Impeders are less necessary when welding non magnetic materials. This because the degree of high frequency “skin effect” varies among different materials and is most pronounced in carbon steels. Below Curie temperature, 90% of the current flow in carbon steel is confined to within 0.005" of the surface. The skin depth is greater for non magnetic materials, typically 0.040" in the case of aluminium. Since currentcannot flow in two opposite directions at the same time, materials that are thinner than twice the skindepth have less need for impeders, and light walled aluminium tubing is often welded without them.There is always some benefit to be gained from using an impeder. In addition to opposing current flow around the inside surface of the tube, an impeder increases the transfer of energy from the coil to the strip by “focusing” the energy in the area where it is needed. The coil has to produce a much stronger (and larger) magnetic field if an impeder is not used, and this causes wasteful and destructive heating of the weld rolls and other steel parts in the vicinity.
There are a few instances where weld quality may be improved by not using an impeder, and the higher power requirements may be a worthwhile trade-off. Under normal circumstances the tube edges are heated to a forging temperature that is well below the melting point of the material, but because the high frequency skin effect concentrates so much energy at the strip edge, the surface will sometimes melt, and this molten material is forced out of the weld area as a result of the pressure applied by the squeeze rolls, causing a large, rough weld bead to occur. This overheating may also cause oxidation and if oxides are not forced out into the bead, weld defects will occur. The heating can be made more gradual by either moving the coil, the impeder or both, or in some cases eliminating the impeder altogether. If impeders are not used, the R.F. voltage across the coil will be much higher, and care must be taken to avoid flashovers.
For tube diameters in the 3/4" to 2 1/2" range, induction welding without impeders will normally require 50% to 100% more power.
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