Cold bending and hot bending in pipe bending

What is cold bending?

Cold bending is the plastic deformation of metals below the recrystallization temperature of metal processing process (not using any added heat). In most cases of manufacturing, such cold forming is done at room temperature. In fact, it has two concepts, cold roll forming, and cold bending.

Cold roll forming

Sheets and strips of metal material are mechanically bent into profiles of a certain shape and size at room temperature. Its products are called cold-formed profiles. The advantages of cold roll forming are: it can produce all kinds of ultra-thin, ultra-wide, and complex shapes that cannot be produced by rolling; save metal materials; the mechanical properties of the products are good.

Cold bending

The stamping forming process of bending metal sheets, plates, and profiles into workpieces with a certain curvature, shape, and size at room temperature. Bending is widely used in the manufacture of high-pressure vessels, boiler drums, boiler tubes, hull steel plates and ribs, various utensils, instrumentation components, and cabinet inserts.

Although the cold bending deformation is limited to a local area of ​​the material, the rebound effect affects the accuracy of the bending part. There are many factors that affect the spring back, and these factors are difficult to control. The accuracy of the bending part caused by the Springbank has always been the key to cold bending production.

According to the process characteristics, cold bending can be divided into press bending, roll bending, rotary draw bending, and stretch bending.

Press bending

Press bending is the most commonly used bending method. Most of the equipment used are general-purpose mechanical presses or hydraulic presses, and there are also special bending presses.

Roll bending

The commonly used roll bending equipment is the plate bending machine and angle roll/angle roller (aka, profile bending machine). The roll bending machine continuously bends metal profiles or plates according to the principle of three points to determine a circle or arc.

Roll bending is a process whereby we obtain cold process deformation with a wider bend radius that theoretically can range from 5 times the cross-section to infinity. To achieve this process, the equipment used consists of plate bending machines.

Stretch bending

For bending parts with high precision requirements, larger length and radius of curvature, and smaller lateral dimensions, they can be stretched and bent on a certain stretch bending machine. During bending, the entire thickness of the plate is subjected to tensile stress, so only elongation deformation occurs. The deformation caused by a rebound after unloading is small, and it is easy to ensure accuracy.

Rotary Draw Bending

The mold of Rotary Draw Bending is installed on the main shaft; the workpiece is clamped by the clamping mold to prevent the workpiece from moving axially; the pressure mold is composed of a guide mold and a follow-up mold. On the wrinkle die, the movable die part moves with the workpiece when bending; the mandrel fills the inner cavity of the workpiece to prevent wrinkling, flattening, thinning, and other failures during bending.

When the spindle rotates, the workpiece is wound around the bending die to be formed with the rotation of the spindle. Then the workpiece is fed, the space corner is ready for the next bending, and so on. The radius of the bending die determines the bending radius. If you want to get a different bending radius, just replace the bending die with a different radius.

Pros of cold bending and cold roll forming

• No heating required
• Better surface finish obtained
• Superior dimension control
• Better reproducibility and interchangeability of parts
• Improved strength properties
• Directional properties can be minimized

Cons of cold bending and cold roll forming

• Higher forces required for deformation
• Heavier and more powerful equipment required
• Less ductility available
• Metal surfaces must be clean and scale-free
• Strain hardening occurs (may require intermediate anneals)
• Imparted directional properties may be detrimental
• May produce undesirable residual stresses

Before bending pipes and tubes

WHY USE BENT PIPES AND TUBES?

Pipe bending allows systems to minimize pressure changes while still routing materials through complex piping systems.

The sheer variety of pipe bend sizes and materials also make them suitable for routing everything from hot or caustic liquids to maintaining pressure and movement in high viscosity liquids or those with suspended solids, such as Oil Sands slurry lines which contains a high concentration of silica sand.

Finally, as most pipe bending methods are quite cost-efficient, using bent pipes and tubes will have a minimal impact on the overall design budget when using the ideal length and sizing for your application.

As most bent pipes do not alter the ends of the piping, pipe bends are often easy to implement within a processing system using standard welding processes, flanges, or other connection methods.

Video of roll bending pipe

hot bending and cold bending.

Pipe bending is a kind of processing technology in the bending of profiles. Good pipe bending is an art. It requires us to think in many ways before performing the bending.

When bending a pipe, you need to consider many different factors to get the job done as well as possible.

You need to make sure that you have enough experience to do the job well, you have all the right equipment, and you are using the right method to bend.

There are many types of bending, but the simplest classification method is to divide them into hot bending and cold bending.

There are pros and cons to both hot and cold bending, depending on the type of material you need to be bent and the angle of the bend required.

Cold bending methods often rely on sheer physical force to help bring the pipe to its final shape while hot bending methods use careful heating to reduce the force required.

Each method has unique benefits and will determine the degree of bending possible and the final shape of the pipe.

Cold bending of pipe bending

Cold bending is referring to all types of bending that do not use any added heat to bend the pipes. The procedure is usually performed by wrapping a pipe or other material around a die or shape that causes the pipe to bend.

Cold bending of pipe bending procedure is rather fast and even more so as it doesn’t need to cool down or be taken care of in any particular way after the bending has been performed. As the machines used for cold bending aren’t very expensive in general, due to not being particularly complex or advanced, cold bending can be a good option for smaller companies that don’t have a large budget. It can also be a good method for companies that aren’t specialized in bending but only needs to perform it every once in a while.

The negative aspect of all types of cold bending is that it’s not possible to bend too radically. In most instances, you will have to fill the machine with filler material, like sand, for the pipe not to crease or break due to a radical angle.

Cold bending machine vs hot bending machine

Normally, the workpieces are bent under cold conditions (cold pipe bending).

The workpiece has to be heated only when working with very thick sheet metal or very small bending radii, in order to keep the required bending forces down and to avoid brittleness of the material due to low temperature.

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.

WHO NEEDS COLD BENDING?

Cold bent steel has many applications.

• Curved steel formed by cold bending is frequently used in the construction of buildings and bridges and is especially impressive when left exposed to view;
• Shipbuilders, railroads, and automobile manufacturers also use cold-bent steel products;
• The petrochemical industry uses curved and coiled piping for processing and transporting their products;
• Cold bent steel also has many other industrial and food-processing uses as well.

Features of pipe bending for cold bending

The cold bending process is usually performed by wrapping a tube or other material around a mold or shape, causing the tube to bend. The process is quite fast, or even faster because there is no need for cooling or processing in any particular way after performing the bending.

Since the machines used for cold bending are usually not very expensive, and because they are not particularly complex or advanced, cold bending is a good choice for small companies without a large budget.

This is also a good method for companies that do not specialize in bending but only need to perform it every once in a while.

Advantages of cold bending of pipe bending

Metal properties

Due to the work hardening of the metal during cold bending, the metal tube after cold bending is much harder than the metal tube after hot bending, but cold bending will not destroy the original properties of the metal. After cold bending, there is no need to clean and remove the oxide scale, and there will be no thermal deformation.

Energy consumption

Compared with hot bending, cold bending requires more bending power, and the spring back and residual stress are significantly increased. And cold bending cannot bend sharp bends with a small radius of curvature.

Disadvantages of cold bending

The downside of all types of cold bending is that it is impossible to bend completely.

Hot bending

Hot bending generally refers to different types of induction bending. Hot bending is highly effective at bending pipes because they are fast, precise, and make few errors.

The induction bending process, also known as high-frequency bending, incremental bending, or hot bending, uses inductors to locally heat steel by induction. This results in a narrow heat band in the shape to be bent. The shape is firmly held by a clamp at the desired radius, which is mounted on a free pivoting arm. The shape is pushed through the inductor by an accurate drive system which causes the hot section to form the induction bend at the set radius. The bent part is then cooled by water, forced or still air to fix the bent shape.

Hot bending of the pipe bending

Hot bending is generally only referring to different types of induction bending.

Induction bending is a highly effective way of pipe bending, as it is fast, precise, and with few errors. The induction bending process is performed by heating a certain point of the pipe up to where it then can be bent without much effort. It doesn’t require any filler material and the result of the bending tends to keep distortion to a minimum.

Many induction benders have also chosen this type of bending because of its energy sufficiency. After the heating process has been done, the bending doesn’t take a lot of time at all.

Features of pipe bending for Hot bending

Induction bending is a very effective method of pipe bending because it is fast, accurate, and almost error-free.

The induction bending process is performed by heating a certain point of the pipe, which can then be bent effortlessly. It does not require any filling material, and the result of bending tends to keep deformation to a minimum.

Many induction bending machines also choose this type of bending because of its sufficient energy. The heating process is the most time-consuming element of the process, after the heating process is completed, bending does not require much time at all.

Advantages of hot bending of pipe bending

Hot bending has the incomparable adaptability of cold bending.

• For example, the straight line distance between two adjacent elbows on a pipe can be kept small, and even continuous bending can be carried out without leaving straight pipe sections;
• Can process materials with poor cold ductility into elbows;
• It can process elbows that require a lot of mechanical energy during cold bending and can bend brittle materials that are easy to break during cold bending. Hot bending can be bent into a small radius elbow on the pipe.
• For carbon steel pipes and most alloy steel pipes, the bending radius of hot bending is much smaller than that of cold bending, and the bending radius can be as small as 0.7 to 1.5 times the outer diameter of the pipe.

Disadvantages of hot bending

• The downside of hot bending may be that the material must be cooled later, increasing the time spent on each pipe, and the machines tend to be more expensive than cold bending equipment.
• The negative aspects of hot bending can be that the material does have to cool off afterward, adding to the time spent on each pipe and that the machines tend to be more expensive than cold bending appliances.
• The equipment is complex, the processing cost is high, the production efficiency is low, and the surface finish is poor.
• For copper pipes, the cold bending process is used, which eliminates the possibility of “hydrogen disease” due to the elimination of high-temperature heating.

Induction Pipes and tubes Bending

Hot bending or induction bending:
While there are slight variances to different hot pipe bending methods, nearly all are a form of induction bending.

This method precisely heats the pipe using an induction heating coil before applying pressure to make the intended bend.

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.

What is Induction Bending?

Induction Bending is a precisely controlled and efficient piping bending technique. Local heating using high frequency induced electrical power is applied during the induction bending process. Pipes, tubes, and even structural shapes (channels, W & H sections) can be bent efficiently in an induction bending machine. Induction bending is also known as hot bending, incremental bending, or high-frequency bending. For bigger pipe diameters, when cold bending methods are limited, Induction bending 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.

Induction Bending Process

The following steps are performed for induction bending of piping or pipeline system:

• The pre-inspected pipe or pipeline to be bent is placed in the machine bed and clamped hydraulically.
• Around the pipe, induction heating coils and cooling coils are mounted. To ensure uniform heating, the induction coil can be adjusted with a 3-plane movement.
• By adjusting the radius arm and front clamp, the required bend radius can be fixed. There is one pointer to display the correct degree of turning.
• Arc lengths are marked on the pipe. The pipe can be moved slowly whilst the bending force is applied by a fixed radius arm arrangement.
• Once everything is set as required, hydraulic pressure, water level, and switches are inspected and then the induction bending operation is started.
• Upon reaching the required temperature range, the pipe is pushed forward slowly at a speed of 10-40 mm/min, and the operation is stopped when the specified bend angle and pre-determined arc length is reached.
• Just beyond the induction coil, the heated pipe material is quenched using a water spray on the outside surface of the pipe.
• In the next step, the induction bend is removed and sent for inspection and measurement of tolerances.
• The final step for the induction bends is the use of post-bend heat treatments for stress relieving, normalizing, etc.

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. Fig. 2 below provides a schematic diagram of the induction bending mechanism.

Seven important parameters that affect the induction bending square pipe process

• Pipe Diameter
• Surface Contamination
• Process Parameters like Temperature, Speed, Cooling rate, etc
• Bend Radius
• Bend Angle
• Process Interruptions
• Hardenability of the Pipe Material, etc.

Induction Bending Standards

As the complex induction bending process involves various steps for producing bends, it must be controlled precisely to produce quality items. Different codes and standards govern this process. The most conventional and widely used standards for the induction bends are the ASME B16.49 and ISO 15590-1(en).

Advantages of Induction bending

The major advantages of induction bending are:

• Lower

• Cost efficiency. Straight material is less costly than standard components (e.g. elbows) and bends can be produced faster than standard components can be welded.
• Elbows can often be replaced by larger radius induction bends, reducing friction, wear, and required pump capacity.
• Induction bending reduces the number of welds in a system.
• No welds at the critical points thanks to the tangents.
• Less non-destructive testing, saving cost.
• Induction bends are stronger than elbows with uniform wall thickness
• The stock of elbows and standard bends can be greatly reduced.
• Straight pipe is more readily available than elbows, reducing time to market
• Induction bends can be made from the same base material as the straight pipe.
• Lower risk of wall thinning and deformation of the cross-section
• Thin-walled pipes can easily be bent.
• Uniform hardness and thickness.
• No pipe wrinkles.
• Only a straight pipe is required for induction bending.
• Precise bending radius and angle.
• Induction bending does not need bend dies or mandrels. A simple clamping/ inductor set covers a wide range of radii and wall thicknesses.
• The induction bending process is a clean process. No lubricants were necessary.
• Diverse bendings: square pipe, flat bar, I-beam, H-beam, channel section, etc.

Applications of Induction bends

The majority of the Induction bends are found in the pipeline systems for liquid and gas transportation. Additionally, they are found in applications requiring large diameter bends with precision and reliability and where the laminar smooth flow is required. Typical applications of induction bends include the following industries:

Induction bends find their way into the following industries:

• Petrochemical
• Chemical
• Power Generation (conventional and nuclear)
• Oil and gas (incl. expansion joints)
• Compressor and pump stations (fluids and gasses)
• Offshore
• Shipbuilding
• Construction

Induction Bending Materials

The induction bending technology allows the bending of an almost unlimited variety of materials.

The only requirement is that they can be heated by induction.

Common material groups are:

Carbon steels

• Low alloyed steels
• High alloyed steels
• Fine-grain steels

Stainless steels

• Austenitic
• Martensitic
• Ferritic
• Duplex

Other

• Special alloys
• Clad pipe
• Aluminum
• Titanium

Getting the Bending Formula Right

Another important step when preparing for bending is formula calculation. When you spend time calculating things like the cross-sectional area moment of inertia of the shape and size tube, you will bend, which can save a lot of time and effort. Make sure that you give yourself enough time to complete all the bending formulas that will help you set up the machine correctly and prevent many common tube bending problems before they occur.

PIPE BEND SIZING AND FIT

In most cases, pipe bends are measured in relation to the nominal pipe size or diameter (D).

Long radius elbows, for example, have an end-to-center dimension equal to 1.5 times the diameter (sometimes noted as >1.5D).

Short radius elbows feature an end-to-center dimension equal to the pipe diameter.

You can determine the radius of the centerline of bent pipes and tubes by multiplying the D designation by the diameter of the pipe.

For example, a 5D pipe with a 10-inch D will have a centerline radius of 50 inches.

180-degree Pipe Bends use a different measurement based on the center-to-center dimension to allow for a better idea of the space required and how the pipe bends will fit into the system.

Similar to elbows, multiplying the diameter of 180-degree pipe bends by the D designation will give you the center-to-center dimension.

Short radius 180-degree pipe bends are 2D while long radius pipe bends are 3D.

This means a 4-inch short radius pipe would have a center-to-center dimension of 8-inches, while the same 4-inch pipe with a long radius bend would have a center-to-center dimension of 12-inches.

Whether you’re looking at elbows or 180-degree bends, the tangent ends of induction bent pipe are often unaffected by the bending process and can be matched to existing piping by diameter, flange, valve, or fitting requirements.

While fitting and sizing bent pipes might seem complex at first, a basic understanding of the measurements used makes matching them to your existing system or integrating them into a new design straightforward.

Further reading:

PIPE ENDS: EXPLAINED

While size is an important factor when choosing flanges, elbows, and other components of your piping process, pipe ends are a critical consideration to ensure a proper fit, a tight seal, and optimal performance.

COMMON PIPE ENDS

The type of pipe end chosen will determine how it connects to other components and which applications and components the pipe is best suited for.

Pipe ends typically fall into one of four categories:

Plain Ends (PE)
Threaded Ends (TE)
Beveled Ends (BW)
Grooved Mechanical Joints or Grooved Ends
A single pipe can also have multiple end types. This is often designated in the pipe description or label.

For example, a 3/4-inch SMLS Schedule 80s A/SA312-TP316L TOE pipe has threads on one end (TOE) and is plain on the other.

In contrast, a 3/4-inch SMLS Schedule 80s A/SA312-TP316L TBE pipe has threads on both ends (TBE).

PLAIN END (PE) PIPE USES AND CONSIDERATIONS

PE pipes feature ends typically cut at a 90-degree angle to the pipe run for a flat, even termination.

In most cases, plain end pipes are used in combination with slip-on flanges and socket weld fittings and flanges.

Both styles require fillet welding on either one or both sides of the fitting or flange and at the base of the fitting or flange.

Where applicable, the plain end will be placed typically ⅛” from where the pipe rests to allow for thermal expansion during welding.

This makes them ideal for small diameter piping systems.

THREADED END (TE) PIPE USES AND CONSIDERATIONS

Typically used for pipes with a nominal size of three-inch or smaller, TE pipes allow for an excellent seal.

Most pipes use the National Pipe Thread (NPT) standard which describes the tapered threads used on the pipe with the most common taper measuring 3/4-inch per foot.

This taper allows the threads to pull tight and create a more effective seal.

However, connecting the threads on a TE pipe properly is essential to avoid damaging pipes, fittings, or flanges.

Improper assembly or disassembly may lead to galling or seizing.

Once unseized, damage to the threads or pipe could further reduce corrosion resistance and hygienic properties — two popular reasons for choosing stainless steel pipe.

Fortunately, avoiding these concerns is often as simple as preparing the threads before assembly.

We recommend and sell Unasco stainless steel thread sealing tape.

Impregnated with nickel powder, the tape keeps the surface of male and female thread ends separately while also lubricating the connection for easier assembly and disassembly.

BEVELLED END (BW) PIPE USES AND CONSIDERATIONS

Used with buttwelding, BW pipe fittings commonly feature a 37.5-degree bevel.

These bevels are often applied by fabricators by hand or through automated processes to ensure consistency.

This allows for a perfect match with BW pipe fittings and flanges and easier welding.

GROOVED END PIPE USES AND CONSIDERATIONS

Grooved mechanical joints or grooved end pipes use a formed or machined groove at the end of the pipe to seat a gasket.

Housing around the gasket is then tightened to secure the connection and ensure optimal seal and performance.

The design allows for easier disassembly with a reduced risk of damaging piping components.

COMMON PIPE END ABBREVIATIONS AND STANDARDS

Pipe end connections typically used for pipe nipples — are often denoted using abbreviations.

In most cases, the first letter denotes the type of end users while the following letters let you know which ends are finished.

Common abbreviations include:

1. BE: Bevel End
2. BBE: Bevel Both Ends
3. BLE: Bevel Large End
4. BOE: Bevel One End
5. BSE: Bevel Small End
6. BW: Buttweld End
7. PE: Plain End
8. PBE: Plain Both Ends
9. POE: Plain One End
10. TE: Thread End
11. TBE: Thread Both Ends
12. TLE: Thread Large End
13. TOE: Thread One End
14. TSE: Thread Small End

If you’d like to know more about common pipe end standards, you can find information from ASME below:

• ASME B1.1 – Unified Inch Screw Threads
• ASME B16.25 – Buttwelding Ends

China’s marine elbow technical conditions

• The pipe bending should be cold bending as far as possible, and hot bending is only allowed under the following conditions:
• The bending radius of the pipe is smaller than the bending radius specified by the cold bending, or smaller than the bending radius of the existing mold.
• The pipe shape is complicated or there is no straight pipe section between the elbows, so it cannot be tightened on the pipe bender.
• If the tube wall is too thin, it is prone to large collapse and wrinkles after cold bending.
• For pipes with larger diameters or infrequently used, there is no such mold at present.
• When the pipe wall is too thick to be cold-formed.

THEORY OF THE COLD PIPE BENDING PROCESS

VVideo of cold-formed Φ168mm steel pipe (Malaysia)

Video of cold-formed Φ510mm steel pipe (UAE)

Cold pipe bending machine

PBH & MS Series cold Pipe bending Machine

Can bend square pipe/tube into circle or arc.

Focus on pipe diameters for bending up to 20 inches, with cross-section bending combination mold can bend various profiles, multi-purpose section bending machine.

• Pipe. Dia (mm): Ø25-510
• Max.Wall thickness (mm): 1.5-20
• Bending. DIA(mm): Φ400-10000

Note: A larger size cold pipe roll bending machine can be customized.

MORE DETAILED PARAMETERS

induction bending machine

It is used for all kinds of round or square steel pipes, stainless steel pipes, I-beam, H-beam, U-channel hot bending, suitable for petroleum, chemical industry, metallurgy, steel structure, boilers, and other projects.

MORE DETAILED PARAMETERS