Seamless Square Tube Welding Process

1. Preheating

Preheating helps reduce the peak hardness of the heat-affected zone (HAZ) of medium-carbon steel and prevents cold cracking. It is a key process measure for welding medium-carbon steel. Preheating also improves joint plasticity and reduces post-weld residual stress. Typically, the preheating temperature for 35 and 45 steels is 150-250°C. For higher carbon content or due to thickness and stiffness, which increases cracking propensity, the preheating temperature can be increased to 250-400°C.

If the weldment is too large to preheat the entire weld, local preheating can be performed. The heating range for local preheating is 150-200mm on each side of the weld.

2. Welding Rods

Basic welding rods are preferred when conditions permit.

3. Groove Form

U-shaped grooves should be used for welding. If the weldment is a casting defect, the groove should be smooth. This reduces the amount of base metal melted into the weld metal, thereby reducing the carbon content in the weld and preventing cracking.

4. Welding Process Parameters

Since the percentage of base metal melted into the first layer of weld metal can reach up to approximately 30%, low current and slow welding speed should be used during the first weld to minimize base metal penetration.

5. Post-Weld Heat Treatment

It is best to perform stress relief heat treatment immediately after welding, especially for thick welds, highly rigid structural parts, and welds subjected to severe conditions (dynamic or impact loads). The tempering temperature for stress relief is 600-650°C. If stress relief heat treatment is not possible after welding, post-heat treatment should be performed immediately.