Four production processes for large-diameter square and rectangular tubes

There are currently four main processes for producing large-size square tubes and rectangular tubes:

1. Flat plate tailor welding process

Traditional large-sized square and rectangular tubes are made of four steel plates welded together. The advantage is that the bending process can be omitted and it is not limited by wall thickness. The disadvantages are slow production speed, unstable quality, low strength, and four welds. Currently, this production process is rarely used.

2. High-frequency resistance welding (ERW) round-first-last process

This is a basic process for producing square and rectangular steel pipes. Based on the process of producing round pipes by high-frequency resistance welding units, a square and rectangular hole pattern is added to the sizing machine to cool and process the round welded pipes into square and rectangular welded pipes. The advantages of this process are high dimensional accuracy and a slight increase in wall thickness at the 90° bend. Regarding the shortcomings, some literature points out that in the process of changing from circle to square, the force of the roller is always concentrated at one point, and the stress is concentrated. Often when the wall thickness is greater than 12.7mm, microcracks may appear during the bending process of the profile, and even breakage may occur in severe cases. In addition, when producing small batches and multiple varieties, the number of roll changes is high.

3. Direct square forming process of high frequency resistance welding (ERW)

This process uses a special forming unit to directly bend the strip steel into a square rectangular tube. Its advantage is that it effectively solves the problem of micro-cracks at the corners of thick-walled and high-strength products. At the same time, the number of roll changes is less when producing small batches and multiple varieties; its disadvantage is that the wall thickness becomes thinner at the 90° bend.

4. Submerged arc welding (SAW) two-channel steel butt welding process. 

This process is to bend the plate coils into "[" shapes respectively, and then use submerged arc welding (internal and external welding) to butt-weld the two "[" shapes into double welds. Sew square rectangular tube. This forming process has two characteristics: first, partitions (stiff plates) can be added inside square and rectangular tubes; second, the cross-section can range from 200mm×200mm to 1000mm×1000mm, breaking through the high-frequency resistance welding (ERW) welded pipes. The cross section can only reach the limit of 550mm×550mm. However, its disadvantage is that the double welding seam process uses butt welds and post-weld off-line heat treatment and correction processes are more complicated; at the same time, since it is still roll bending in advance, the maximum wall thickness generally does not exceed 20mm; and, since the two welds are The seam is on the central section, which is very disadvantageous when used as a stress-bearing member to make the weld bear shear force.

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