Medium-frequency induction furnaces convert three-phase industrial-frequency AC power into DC power via rectification, then transform that DC into an adjustable medium-frequency alternating current. This current flows through a circuit comprising capacitors and an induction coil, generating high-density magnetic flux lines within the coil. These lines cut across the metal material placed inside, inducing strong eddy currents within the metal. These eddy currents exhibit the characteristics of medium-frequency current; specifically, the flow of free electrons through the resistive metal body generates heat. For instance, when a metal cylinder is placed inside an induction coil carrying medium-frequency alternating current, the cylinder does not make direct contact with the coil. While the energized coil itself remains cool, the surface of the cylinder heats up until it glows red or even melts-processes that can be precisely controlled by adjusting the frequency and current intensity. If the cylinder is positioned at the center of the coil, the temperature distribution around its periphery is uniform; furthermore, the heating and melting processes generate no harmful gases or intense light pollution. Medium-frequency electric furnaces are widely used for melting ferrous metals-such as steel, alloy steel, specialty steel, and cast iron-as well as non-ferrous metals like stainless steel, zinc, copper, and aluminum. They are also employed for heating, temperature maintenance, and holding, and can operate in tandem with blast furnaces. Applications in forging include through-heating, temperature compensation, and online heating for bar stock, round steel, square steel, and plates; localized heating; online forging (e.g., precision forging of gears, half-shafts, connecting rods, and bearings); extrusion; hot rolling; pre-heating for shearing; heating for thermal spraying; and thermal assembly, as well as bulk heat treatment processes such as quenching and tempering, annealing, and tempering. Heat treatment services cover a wide range of mechanical and automotive/motorcycle components-including shafts (straight, stepped, camshafts, crankshafts, gear shafts, etc.), gears, sleeves, rings, discs, machine tool lead screws, guide rails, flat surfaces, ball joints, and hardware tools-offering both surface heat treatment and bulk treatments such as quenching and tempering, annealing, and tempering.
Based on the principle of electromagnetic induction, medium-frequency induction heating generates heat directly within the workpiece. This allows general operators to begin continuous forging tasks just ten minutes after starting the furnace, eliminating the need for specialized furnace operators to handle pre-heating and sealing procedures. The rapid heating rate minimizes oxidation; oxidative loss for medium-frequency heated forgings is only 0.5%, compared to 2% for gas furnaces and 3% for coal-fired furnaces. This process conserves materials, saving at least 20–50 kg of steel raw material per ton of forgings compared to coal-fired furnaces.
Innovation continues in specialized fields such as scrap steel processing; for instance, in September 2025, Fujian Shengli Intelligent Industrial Technology Co., Ltd. filed a patent for a "medium-frequency furnace scrap steel processing device utilizing natural gas preheating combined with vibration feeding" (Publication No. CN121450873A).