How to Bend A Square Tube?

Square tubes (SHS steel) are tubing with a square cross-section. They are typically made by bending steel strips or plates and then welding them. Seamless square tubes are also available, manufactured through cold drawing or hot rolling processes. The main characteristics of square tubes are symmetrical structure and good load-bearing capacity, making them widely used in many applications requiring support or frame structures.

Bending square tubes is a common and technically demanding process in structural manufacturing. Because square tubes have a closed cross-section, wrinkling, twisting, and cross-sectional deformation are prone to occur during bending. Therefore, mastering the correct process and parameters is crucial.

The Basic Principle of Square Tube Bending

Bending a square tube is the process of plastically deforming the tube under external force, thereby changing its axial shape.

During bending, the outer side of the square tube is subjected to tensile stress, the inner side to compressive stress, while the neutral layer remains essentially unchanged.

Because the rigidity of the four side walls of a square tube differs from that of a round tube (CHS), cross-sectional deformation and wrinkling are easily caused during bending. Therefore, it is necessary to select an appropriate process based on factors such as material thickness and bending angle.

Common Methods of Bending Square Tube

The commonly used methods for bending square tubes are as follows:

1. Cold Bending

Cold bending is a bending process performed at room temperature.

Suitable for: Square tubes with thin walls and low strength.

Common cold bending equipment: Manual tube bending machine, hydraulic tube bending machine, etc.

Properties of cold bending: Simple operation, low cost, but the bending radius should not be too small, otherwise it is easy to cause deformation or cracking of the tube.

2. Hot Bending

Hot bending is a method of bending the square tube by heating it to soften it.

Suitable for: Materials with thick walls or high strength.

Heating methods: Local heating or overall heating; commonly used tools include flame heating, induction heating, etc.

Properties of hot bending: Can reduce the internal stress of the material and reduce the risk of cracking, but the heating temperature needs to be controlled to avoid degradation of material properties.

Hot bending temperature reference (carbon steel square tube): 300°C – 650°C is the commonly used plastic deformation range; temperatures exceeding 700°C will affect the metallographic structure and should not be overheated.

Note: Heating must be uniform; straightening and welding repair may be necessary after heating.

3. Die Bending Method

Die bending is a method of forming square tubes using a special die.

Applicable to: Mass production.

Common equipment: CNC bending machine, die bending machine.

Properties of die bending: Die bending can ensure bending accuracy and consistency, but the die cost is high, making it suitable for large-scale processing.

Factors Affecting The Bending Quality of Square Tubes

1. Material Properties

The size, material, wall thickness, and hardness of the square tube will affect the bending effect.

Generally speaking, MS square tubes are easier to bend, while high-carbon steel or alloy steel require higher process requirements.

The softer the material, the easier it is to bend. The bendability of different materials is as follows:

2. Bending Radius

A bending radius that is too small will cause excessive stretching on the outside and excessive compression on the inside of the square tube, resulting in cracks or wrinkles.

It is generally recommended that the bending radius be no less than twice the outer diameter of the tube.

Recommended Bending Radius Table:

3. Bending Speed

Excessive bending speed can easily lead to localized stress concentration in the material, increasing the risk of deformation.

Insufficient bending speed may affect production efficiency.

A suitable bending speed should be selected based on the material properties.

Common Problems and Solutions for Square Tube Bending

1. Cross-sectional Deformation

After bending, the cross-section of the square tube may become elliptical or other irregular shapes.

Solutions include using mandrels to support the internal structure and increasing mold constraint.

2. Wrinkling

The inner side of square tubing is prone to wrinkling under pressure during bending.

This problem can be reduced by adjusting the bending radius, using hot bending processes, or using anti-wrinkle molds.

3. Springback

Due to the elasticity of the material, the bent square tubing may partially return to its original shape, resulting in angular deviation.

Springback can be compensated for by over-bending or using correction processes.

Applications of Square Tube Bending

1. Construction

Square tubing is commonly used to manufacture stair handrails, railings, and steel structural frames.

Bent square tubing meets strength requirements while achieving an aesthetically pleasing shape.

2. Furniture Manufacturing

Many modern furniture pieces use square tubing as a supporting structure.

Bending allows for various curved or irregular shapes, enhancing the aesthetics and practicality of square tubing.

3. Machinery Equipment

In machinery equipment, bent square tubing is often used to manufacture components such as conveyor frames and protective covers.

It saves space while ensuring structural stability.

Cost Estimation for Square Tube Bending

The cost of square tube bending mainly includes material costs, processing costs, and mold costs. Specific costs vary depending on material specifications, bending difficulty, and processing quantity. Generally, cold bending is the least expensive, hot bending is moderately expensive, and mold bending is relatively expensive, but the cost is lowest for large-scale production.

How to Choose a Square Tube Bending Service

1. Equipment Capabilities

Does the company have CNC tube bending machines, hot bending equipment, and internal support mandrels to ensure they can meet bending requirements?

2. Provide Complete Drawings

The drawings must include bending angles, radius (R), allowable tolerances, and appearance requirements.

3. Trial Bending Recommended for Special Materials

For materials such as stainless steel, high-strength steel, and thick-walled parts, sample confirmation is required before mass production.

4. Inspect Processed Samples

Including weld integrity, no surface indentations, and angle error less than ±1°.

FAQ

1. Which is Better, Cold Bending or Hot Bending of Square Tubes?

Cold bending: Low cost, high efficiency, but prone to deformation at small radii.

Hot bending: Suitable for thick-walled, high-strength steel, allowing for smaller bending radii.

Neither is absolutely better or worse; it depends on the material and bending requirements.

2. Can Thick-Walled Square Tubes Be Cold-Bent?

Yes, but prone to: outer cracking, inner wrinkling, and cross-sectional distortion.

Hot bending or using a mandrel for cold bending is generally recommended.

3. How to Bend A Square Tube by Hand?

Positioning Marks: Draw lines at the bending point and heat evenly with a spray gun until dark red (wall thickness ≤ 3mm can be cold-bent).

Gradual Force Application: Use a lever tool to apply force slowly in 3-4 stages, holding each stage for 10 seconds.

Angle Correction: Gently tap the over-bent area with a mallet to fine-tune the angle using the rebound principle.

Summary

Bending square tubes is a seemingly simple but highly technical process. Whether used in architecture, aesthetic design, or mechanical manufacturing, proper selection of processes, bending radii, and processing equipment can effectively prevent deformation and cracking, ensuring structural strength and appearance quality.

Read more: SHS Steel vs RHS Steel or Advantages of Using Square Tubes