Bent pipe fittings are widely used in a variety of products in the mechanical industry and oil field capacity construction, such as chemical container manufacturing, water supply piping installation, and boiler pressure vessel manufacturing.
However, both cold and hot bending can result in various defects of different degrees due to improper process conditions or operations. These defects directly affect the safety and appearance quality of the final product.
For example, water-cooled wall tubes and boiler convection tubes are corroded by water and steam on one side and washed and corroded by combustion gases on the other side. Furthermore, the range of pressure and temperature changes is quite large.
If there are defects in the pipe connections, it will affect the safe operation of the entire unit. Pressure vessels, due to their pressurized operation, will have reduced pressure resistance if the pipe connections are defective, thus affecting their safety.
Therefore, it is particularly important to identify the causes of various types of defects and take corresponding preventive measures.
1. Common defects in bent pipes
The common defects of bent tubes mainly include the following forms: severe flattening on the arc (elliptical), severe thinning of the outer wall of the arc, breakage on the outer side of the arc, and wrinkling on the inner side of the arc, as shown in Figure 1.
These defects do not necessarily occur simultaneously, and the shapes and locations in which they occur may vary with the radius of the bent tube.
However, corresponding measures must be taken to prevent them from bending.
Figure 1: Types of common defects in pipe bending
From a process point of view, it is known that when a tube with external diameter D and wall thickness S is subjected to an external torque M, causing it to bend, the external side of the neutral layer of the tube wall becomes more thin due to tensile stress. , while the inner side thickens due to compressive stress (see Figure 2a).
At the same time, the combined forces deform the cross-section of the fold into a roughly elliptical shape (see Figure 2b). Under the influence of compressive stress, the inner wall of the tube may become unstable and wrinkle (see Figure 2c).
Generally, for bend radii ≥3D, satisfactory bent tubes can be achieved. However, when the radius of curvature <2D, especially when R/D or S/D decreases (i.e., the radius of curvature R is small, the pipe diameter is larger, and the wall is thinner), it is likely that mentioned defects occur.
In addition, during the operation process, improper mold installation and other factors can also lead to various types of defects in the bent tubes.
Figure 2: Stress and Strain in the Pure Bending of a Tube
2. Causes
Below, we carry out a detailed analysis of the causes of defects that occur during the bending of small radius tubes:
2.1 Severe flattening in the arch
During pipe bending, the combined forces F1 and F2 cause the cross-sectional area in the arch to tend to have an elliptical shape.
For a tube with the same specification, the smaller the radius of curvature, the greater these forces will be and the more apparent the flattening will be. If the tube is bent without a mandrel, the flattening will be more severe.
If a mandrel is used but its diameter is too small or it is heavily worn, causing a large gap between the mandrel and the inner wall of the tube, the outer side of the arc can also easily flatten.
Even if the clearance between the mandrel and the pipe wall is reasonable, if the mandrel is installed too early, the pipe wall will not be well supported and the outer side of the arc will also be flat.
In addition, if attention is not paid during mold installation and the faces of the bending mold and the compression mold (wheel) are misaligned, the tube in the arch will also become flat.
2.2 Excessive grinding on the outer side of the arch
When the bending radius is small, if the tube bender does not have an outer side reinforcing device and a rear end pushing device, the resistance of the compression mold will increase the tensile stress on the outer side of the arc, causing the neutral layer to shift inside.
This results in thinning of the outside of the bent tube, and the greater the compression force, the greater the resistance and the greater the thinning. This is especially true when the relative radius of curvature R/D and the relative wall thickness S/D are smaller, which makes thinning more apparent.
In addition to factors such as pipe material and bend radius, factors such as oversize mandrel installation and poor lubrication will also increase the thinning of the outside of the pipe wall.
2.3 Cracks on the External Side of the Arch
Cracks or breaks on the outer side of the arch during tube bending can be caused by several factors.
These include inadequate heat treatment of the tube material, excessive grain size, excessive pressure from the compression die (wheel) causing high bending resistance, too small a gap between the mandrel and the inner wall of the tube causing high friction, poor lubrication, excessive bending force causing machine vibration and others.
2.4 Wrinkling on the Inner Side of the Arc
There are three common situations in which wrinkling occurs on the inner side of the arch: wrinkling at the front tangent point, the back tangent point, or the entire inner side of the arch.
Wrinkling at the front tangent point is generally due to undersized pre-installation of the mandrel, where the tube wall at the front tangent point is not well supported during bending.
Wrinkling at the back tangent point typically occurs because no anti-crease mold was installed or the anti-crease mold and bend mold were not properly aligned at the tangent point.
Full wrinkling usually occurs if the diameter of the mandrel is too small, creating a large space between the mandrel and the tube wall, leading to easy wrinkling.
It may also be due to insufficient pressure of the compression wheel, preventing proper alignment of the tube with the bending die and anti-wrinkle mold during bending, leading to instability and wrinkling under compression stress.
3. Policies to Prevent Pipe Bending Defects
To prevent or mitigate pipe bending defects and produce pipes that meet the requirements, appropriate countermeasures must be adopted during the bending process.
Within the allowable range of the product design structure, a larger bending radius should be used as much as possible. For the common defects mentioned above, specific measures must be taken.
3.1 For pipes with severe flattening on the outer side of the arc, when performing bending without mandrel, the compression mold (wheel) can be designed with an anti-deformation groove structure to reduce the degree of flattening during bending.
For mandrel bending, mandrel wear should be checked periodically to ensure that the one-sided clearance between the mandrel and the inner wall of the tube is not more than 0.5 mm. Furthermore, the pre-installation size of the chuck must be appropriate.
3.2 Thinning on the outer side of the arch during bending of small radius tubes is a characteristic of the bending process and is inevitable.
However, measures must be taken to overcome excessive thinning. A common effective method is to use a bender with a side gusset and a back pusher.
During operation, the booster or pusher propels the tube forward, compensating for some resistance during bending, improving the stress distribution in the tube's cross-section and moving the neutral layer outwards, thus reducing thinning on the outer side of the arc.
The speed of the booster and pusher is determined according to the actual bending conditions so that it corresponds to the bending speed.
3.3 For situations where the tube cracks at the outer bend of the arc, it is essential to first ensure that the tube material is in good heat treatment condition.
After excluding factors related to the pipe material, check whether the clamping mold pressure is too high and adjust it to be correct.
Check whether the diameter of the central rod is too large, and if so, grind it, ensuring that the central rod and the inner wall of the tube have good lubrication to reduce bending resistance and friction. At the same time, take appropriate measures to prevent machine tool vibration, etc.
3.4 For wrinkles on the inner face of the arch, measurements must be taken according to the location of the wrinkles.
If wrinkles form at the front cutting point, adjust the position of the center rod forward, making the advance of the center rod appropriate to obtain reasonable support for the tube during bending.
If wrinkles form at the back cutting point, install an anti-wrinkle block, ensure its correct installation position, and adjust the clamping die (wheel) pressure to be appropriate.
If there are wrinkles along the entire inner side of the bow, in addition to adjusting the clamping die (wheel) to make the pressure suitable, also check the diameter of the center rod – if it is too small or very worn, replace the center rod.
4. Conclusion
In the tube bending process, different types and degrees of defects may occur due to inappropriate techniques and operations.
Therefore, the causes of various types of defects must be specifically analyzed and corresponding measures must be taken according to different situations.
This can reduce or even eliminate the occurrence of defects, thus producing better bent tubes.
