Characteristics of elastic-plastic bending of large diameter seamless pipe

Large-diameter seamless pipe is an important industrial pipeline material, widely used in petroleum, chemical, natural gas and other fields. In some special projects, large-diameter seamless pipes need to be bent to adapt to complex pipeline layouts. Elastic-plastic bending is a commonly used pipeline forming method with advantages such as low cost, reliable quality and simple process.

Process flow analysis

The elastic-plastic bending process of large-diameter seamless pipes mainly includes the following steps:

1. Material preparation: Select suitable large-diameter seamless pipes as raw materials. The quality and performance of the materials directly affect the bending effect of the finished product.

2. Pipe bending: Put the large-diameter seamless pipe into the pipe bending machine, and bend the pipe under the action of the pipe bending mold through the action of pressure and torque.

3. Heat treatment: Heat treat the large-diameter seamless pipe after bending to eliminate stress and improve the strength and toughness of the pipe.

4. Finished product inspection: Inspect the size and quality of the large-diameter seamless pipe after bending to ensure that it meets the relevant standards and requirements.

Advantages

Elastic-plastic bending of large-diameter seamless pipes has the following advantages:

1.Low cost: Compared with other forming methods, the equipment investment and process cost of elastic-plastic bending are lower, and it is suitable for mass production. 

2.Reliable quality: By controlling the process parameters and material quality, elastic-plastic bending can achieve precise bending angles and radii to ensure the quality of the finished product. 

3.Simple process: The elastic-plastic bending process is simple, easy to operate, and does not require complex molds and equipment. 

4.Strong adaptability: After elastic-plastic bending, large-diameter steel pipes can adapt to complex pipeline layouts and improve the overall performance of the pipeline system. 

Quality defects

After the plastic bending of the pipe is unloaded, common quality defects include: rebound, cross-sectional distortion, thinning or even fracture of the outer pipe wall, and thickening or even wrinkling of the inner pipe wall. Take different measures to control quality and prevent defects according to different causes of defects.After the plastic bending of the pipe is unloaded, the common quality defects are:

1. Rebound

2. Cross-sectional distortion

3. Thinning or even fracture of the outer wall

4. Thickening or even wrinkling of the inner wall

Under the condition that the outer wall is not broken and the inner wall is not wrinkled, the performance of the bending forming is mainly evaluated from the following two aspects:

1.The shape and size accuracy of the local bending deformation part, mainly with the outer wall thinning rate and cross-sectional distortion rate as evaluation indicators.

2.The overall shape and size accuracy of the pipe after bending, mainly with the rebound rate after bending deformation as evaluation indicator.
Springback

Springback is caused by the elastic recovery of the material in the elastic deformation zone and the elastic recovery of the elastic deformation part of the material in the plastic deformation zone.

After the tube is bent and unloaded, the actual bending angle 0' of the rebound tube is less than the pre-bending forming angle 0.

Springback angle ∆θ

Usually, the major axis change rate θ and the minor axis change rate s are used to characterize the degree of cross-sectional distortion.

Calculation of distortion degree

Where D is the original outer diameter of the pipe, Dmax is the length of the major axis after the cross-section is distorted, and Dmin is the length of the distorted minor axis. For bending with a mandrel, the material on the side close to the bending die is affected by the mandrel and the bending die, and the deformation is very small and can be ignored. The outer material is only supported by the mandrel, and the distortion is more obvious. For coreless bending, the degree of distortion may be aggravated because there is no support inside. For pipe fittings with severe cross-sectional distortion, when performing coreless bending, the die can be designed with an anti-deformation groove structure to reduce the degree of distortion during bending. For bending with a mandrel, the wear of the mandrel should be checked in time to ensure that the bilateral clearance between the mandrel and the inner wall of the pipe fitting is not greater than 0.3mm, and the appropriate mandrel extension should be set.

Thinning of the outer wall thickness and thickening of the inner wall thickness

During the plastic bending process of the pipe, the outer material is thinned by tension, and the inner material is thickened by compression. The wall thickness thinning rate ∆t1 and thickening rate ∆t2 are usually used to test the change in the wall thickness of the bent pipe:

Calculation of wall thickness change:

Where t is the original wall thickness of the pipe.

When the wall thickness reduction rate ∆t1 is too large, cracks will occur on the outermost wall of the pipe, and the product will not meet the requirements and will be scrapped.

When the wall thickness increase rate ∆t2 is too large, it exceeds the compression instability limit of the material and causes wrinkling, and the pipe fittings will not meet the technical requirements.

The thickness change of the pipe wall is affected by the combined influence of geometric parameters, material parameters and process parameters. Usually, in order to reduce the wall thickness reduction rate, the thrust can be increased on the outside of the pipe to push the material to flow from the non-deformed area to the deformed area to achieve the purpose of reducing the outer wall thinning rate.

Pipe outer cracking

During the plastic bending process of the pipe, the outer wall is prone to cracks or fractures, especially thin-wall bending forming.

The reasons for the cracking of the outer wall are:

1. Improper heat treatment of seamless steel pipe

2. The die pressure is too large, and the material flow resistance is too large during the pipe bending process.

3. The gap between the mandrel and the inner wall of the pipe is too small, resulting in too much friction.

4. Excessive extension of the mandrel, etc.

To prevent the outer wall of the pipe from cracking, in addition to excluding factors such as heat treatment and the material itself, the die pressure, inner wall gap, mandrel extension and lubrication conditions all need to be checked. Wrinkling on the inner bending side of the pipe

Wrinkling of the bent pipe mainly occurs on the inner bending side, which is usually divided into three situations:

1.Wrinkling at the front tangent point: Generally, the elongation of the mandrel is too small when it is installed, and the pipe wall cannot be supported by the mandrel during the bending process. 
2.Wrinkling at the rear tangent point: Generally, the anti-wrinkle die is not installed or the anti-wrinkle die is installed in the wrong position. 
3.Full wrinkling on the inner side of the arc:

The reasons for full wrinkling are more complicated, mainly because:
1.The anti-wrinkle die is located at the back or the shape groove size is too large, and the anti-wrinkle die cannot support the pipe wall. 
2.The die pressure is too small, and the gap between the pipe and the anti-wrinkle die is too large. 
3.The mandrel diameter is too small and the position is unreasonable, etc. 

To prevent the inner side of the bent pipe from wrinkling, if the front tangent point wrinkles, the mandrel position should be adjusted forward; if the rear tangent point wrinkles, an anti-wrinkle die should be installed, and the reasonable inclination angle and die pressure should be adjusted; if all wrinkles, in addition to adjusting the die pressure, the mandrel diameter should also be checked. If the diameter is too small or severely worn, it needs to be replaced in time.