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.
How To Avoid Corrugation On The Surface Of The Bent Pipe
It’s worth noting that there are distinctions when specifying pipe and tubing. Unlike tubing, the pipe isn’t identified by its outer diameter (OD) size and doesn’t come with easily discernible thickness information.
Similar to square and rectangular tubing, both round tubing and pipe can undergo bending using a variety of processes available with the BIT series pipe rolling machine. These processes include draw bending, roll bending, incremental hot bending, incremental cold bending, and induction bending.
The PBH series section bending machine excels at bending a wide range of pipes and tubing, encompassing aluminum, stainless steel, and carbon steel. It can also handle specialty materials like zirconium and titanium. For reference, carbon pipes (mill-produced) fall under ASTM A53 classification, usually resulting in a minimum yield point of 30,000 psi for grade A and 35,000 psi for grade B. Carbon tubing (mill-produced) is commonly categorized as ASTM A500B, yielding a minimum of 46,000 psi for Grade B.
Related Reading: Similarities Between Bending Round and Rectangular Tubing
Pipes and tubing serve a multitude of applications, with typical uses including railings, recreational items such as playgrounds, bike racks, and goalposts, industrial purposes like boiler tubes, heat exchangers, and piping systems, as well as the creation of curved components for canopies and awnings.
Related Reading： Square and rectangular tubing is also known as HSS (hollow structural steel).
Why Are There Rippling On The Surface When Bending Pipes
Rippling on the surface of a pipe is a somewhat common problem that occurs during bending operations.
Ripples are caused by high levels of compressive force on the inside of a pipe or tube with thin wall thickness. Thicker pipe walls can withstand higher compressive forces on the inside of a bend without distorting or wrinkling. With custom rolled pipe, the material on the outside of the bend stretches – this causes wall thinning. The material resists this thinning and as a result, the outside surface of the bend wants to cave in.
For precision work, the material cannot have ovality or distortion because there may be a need to splice a bent pipe into a straight pipe. If the profiles of these two mating parts are different, there may be an issue fitting them up to each other.
To avoid distortion when bending pipe, it starts with evaluating the material itself. The pipe comes in many sizes with varying wall thicknesses. It is important to keep in mind that the wall thickness can have nominal variations.
Some pipe sizes perform better in minimum wall thicknesses and above. With cold rolling specifically, a large diameter pipe with a thin wall does not bend well and can even be difficult to bend to large radiuses with a slight curvature. This is because the ratio of diameter to wall thickness of the material is too large and translates to an unstable profile shape.
How To Avoid Corrugation On The Surface Of The Bent Pipe
Pipe bending techniques include, first, looking at the inside and outside bend radii of the task – this will help identify the best machine and tooling to successfully bend pipe. If the pipe requires a tight bend radius, usually with a radius from 1 to 8 times the diameter of the pipe, then a heat induction, rotary draw, or push bending operation with a die matching the shape and size of the pipe should be used. These methods offer the most support.
Bending pipe to any radius 8 or more times larger than the pipe diameter in inches should be able to withstand bending on a section rolling machine with a thick enough wall for stability under opposing compressive and tensile forces.
When bending the pipe, spring back can occur leading to radial growth. To account for this, pipes are usually slightly over-bent or bent to a slightly smaller radius than the print or design requires depending on how small the radius is and what material is being used. Steel members with abnormally high tensile and yield strengths require rolling to a much tighter radius than the design requires to achieve the true design radius. This is because you must strain the material more to force it to hold a shape. Typical pipe steel grades and pipe sizes don’t require severe over-bending in most cases to achieve the desired results.
