Bending Process for Seamless Pipe

The seamless pipe is widely used in many fields, and their bending and forming processes are crucial for meeting diverse engineering needs. The following will introduce several common seamless pipe bending and forming processes.

What is Pipe Bending

Pipe bending refers to the process of shaping metal pipes (such as steel pipes) into desired curved shapes (such as arcs, circles, U-shapes, etc.) through mechanical or thermal action. It is widely used in construction, power, petroleum, chemical, shipbuilding, bridge construction, and other fields for manufacturing pipe elbows, irregularly shaped components, etc.

Specification of Pipe Bends

Seamless Pipe Bending Process

1. Cold Bending Process

Cold bending is a process of bending seamless pipes at room temperature. This process has many advantages. First, it better maintains the surface quality of the seamless pipe because, without the influence of high temperatures, defects such as oxide scale will not appear on the steel tube surface. Second, the cold bending process allows for precise control of the bending angle and radius, meeting high-precision engineering requirements.

Common cold bending methods include:

- Roll Bending

Roll bending uses three rollers to bend the seamless pipe. Two are driving rollers, and one is a driven roller. The steel tube moves forward under the rotation of the rollers, while the driven roller applies pressure to bend the tube.

This method is suitable for seamless pipes with large bending radii, enabling continuous bending and offering high production efficiency.

- Press Bending

Pressure bending uses a press and a die to bend the seamless steel pipe. The steel tube is placed in the die, and pressure is applied by the press, causing the tube to bend under the action of the die.

Press bending can achieve various shapes of bending, such as U-shapes and V-shapes, but it can only bend one angle at a time, resulting in relatively low production efficiency and suitability for small-batch production.

- Stretch Bending

Stretch bending is the bending of seamless steel pipes under the combined action of tension and bending moment. One end of the steel pipe is fixed, tension is applied to the other end, and a bending moment is applied to the middle of the pipe, causing it to bend.

Stretch bending effectively reduces springback during bending, improving bending accuracy, and is suitable for applications requiring high bending precision.

2. Hot Bending

Hot bending is a process of bending seamless pipes after heating them to a certain temperature. The purpose of heating is to reduce the hardness of the steel pipe, increase its plasticity, and make it easier to bend. Hot bending is suitable for seamless pipes with small bending radii and large wall thicknesses.

Common hot bending methods include:

- Medium Frequency Induction Heating Bending

Medium frequency induction heating bending utilizes the induced magnetic field generated by medium frequency current to locally heat the steel pipe.

A steel pipe is passed through an induction coil. When current passes through the coil, an induced current is generated inside the pipe, rapidly heating it to the desired temperature. The heated pipe is then bent using a mechanical device.

This method offers fast heating, concentrated heating, precise control of heating temperature and bending angle, and high bending quality.

- Flame Heating Bending

Flame heating bending utilizes a flame to heat the steel pipe. Typically, a flame generated by the combustion of a mixture of gases such as oxygen and acetylene is used. After reaching a certain temperature, the pipe is bent manually or mechanically.

Flame heating bending equipment is simple and inexpensive, but the heating speed is slow, heating is uneven, and bending quality is relatively poor. It is suitable for applications where high bending quality is not required.

3. Other Bending Forming Processes

- Hydraulic Bending

Hydraulic bending uses pressure provided by a hydraulic system to bend seamless pipes. A hydraulic cylinder pushes a bending die, causing the steel pipe to bend under the action of the die.

Hydraulic bending features high bending force and high bending precision, making it suitable for bending large-diameter, thick-walled seamless pipes.

- CNC Pipe Bending

CNC pipe bending utilizes a computer control system to precisely control the pipe bending process. By inputting bending parameters, the computer controls various actions of the bending equipment to achieve automated bending.

CNC pipe bending improves bending accuracy and production efficiency, and is suitable for mass production and bending of complex shapes.

Bending Radius of Seamless Pipes

The bending radius of seamless steel pipes needs to be determined based on the pipe material, wall thickness, pipe diameter, and bending process. It directly affects the bending quality and performance. Generally, the principle of "safe bending radius" is followed to avoid cracks and deformation during bending.

1. Minimum Bending Radius Coefficient

First, determine the minimum bending radius coefficient based on the material (1.5-2.5 for carbon steel, 2.0-3.0 for stainless steel, and 2.5-3.5 for alloy steel).

Minimum Bending Radius = Bending Radius Coefficient × Pipe Diameter

For example, for a φ50mm carbon steel seamless pipe, the minimum bending radius = 2.0 × 50 = 100mm. The actual bending radius must be ≥ the minimum bending radius to avoid excessive stretching on the outer side of the pipe, which could lead to cracks.

For seamless pipes with thinner walls (≤5mm) or larger diameters (≥200mm), the bending radius coefficient needs to be appropriately increased (by 0.5-1.0) to prevent collapse during bending.

The bending radius for cold bending should be greater than that for hot bending (the cold bending coefficient should be 0.3-0.5 greater than the hot bending coefficient) because the metal has lower plasticity during cold bending, making it prone to springback.

2. The Influence of Bending Radius

An excessively small bending radius will lead to thinning of the outer wall thickness of the pipe (thinning ≤10% is acceptable), exceeding which will affect pressure resistance; it may also cause internal wrinkles, affecting fluid transport efficiency. An excessively large bending radius will increase the space occupied by the pipe, making installation in confined areas difficult, and may also increase the deviation of the straight section of the pipe after bending, affecting assembly accuracy.

In practical applications, the bending radius needs to be determined comprehensively based on the installation space and operating pressure. The bending radius of important pipelines (such as high-pressure pipes) needs to be verified through bending tests to ensure that there are no defects after bending and that the requirements for use are met.

Factors Affecting the Bending Degree of Seamless Pipes

1. Material

The material of the seamless pipe directly affects its bending degree. Different materials have different strengths and toughnesses; therefore, when selecting and bending seamless pipes, it is necessary to choose the appropriate material based on specific industry requirements.

2. Diameter

The larger the diameter of the seamless steel pipe, the greater the force required for bending, and the larger the bending radius will be. Therefore, diameter is one of the important factors affecting the bending degree of seamless pipes.

3. Wall Thickness

The thinner the seamless pipe thickness, the easier it is to deform during bending, and therefore its bending degree is relatively larger. However, excessively thin walls may also lead to problems such as cracking or uneven deformation during the bending process.

Seamless Pipe Bending Processing Skills

1. Choosing an Appropriate Bending Radius

Generally speaking, the larger the bending radius, the easier it is to bend the steel pipe and the lower the internal stress; the smaller the bending radius, the more difficult it is to bend the steel pipe and the more prone it is to defects such as cracks.

When choosing the bending radius, it is necessary to comprehensively consider factors such as the actual engineering requirements, the material of the steel pipe, its diameter, and its wall thickness.

2. Proper Lubrication

During the bending process, proper lubrication of the steel pipe and die can reduce friction, minimize surface damage, and improve bending quality.

Common lubricants include graphite lubricant and machine oil. When applying lubricant, it should be evenly applied to the contact surfaces of the steel pipe and die.

3. Controlling Bending Speed

Bending speed significantly affects the bending quality of seamless pipes. Excessive bending speed can lead to defects such as cracks and wrinkles; insufficient bending speed reduces production efficiency.

In practice, the bending speed should be rationally controlled based on factors such as the steel pipe's material, diameter, wall thickness, and bending radius.

4. Stress Relief Treatment

Whether hot or cold bending, seamless pipes will generate internal stress after bending. This stress affects the service life and performance of the steel pipe. Therefore, stress relief treatment is necessary after bending.

Common stress relief methods include heat treatment and mechanical vibration. Heat treatment involves heating the steel pipe to a certain temperature, holding it at that temperature for a period of time, and then slowly cooling it. Mechanical vibration involves vibrating the steel pipe with a vibrating device to release internal stress.

Key Points for Seamless Pipe Bending

1. Material Selection

Before bending seamless steel pipes, it is necessary to select a suitable steel pipe material based on the specific application requirements.

Different steel pipe materials have different mechanical and processing properties. For example, carbon steel seamless pipes have high strength and toughness, suitable for general industrial applications; stainless steel seamless pipes have good corrosion resistance, suitable for chemical, food, and other industries.

In addition, the wall thickness and outer diameter of the steel pipe must be considered to ensure they meet the requirements of bending processing.

2. Bending Radius Control

The bending radius is an important parameter in seamless steel pipe bending. A bending radius that is too small will result in thinning of the outer wall and thickening of the inner wall, and may even lead to defects such as wrinkles and cracks; a bending radius that is too large will increase the pipe's footprint and cost.

Therefore, when determining the bending radius, it is necessary to comprehensively consider factors such as the steel pipe's material, wall thickness, and application requirements. Generally, the bending radius should be no less than 3-5 times the outer diameter of the steel pipe.

3. Die Design and Manufacturing

The die is a key tool in the bending process of seamless pipes. The design and manufacturing quality of the die directly affect the accuracy and quality of the bending process. The die dimensions should match the outer diameter and bending radius of the steel pipe, and the surface should be smooth, free of burrs and scratches.

When manufacturing the die, suitable materials should be selected, and advanced processing technology should be adopted to ensure the strength and wear resistance of the die.

4. Quality Inspection

After the seamless pipe bending process is completed, strict quality inspection is required. Inspection includes bending angle, bending radius, pipe wall thickness variation, surface quality, etc.

Common inspection methods include measuring tools and non-destructive testing. Through quality inspection, problems that occur during the bending process can be identified in a timely manner, and corresponding measures can be taken for rectification to ensure that the quality of the seamless steel pipe meets the usage requirements.

Summary

Different seamless pipe bending processes have their own characteristics and applicable scopes. In actual production, it is necessary to select a suitable bending process based on parameters such as the material, diameter, wall thickness, and bending radius of the seamless pipe, as well as factors such as processing cost and production efficiency, to ensure that the quality and performance of the bent pipe meet the usage requirements.