The steel fin, that may be solid or serrated, is helically wound and continuously welded to the tube by high frequency electric resistance welding. This produces an homogeneous, clean and continuous fin-totube bond with no additional weld material, for a very efficient heat flow and resistance to corrosion. There are some applications for which one or the other fin type is preferred. While in some cases fouling and cleaning considerations will favour solid fins, serrated fins are superior with respect to weight, compactness and price. The use of high-frequency current results in very localised heating of the tube surface and the edge of the fin. This skin effect produces higher weld speeds, hence post-weld heat treatment is not considered necessary because of the superficial nature of the changes in the microstructure. The main uses for high frequency welded finned tubes are in the heat recovery associated with boilers for power generation and in furnace applications for the petrochemical industry.