Advantages and Disadvantages of High-frequency Welded Pipe Forming Methods

The high-frequency welded pipe (ERW steel pipe) forming roll 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 before high-frequency welding is usually called preforming.

1. Edge forming

Edge forming is the first step in the forming process of high-frequency welded pipes. Its purpose is to bend the edge of the strip to the curvature of the squeeze roll radius. The traditional process uses single radius forming, and the modern process uses double radius forming.

The initial double-radius forming was a simple double-radius upward bending, which was later developed into W forming. The bending of the middle part of the molding method uses reverse protrusions to more fully bend the edges, and the linear velocity difference of forming surrounding holes during molding is small.

Edge forming generally adopts the solid bending method. In cold bending, solid bending usually refers to the bending of the strip under the force on both sides, while the empty bending is the bending under the force on one side. In edge forming, there are three basic types of solid curved holes: single radius, multiple curves and involutes.

1. Single radius solid bending forming

The single-radius solid-curved hole pattern is in online contact with the strip material, and the strip material is pushed into the hole shape under the action of an external force, and then bends. The pass of this kind of roll is a single radius of curvature, which can only bend the radius of curvature of a specific specification, and the roll has no versatility.

The advantages of single-radius solid bending forming: the strip has good centering and guiding ability, and it is not easy to deviate from the rolling center; the curved strip receives the larger forming force of the upper and lower rolls, the bending is sufficient, and the forming area is small. Rolling through the curve is simple and easy to handle.

Disadvantages: poor commonality of rolls; large difference in linear speeds on the pass pattern, which can easily cause scratches on the pass pattern and the surface of the strip, and it is difficult for the strip to enter the cavity.

2. Multi-curve real bending forming

The bending passes of the multi-curve entity are the same as the strip steel, but the upper and lower rolling passes have become multiple curves with different radii of curvature. Use a curve when forming (shorter than the traditional solid bending forming line). 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, and its center distance is adjusted to correspond to the hole curve of the upper roller.

The advantages of multi-curve solid bending forming: the roll has a certain versatility; the adjustment is more convenient; the contact surface between the pass and the strip is small, and it is not easy to scratch.

Disadvantages: The curve is more complicated and the processing is more difficult; the frame structure is more complicated. The strip center is easy to deviate from the rolling center, and the forming springback is large.

3. Involute solid bending forming

The involute solid bending forming pass is the point contact with the strip, the upper roller table is involute, and the lower roller table is an arc or even a straight line with a large radius of curvature. 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 the involute moulding are usually rotating structures with adjustable angles.

The advantages of involute solid bending: the sharing of rolls is the best; the wireless speed of the strip surface is poor, avoiding scratches.

Disadvantages: difficult roll processing; complicated frame structure; high manufacturing precision requirements; strip center is easy to deviate from the rolling center, and the forming flexibility is large.

2. Overall molding

The overall bending is the second step of high-frequency welded pipe forming. The purpose is to bend the entire pipe to the curvature of the closed introduction roller. Traditional overall bending is formed by alternating flat and vertical rollers. Modern advanced integral forming process uses row roll forming and concentrated hollow bending, while FFX forming process uses involute vertical roll cluster hollow bending to achieve integral forming.

1. Alternate forming of flat rolls and vertical rolls

Flat/vertical rollers alternately form holes and bands for closed air bending. The 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 roller and the vertical roller are bent one by one.

The advantages of alternate plane/vertical roll forming: good formability, uniform and stable forming force distribution, good strip centering, and it is not easy to deviate from the rolling center.

2. Form a set of row rollers/involute vertical rollers

The holes and bands formed in the row roller set/involute vertical roller set are open air bending. The strip is continuously concentrated and freely bent by the row roller group or the involute vertical roller group.

The advantages of forming in a row of roll groups/involute vertical roll clusters: the strip is easy to enter the hole, the surface is not easy to scratch, the forming path is short, and the roll sharing is good.

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

Tips: ASTM A53 Grade B is the grade in ERW high frequency welded pipe, divided into A and B grades. ASTM is only a set of specifications. ASTM steel pipe A53 A corresponds to the Chinese standard GB8163 raw material is 10# steel, and A53 B corresponds to the Chinese standard GB8163 raw material 20#.

For example: ASTM (standard) A53 (grade) gr (abbreviation of grade) b (grade B)