (such as E4303): The coating contains a large amount of oxides. They have good processability (stable arc, little spatter, easy slag removal), are not sensitive to oil and rust, can be used with both AC and DC power supplies, but the plasticity and toughness of the weld are slightly lower than those of alkaline welding rods. They are suitable for general structures, thin-walled pipes, situations with low requirements or general cleanliness.

3. Efficient alternative solutions:
◦ For mass production, gas shielded welding with solid wire (such as ER50-6) or flux-cored wire is a superior choice. It is highly efficient, has minimal deformation, and produces a neat and aesthetically pleasing weld, making it particularly suitable for continuous welding of square and rectangular tubes.

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