THE PROCESSTo bend sections, there are two main processes – cold bending and induction bending. Cold bending is accomplished by passing the steel member back and forth between sets of rolls. The point load applied by the central roller is sufficient to take the steel past its yield point and introduce a permanent set.
The bending takes place between three points. As the diagram shows, no bending takes place until the section touches all three points or rollers.
When the section, which is driven through the rollers, meets the third roll it will deflect. The deflection will depend upon the position of the rollers.
THE METALLURGY OF CURVING STEELThe more def Cold bending takes the material past its yield point which strain hardens the steel to some degree. Some of the yield plateau has to be used, so in general plastic design is not recommended. The toughness of the steel can also change, particularly at small radii. It is worth emphasising that at most radii found in structural applications, the changes are modest. lection the more the section will bend. I.e. the smaller the radius.
For normal low carbon steels and including structural steel, the strain induced during the bending process produces no real problems, as he material exerts the same elastic characteristics in the elastic range.
As mentioned, during the rolling bending process, the material must have exerted on it a stress greater than its yield strength or elastic limit. This is the maximum stress that the material can be subjected to and still spring back, or return to its original length. The yield point or elastic limits is shown as point ‘A' on the figure below, a typical stress-strain curve. A stress less than the yield strength will not permanently bend the material. The amount of stress to apply to the material being bent is in area ‘C', which is the plastic region. These lines show how, when the stress is removed, the material will spring back to a length somewhat smaller than when the stress was being applied.
The steel sections become work hardened when using the cold bending process. The amount of work hardening is dependent on the radius required and the geometry of the section. The results in a ‘flattened-out' stress-strain curve as shown in the diagram above. A tensile test on a sample of steel that has been cold-roller bent will show a small loss in ductility, but a higher Ultimate Tensile Strength, which results in a loss of some ductility. Even though there is a loss of some ductility, for normal structural applications, the effect is minimal and can be ignored.
Often, the most important effects of the curving process are aesthetic, rather than structural. The steel on the outside of the curve tends to get stretched (and therefore thinner) whilst the steel on the inside of the curve tends to become thicker. There can be some visible distortion on sections caused by the bending process, for example with thin hollow sections, the bending process can cause visible ripples at small radii, therefore our expert staff will more that often advise when it is more appropriate to supply a thicker wall section to reduce the impact caused by the bending process.
MINIMUM RADII AND TOLERANCESThe minimum radius to which a section can be bent without any meaningful distortion depends on the section properties and bending methods being used.
As the years have gone by these minimum radii have been reduced as new techniques have been developed, so the minimum has continued to get smaller.
Normal bending tolerances for single radius bends are in line with those specified in the National Structural Steelwork Specification. Tolerances for multi-radius bends or other complex curves are best discussed with one of our specialists at the design stage.
It is not easy to provide a definitive and comprehensive list of the radii to which every section can be curved. There are large numbers or standard sections (each with different bending characteristics), there are different methods of bending (hot and cold), and the end-uses vary widely. Also, with continuing technical developments, ‘minimum radii' also change.
In general, sections, tubes and hollow sections can be curved to single radius curves, to multi-radius curves, to parabolic or elliptical curves, or even to co-ordinates. They can also, within limits, be curved in two places or to form spirals. There are, however, a number of physical constraints which limits the degree to which three-dimensional curvature is possible in practical terms. It is important therefore, that any requirements for three-dimensional bends are discussed in detail at enquiry stage.