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Cold pipe bending process

Profile bending machine bending tubes

Almost every industry has a large amount of cold bending demand.
Steel and other metals can be formed in a variety of ways. Cold bending is one of the most common methods of shaping steel into many different configurations.

Theory of the Cold pipe Bending Process

When metallic working materials are bent in cold condition (below their recrystallization temperature), at first an elastic shape alteration takes place, which is replaced by a ductile shape alteration from a certain degree on. If the reshaping capacity is run down, the work piece breaks.

Elastic-plastic behaviour

This elastic-plastic behaviour of metallic materials is reflected in the stress-strain- diagram (see fig.2) determined by tensile tests. Within the range of the elastic line (Hooke’s law), a tensile sample is reshaped elastically; as soon as the strain is relieved, the body returns to its original shape.

elastic-plastic behaviour of cold pipe bending
elastic-plastic behaviour of cold pipe bending

However, if the applied tension exceeds the elasticity limit, the shape of the sample is permanently altered. The degree of resiliency after the strain has been removed, results from the elastic part of the shape altering process, which has been stored in the sample as potential energy beforehand.

The altera- tions of shape occuring when metal pipes are being bent are mainly deter-mined by the material-specific parameters modulus of elasticity and yield stress.

Spring-back

Due to the elastic-plastic behaviour of metallic materials, the pipe springs back by a certain angle after every bending attempt.

Besides the resiliency, there are also other inevitable phenomena to regard, where shaping by bending is concerned: the spring-back of the radius, oval deformation of the cross section (round pipes and tubes) as well as changes in the length of the work piece and formation of wrinkles.

spring-back of cold pipe bending

The elasticity is the reason for the spring-back of the pipe after the bend-ing process has been completed (fig.3). While in the valid range of “Hooke’s law” (elastic line), the shaping energy is completely given back as work of elastic strain in the form of resiliency. But after the external strain has been re-moved, it is partly dissipated as work of plasticity when per- forming the elastic-plastic shaping.

In this case, the extent of spring-back is only caused by the elastic (reversible) part of the shaping work, which is stored in the pipe as potential energy during the bending process. Spring- back is an inevitable phenomenon of bending, and can only be compen- sated by overbending the work piece.

spring-back curve of cold pipe bending

In the so-called bending curve (see fig.4) the spring-back, depending on the bending angle with otherwise identical bending parameters, is dis- played. Always the same typical progression is recognisable. A steeply ris- ing linear phase (purely elastic forming) is followed by a non-linear phase (elastic-plastic bending phase, plasticizing in cross section) and then by the weak linear rise of a further range (plasticizing in the longitudinal section only) up to the end of the bending process. Spring-back of the pipe after relief of the strain is also followed by a slight increase of the bending ra- dius, but this can already be considered when fabricating the bending tools.

Oval distortion

While bending round pipes, radial components of the longitudinal bending stress lead to oval distortion of the circular pipe cross section. The outer side of the bend has an inclination to the central line, thus flattening the pipe.

oval distortion of cold pipe bending

Regarding the equilibrium of forces active during the bending process (see fig.5) you can see, that the pressure forces resulting from the bending moment in the inner area of the pipe bend and the traction forces in the outer area of the pipe bend work in opposite direction, thus favouring a compression of the original circular cross section.

oval distortion of cold pipe bending
  • The measuring size for oval distortion is the eccentricity.
  • The oval distortion grows stronger, if thinner pipe walls and smaller bending radii of the work piece have been selected.
  • The alteration of the cross section shape has an influence on the free circulation cross-section and the consistency of the pipe when exposed to inner pressure.

Unlengthened layer and the stress-free layer

With every bending process, the inner layers of the work piece suffer pres-sure stress in connection with material compression, while the outer layers are exposed to tensile stress and stretched in the direction of the leg. Under the consideration of plastic bending, we must differ between the unlengthened layer (neutral axis) and the stress-free layer.

The unlengthened layer has maintained its original length after the bending process is completed, permanent stretching equals zero. The position of this layer does not correspond with the neutral circular arc layer (theoreti- cal bending radius), but is displaced indirection of the bending axis.

For this reason, every pipe suffers a certain elongation during the bending process, but it is possible to approximately determine the corresponding cutting length with the help of mathematical calculation. The stress-free layer is positioned even further inside, it is the layer, which shows no longitudinal stress at all after ductile forming.

nlengthened layer (neutral axis) and the stress-free layer.

An overview of the geometric terms and relationships concerning bent pipes can be seen in fig.7.

Wiper die

If thin-walled pipes are bent to small radii using the rotary draw bending method, the material on the inside of the bend is pressed back behind the line of tangent, where it is no longer supported by the bend die and there- fore susceptible to wrinkling.

Cold pipe bending process

This unwanted phenomenon is best avoided by using a wiper die (comp. fig.11).

The wiper die is a form part, which is mounted inside the bend behind the bend die and has a sharped-edged end, which is placed in positive fit into the pipe groove of the bend die and pushed to the line of tangent as closely as possible, however without ex- ceeding it. Flow of the material behind the line of tangent is avoided, thus minimising wrinkling.

However, if wrinkles have already been formed, they cannot be eliminated after bending.

Whether a pipe with defined dimensions can be bent at all?

Whether a pipe with defined dimensions (outer diameter and wall thickness) can be bent at all, can be drawn from material-specific graphs like in fig.8.

Cold pipe bending process

Bending is impossible below the bending limit determined by stretching and it comes to work piece failure as a result. The bending limit due to wrin- kling separates the range, where bending with mandrel (and wiper die) is possible, from the range, in which the pipes can also be bent without a mandrel.

The larger the relation between the outer diameter and the wall thickness of the pipe and the smaller the bending radius, the stronger is the inclination of the pipe to gather wrinkles while bending.

Three minutes to read

Hot and cold forming

Heat can be used for forming tube rolling to soften the metal and make the forming easier to bend.

Cold forming is also used for form-constrained rolling. Presses usually do not use heat to bend most of the soft, malleable metal around the shape.

Heat can also help free-form scrolling. Large diameter or thicker pipes are usually softened by heating when they are rolled through a free-form press. For example, heavy lead pipe or iron pipe needs some heat to bend. However, many times free-form rolling is cold-formed.
Sometimes, in the complex bends in free-form rolling, a mixture of the two is useful. A part of the tube is heated with a torch or hot air gun before being freely formed by cold pressing. The use of heat is at the discretion of the technicians and only used when the integrity of the pipeline is not compromised.

Cold bending process

The processing of cold-formed steel is actually carried out at room temperature. It is called cold bending to distinguish it from the hot bending process, in which the steel is heated by a torch or furnace before forming. This process usually uses rollers to press a piece of steel onto a metal forming tool called a mold. It can also be called cold rolling or pyramid rolling (such as metal profile bending using BIT series profile bending machines).

Benefits of cold bending

Cold bending does not require the use of fuel to heat the steel before processing, saving the extra time and effort of heating and then cooling the steel.

Cold rolling produces a smoother, smoother surface, and usually results in less deformation of the processed article.

An additional benefit is the increased strength when cold working steel.

When steel is manufactured at high temperatures and then cooled, it forms an internal crystal arrangement.

Processing steel at ambient temperatures below the crystallization point has been shown to increase strength at the molecular level by compressing and distorting the crystal structure. As the molecules get closer, they cannot move easily, so the steel becomes stronger.

Processing steel at high temperatures above the crystallization point means that crystallization will occur after processing the steel, so the steel will not be stronger than unprocessed steel. Heating steel that has been cold-formed or rolled will cause the material to lose the extra strengthening gained from the process, allowing the steel to regain its internal crystal structure.

The trade-off between the cold working process and the hot working process is that in exchange for the increase in cold working strength, the steel becomes more brittle, while the hot working steel generally maintains greater ductility. Heating steel that has been cold-formed or rolled will cause the material to lose the extra strengthening gained from the process, allowing the steel to regain its internal crystal structure.

Cold bending of different steel structures

Steel of any cross section and size can be cold-formed or rolled, as long as it can be fitted into existing molds and rolls.

Cold bending is most commonly used for pipes, channels, I-beams, angles and rectangular, round and semi-circular steel bars with a diameter less than 10 inches.

It is also possible to process large steel products, such as plates, but the size is limited due to the required force and the size of the rolling equipment required.

The bending process can be used to create gentle large-diameter curves, 90° angles, or long series of coils, where the pipe is continuously bent in a 360° circle.

Uses of cold-formed steel

Cold-formed steel has many uses.

Bending steel formed by cold bending is often used in the construction of buildings and bridges, and is especially impressive when exposed. Shipyards, railway and automobile manufacturers also use cold-formed steel products.

The petrochemical industry uses bent and coiled pipes to process and transport its products. Cold-formed steel has many other industrial and food processing applications.

Minimal deformation to form steel

Cold bending is an excellent method for forming steel with minimal deformation during the bending process.

Cold-formed steel can be used in a variety of applications from common daily necessities to high-tech professional industrial applications.