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.