Cold bending and hot bending
While there is metal profile bending multiple methods available, most fall into one of two categories:
- Cold bending
- Hot bending
Cold bending methods often rely on sheer physical force to help bring the metal profile(e.g. pipe beams) to its final shape while hot bending methods use careful heating to reduce the force required.
There are pros and cons to both the hot and cold bending processes, depending on the type of material you need to be bent and the angle of the bend required will determine the degree of bending possible and the final shape of the metal profile.
Cold bending is done by machines without inducing any heat into the material to be bent. These machines will generally wrap a pipe or sheet of material around a shape with a force that causes the pipe to bend.
This process is quick and the materials don’t need to cool down afterward; however, cold bending can have disadvantages if materials are fragile. Radical bends are also not possible with cold bending. In most cases, the machine will need to be filled with sand to achieve any bend that could be considered extreme.
Cold bending machinery is usually lower in cost than hot bending machinery, as it is less complex. This can make it an attractive option for smaller companies.
4 methods of the cold bending process
- Rotary Draw Bending: A pipe or tube is bent using a combination of dies and other various components working in a rotary action. This action draws the pipe or tube forward making the desired bend. Rotary draw bending can also utilize mandrels.
- Mandrel Bending: A mandrel is placed inside the tube or pipe that is being bent, especially with thinner wall materials, to prevent defects that can occur in the piece’s bend such as rippling, flattening, or collapse.
- Compression Bending: Bending a pipe or tube using a stationary die while a counter die bends the material around the stationary die.
- Roll Bending: Used when large radius bends or curves are required, this method passes a piece of pipe or tube through a series of three rollers in a pyramid configuration to achieve the desired curve.
Benefits of Cold Bending
Cold bending does not require the use of fuel to heat the steel before processing, and the additional time and effort of heating and then cooling the steel are eliminated.
Cold rolling results in a smoother, more finished surface and generally causes less deformation of the item being processed.
One additional benefit is the increased strength imparted by working the steel while cold. When steel is manufactured at high heat and then allowed to cool it develops an internal crystalline arrangement. Working steel at ambient temperatures below the point of crystallization has been shown to enhance strength at a molecular level by compressing and distorting the crystalline structure. As the molecules become compacted they are unable to move as easily and, therefore, the steel becomes stronger.
Cold Bending for Different Steel Configurations
Any cross-section and size of steel can be cold bent or rolled as long as it can fit into existing dies and rollers. Cold bending is most commonly applied to pipes of less than 10 inches in diameter, channels, I-beams, angles, and rectangular, round, and half-round bars. Large scale steel products such as plates can also be worked but sizes are limited due to the force necessary and the size of the rolling equipment needed. The bending process can be used to create gentle, large-diameter curves, 90ｰ corners or long series of pipe coils in which the pipe bends in a succession of 360ｰ circular turns.
Why Cold Bending is Necessary
Cold bending, particularly when it is done using a roll bender, can be used when there is no way to heat metal before it can be bent. It is also utilized when it is necessary to have a large, smooth bend in a pipe or with sheet metal. Pipes, in particular, need to behave gentler curves in order to accommodate water or other fluids that may pass through them, and cold bending is sometimes the only way to create these curves and large bends.
Cold bending is also cheaper in many cases; the equipment and materials needed to work with heated metal are expensive. By working with cold metal whenever possible, pipe fabricators and metal workers can save on production costs. Finally, some metalworkers find that cold bending creates more aesthetically pleasing final products, with smoother finishes and gentler curves that often aren’t possible through other methods.
Hot bending for pipe
For bigger pipe diameters, when cold bending methods are limited, Induction bending of pipe is the most preferable option. Around the pipe to be bent, an induction coil is placed that heats the pipe circumference in the range of 850 – 1100 degrees Celsius. By heating a certain point on a pipe, induction bending enables the pipe to be bent without very much effort. No filler is required in the machine, which prevents a good deal of distortion. The heating process is the most time-consuming element of the process; however, after this, bending is quickly achieved.
- It requires much less physical force than cold bending methods and can produce bends of similar or higher quality with no filler materials, mandrils, or other additions used to avoid distortion.
- While it minimizes diameter reduction at the bend site, induction bending causes some changes in pipe thickness.
- Typically, the intrados — or inner section of the bend — will become thicker while the extrados — or outer section of the bend will become thinner.
- The method is most often used in large diameter piping and tubing and long radius bends.
- However, it also has uses in smaller piping diameters and short radius bends.
Induction bends are normally produced at standard bend angles (e.g. 45°, 90°, etc.). However, depending on the requirement they can be custom-made to specific bend angles. Compound out-of-plane bends in a single joint of pipe can also be produced. The bend radius for induction bending is specified as a function of the nominal pipe diameter (D) like 5D, 30D, 60D bends, etc.
Hot Bending vs Cold Bending
Working steel at high heat above the point of crystallization means that crystallization will occur after the steel has been worked and therefore the steel will not be any stronger than unworked steel. Heating steel that has already been cold-bent or rolled will cause the material to lose the extra strengthening gained from the process by allowing the steel to regain its internal crystalline structure. The trade-off between cold and hot working processes is that in exchange for the added strength of cold working the steel can become more brittle, while heat worked steel generally retains greater ductility.
The main differences between Hot bending and Cold Bending are listed below in a tabular format:
|Induction Bending / Hot Bending||Cold Bending|
|Heat Input is a must in induction or hot bending||No added heat|
|Slow Process||Fast Process as no heating & cooling involved|
|Better control||Less control|
|Highly efficient complex types of machinery are used for hot bending.||Simple machines|
|The hot bending Process is efficient for larger pipe diameters||Not suitable for large pipe diameters. Cold bending is limited to smaller diameter piping only.|
|Induction bending is a Costly process||Cold bending is not expensive.|
|Normally no wrinkling on the pipe surface.||High probability of wrinkling on the pipe surface|
|Required force in hot bending is normally less||Physical Force requirement in cold bending is comparatively more|
When hot bending is used, it is often very time-consuming, labor-intensive, and difficult to do accurately. The operator doesn’t know where the camber will end up until the steel is cooled and fully contracted. If the camber was unsuccessful, the entire process of heating has to be redone. If the camber was too much, it also creates a problem trying to reverse the camber.
With a much less labor-intensive process, the entire process of cambering a beam can be done in a matter of minutes, rather than hours. Any amount of labor and material savings will increase your bottom line. By adding a cambering unit to your operation and production line, your savings are compounded. Take for example a bundle of W24 beams X 30 feet long with a required camber of 50mm.
With the cold bending equipment, one operator could process 40 of these beams in 4 hours, while with the heat method it will be as much as 24 hours plus consumables.