How does High-frequency induction (HFI) welding machine work

High-frequency induction (HFI) welding is used widely by the tube producing industry, but a large number of variables need to be controlled to perform the operation successfully.

In the HFI welding process, high-frequency current is induced in the open seam tube by an induction coil located ahead of (upstream from) the weld point (see Figure 1).
Tube edges are spaced apart when they move through the coil, forming an open V, which has an apex slightly ahead of the weld point. The coil does not make contact with the tube and acts as the primary of a high-frequency transformer. The open seam tube acts as a one-turn secondary.
In high-frequency induction (HFI) welding, a high-frequency electric current is induced in the open seam of a tube by an induction coil.
As in general induction heating applications, the induced current path in the workpiece tends to conform to the shape of the induction coil. Most of the induced current completes its path around the formed strip by flowing along the edges and crowding around the apex of the V-shaped opening in the strip.
High-frequency current density is the highest in the edges near the apex off the V and at the apex itself. The tube is heated rapidly, raising the edges to welding temperature when they arrive at the apex. Pressure rolls force the heated edges together, completing the weld.
The high frequency of the welding current creates concentrated heating along the V edges. A small portion of the total current also finds its way around the back of the formed strip. Unless the diameter of the tube is small compared with the V length, the current tends to flow along the edges of the tube forming the V.

The induction heating take use of the skin effect, proximity effect, ring effect, the High-frequency induction (HFI) welding take use of the induction heating and the cold bending forming . so the formed pipe is high qualified.