# Surface deformation of pipe bending

## Ovality or distortion

Ripple on the pipe surface is a common problem that occurs during the bending operation.
Understanding the reason behind Ripple can help avoid this problem.
Corrugations are caused by high compressive forces inside thin-walled pipes or tubes.
The thicker pipe wall can withstand the higher pressure inside the elbow without deformation or wrinkles.
With a custom-rolled tube, the material on the outside of the bend will stretch-this will cause the wall to become thinner. The material resists this thinning, so the curved outer surface wants to collapse.
This can lead to defective contour shapes, which we call ellipticity or distortion.

For precision work, the material must not have ovality or distortion, because it may be necessary to splice the elbow to the straight pipe. If the contours of the two mating parts are different, there may be problems with mutual fit. This is especially important when you have a pressure application and need to run something through the pipeline.

## What material to use to avoid deformation

To avoid deformation when bending the pipe, first evaluate the material itself.
There are many sizes of pipes with different wall thicknesses. It is important to remember that the wall thickness can vary nominally.
Certain pipe sizes perform better at minimum wall thickness and above.

Especially during cold rolling, the thin-walled large-diameter tube cannot be bent well, and it is even difficult to bend to a large radius with a slight curvature.
This is because the ratio of the diameter of the material to the wall thickness is too large and transforms into an unstable contour shape.

## Bend pipe method to avoid deformation

Due to its symmetry, the pipe can be bent to a uniform radius without deforming, but several things need to be considered to avoid ripples. Once the best material is determined, the correct machines, tools, and methods must be used to avoid deformation.

### Inside and outside bending radius

Tube bending techniques include, first of all, looking at the inner and outer bending radius of the task-this will help determine the best machines and tools for successful tube bending.

If the pipe needs a tight bending radius, usually the radius is 1 to 8 times the pipe diameter, a mold that matches the shape and size of the pipe should be used for thermal induction, rotary drawing or push-bending operations. These methods provide the most support.

### Bending on a profile bending machine

Bend the tube to any radius (in inches) that is 8 times or more larger than the tube diameter, and should be able to withstand bending on a profile rolling mill with sufficiently thick walls to maintain stability under opposing compression and tension forces .

In the selection table of BIT’s profile bending machine, we have specially marked the minimum radius for bending pipes of various wall thicknesses and diameters.

When the pipe is bent, springback occurs, resulting in radial growth.

With this in mind, pipes are usually slightly overbent or bent to a radius slightly smaller than the radius required for printing or design, depending on the size of the radius and the materials used.

Steel components with exceptionally high tensile strength and yield strength need to be rolled to a radius smaller than the design requirement to achieve the true design radius.

This is because you have to strain the material more to force it to maintain its shape.

In most cases, typical pipe steel grades and pipe sizes do not require severe overbending to achieve the desired results.