Summary of advantages and disadvantages of high frequency welded pipe forming methods

The high-frequency welded pipe forming roller is an important part of the welded pipe unit and the technical core of the welded pipe forming process. The forming of high-frequency welded pipe needs to go through five basic processes: edge bending - overall bending - closed introduction - extrusion welding - shaping. In the welded pipe industry, the process preceding high-frequency welding is often called preforming.

 Edge shaping
Edge forming is the first step in the high-frequency welded pipe forming process. Its purpose is to bend the edge of the strip to the curvature of the radius of the extrusion roller. The traditional process uses single radius forming, and the modern process uses double radius forming.
The initial double-radius forming was just a simple double-radius upward bend, and later developed into W forming. The bending of the middle part of this molding method uses reverse protrusions to bend the edges more fully and form a line around the hole during molding. The speed difference is very small.
Edge forming generally uses the solid bending method. In cold bending, solid bending usually refers to the bending of the strip under stress on both sides, while empty bending refers to the bending under stress on one side. In edge forming, there are three basic types of solid curved holes: single radius, multi-curve, and involute.
1. Single radius solid bending forming
The single-radius solid curved hole shape is in line contact with the strip material, and the strip material is pushed into the hole shape under the action of external force and then bent. The pass of this kind of roll has a single radius of curvature, which can only bend the curvature radius of specific specifications, and the roll has no universality.
The advantages of single-radius solid bending and forming: the strip has good centering and guiding capabilities and is not easy to deviate from the rolling center; the bent strip is subject to large forming forces from the upper and lower rollers, and the bending is sufficient and the forming area is small. Rolling through curves is simple and easy to handle.
Disadvantages: Poor roll sharing; large linear speed differences at various points on the pass, which can easily lead to scratches on the surface of the pass and strip, making it difficult for the strip to enter the cavity.
2. Multi-curve solid bending forming
The passes of multi-curve solid bending are the same as those of strip steel, but the upper and lower rolling passes have become multiple curves with different curvature radii. A curved line (shorter than a traditional solid curved forming line) is used when forming. There is a certain degree of commonality. The upper part of the multi-curve forming frame should adopt a rotatable structure, and the required forming curvature can be selected by adjusting the angle. The lower roller is usually a flat roller with its center distance adjusted to correspond to the hole curve of the upper roller.
The advantages of multi-curve solid bending forming: the roller has certain versatility; it is more convenient to adjust; the contact surface between the pass and the strip is small and not easy to scratch.
Disadvantages: The curves are more complex and processing is more difficult; the frame structure is complex. The center of the strip easily deviates from the rolling center, resulting in large forming springback.

3. Involute solid bending forming
The involute solid bending forming pass is a point contact with the strip. The upper roller table is involute, while the lower roller table is an arc or even a straight line with a large curvature radius. The strip roll is bent and formed against the upper roll. Therefore, this method is also called "roll forming". The upper and lower frames of involute molding are usually rotating structures with adjustable angles,
The advantages of involute solid bending forming: the sharing of rollers is the best; the wireless speed of the strip surface is poor and scratching is avoided.
Disadvantages: difficult roll processing; complex frame structure; high manufacturing precision requirements; strip center easily deviates from the rolling center, and forming elasticity is large.
Overall forming
Overall bending is the second step in high-frequency welded pipe forming. Its purpose is to bend the entire pipe to the curvature of the enclosed introduction roller. Traditional overall bending is formed by alternating flat and vertical rollers. The modern advanced integral forming process uses row roller forming and concentrated hollow bending, while the FFX forming process uses involute vertical roller cluster hollow bending to achieve integral forming.
1. Alternate forming of flat rollers and vertical rollers
Flat/vertical rollers alternately form holes and strips for closed air bending. Flat rollers and vertical rollers are arranged alternately. The flat roller is the active roller and the vertical roller is the passive roller. The flat rollers and vertical rollers are bent one by one.
The advantages of alternating flat/vertical roll forming: good formability, uniform and stable forming force distribution, good strip centering, and not easy to deviate from the rolling center.
2. Form a set of row rollers/involute vertical rollers
The holes and strips formed in the row roller set/involute vertical roller set are open air bends. The strip is continuously concentrated and freely bent by row roller groups or involute vertical roller groups.
Advantages of forming in rows of roller sets/involute vertical roller clusters: the strip is easy to enter the hole, the surface is not easily scratched, the forming path is short, and the roller sharing is good.

Disadvantages: The frame structure is complex and difficult to adjust; the row of rollers/vertical rollers are non-power transmission and can only be forced to pass under the action of external force. A row of rollers relies on retaining rollers to prevent the strip from deviating from the rolling center, which can easily lead to thickening of the strip edges.