Why need a cold bending process?
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 conditions (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 workpiece breaks.
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.
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 alterations of shape occurring when metal pipes are being bent are mainly determined by the material-specific parameters modulus of elasticity and yield stress.
Due to the elastic-plastic behavior of metallic materials, the pipe springs back at 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 workpiece, and formation of wrinkles.
The elasticity is the reason for the spring-back of the pipe after the bending 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 the work of elastic strain in the form of resiliency. But after the external strain has been removed, it is partly dissipated as a 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 compensated by overbending the workpiece.
In the so-called bending curve (see fig.4) the spring-back, depending on the bending angle with otherwise identical bending parameters, is displayed. Always the same typical progression is recognizable. A steeply rising 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 radius, but this can already be considered when fabricating the bending tools.
While bending around 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.
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 the opposite direction, thus favoring a compression of the original circular cross-section.
- The measuring size for oval distortion is eccentricity.
- The oval distortion grows stronger if thinner pipe walls and smaller bending radii of the workpiece 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 the inner pressure.
Unlengthened layer and the stress-free layer
With every bending process, the inner layers of the workpiece 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 differentiate 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 (theoretical bending radius) but is displaced in direction 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.
An overview of the geometric terms and relationships concerning bent pipes can be seen in fig.7.
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 a tangent, where it is no longer supported by the bend die and therefore susceptible to wrinkling.
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 a tangent as closely as possible, however without ex- ceeding it. The flow of the material behind the line of tangent is avoided, thus minimizing 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.
Bending is impossible below the bending limit determined by stretching and it comes to workpiece failure as a result. The bending limit due to wrinkling 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 the inclination of the pipe to gather wrinkles while bending.
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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. The 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 is 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
The 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 of fewer 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.