Production technology of large diameter square and rectangular tubes

There are currently four primary processes for manufacturing large-size square and rectangular tubes.

Flat Plate Tailor Welding Process:

Traditionally, large-size square and rectangular tubes are fabricated by welding together four steel plates. The key advantage of this method is that it eliminates the need for a bending process and is not constrained by wall thickness. However, this method has several drawbacks, including slow production speed, inconsistent quality, lower strength, and the presence of four weld seams. Due to these disadvantages, this production method is now rarely used.

High-Frequency Resistance Welding (ERW) with Head and Tail Rounding Process:

This is the fundamental process for producing square and rectangular steel pipes. It starts with the production of round pipes using a high-frequency resistance welding unit, followed by a sizing machine that reshapes the round welded pipes into square or rectangular welded pipes. The advantages of this process include high dimensional accuracy and slight wall thickness increase at the 90° bends. However, a noted drawback is that during the transition from round to square shapes, the roller's force is concentrated at a single point, leading to stress concentration. When the wall thickness exceeds 12.7mm, micro-cracks may develop during the bending process, and in severe cases, the profile may even break. Additionally, this process requires frequent roller changes when producing small batches of diverse products.

High-Frequency Resistance Welding (ERW) Direct Square Forming Process:

In this method, a special forming device is used to directly bend the strip into a square or rectangular tube. The main advantage is that it effectively addresses the issue of micro-cracks at the corners of thick-walled, high-strength products. Moreover, it reduces the number of roll changes required when producing small batches of various products. The downside is that the wall thickness at the 90° bends tends to thin out.

Submerged Arc Welding (SAW) Two-Channel Steel Butt Welding Process:

This process involves bending coils into "[" shapes and then butt-welding the two "[" shapes using submerged arc welding (both internal and external) to create double-weld seams, forming a square or rectangular tube. This method has two notable features: the ability to add partitions (hard plates) inside the tubes and the capacity to produce cross-sections ranging from 200mm×200mm to 1000mm×1000mm, surpassing the high-frequency resistance welding (ERW) limit of 550mm×550mm. However, the disadvantages include a more complex post-weld off-line heat treatment and correction process due to the double welding seams. Additionally, the maximum wall thickness generally does not exceed 20mm. The central positioning of both weld seams is also a disadvantage when the tube is used as a load-bearing member subjected to shear forces.