When the furnace is switched on, current starts flowing at a high rate and at a comparatively low voltage, through the induction coils of the furnace, producing an induced magnetic field inside the central space of the coils where the crucible is located. The induced magnetic fluxes are generated, through the packed charge in the crucible, which is placed centrally inside the induction coil.
As the magnetic fluxes are generated through the scraps and complete the circuit, they generate and induce eddy current. This induced eddy current, flows through the highly resistive bath of scrap, generating tremendous heat and starting the melting. It is thus apparent that the melting rate depends primarily on two things: (1) The density of the Magnetic Fluxes and (2) The Compactness of the Charge. The charge mixed arrangement has already been described. The magnetic fluxes can be controlled by varying the input of power into the furnace, especially the current and frequency.
In a medium frequency furnace, the frequency range normally varies between 150-10K cycles/second. This heat is developed in the outer rim of the metal in the charge, but is quickly carried to the center by conduction. Soon, a pool of molten metal is formed at the bottom, causing the charge to sink. At this point, any remaining charge mixed, is added gradually. The eddy current which is generated in the charge, has other uses. It imparts a molten effect on the liquid steel, which is thereby stirred, mixed and heated more homogeneously. This stirring effect is inversely proportional to the frequency of the furnace, that is selected in accordance with the purpose for which the furnace will be utilized.
The melting continues until all charge is melted, and the bath develops a convex surface. However, as the convex surface is not favorable to slag treatment, the power input is then naturally decreased to flatten the convexity and to reduce the circulation rate while refining under a reducing slag. The reduced flow of the liquid metal accelerates the purification reactions by constantly bringing new metal into close contact with the slag. Before the actual reduction of steel is done, the liquid steel which might contain some trapped oxygen is first treated with some suitable deoxidizer. When no purification is attempted, the chief metallurgical advantages of the process that can be attributed to the stirring action, are uniformity of the product, control over the super heat temperature and the opportunity afforded by the conditions of the melt, to control de-oxidation through proper addition.
As soon as the charge has melted and the de-oxidising ions have ceased, the objectionable slag is skimmed off, and the necessary alloying elements are added. When these additives are melted, and diffused through the bath, the power input may be increased to bring the temperature of metal to a point that is most desirable for pouring. The current is then turned off, and the furnace is tilted for pouring into a laddle. As soon as pouring has ceased, any slag adhering to the wall of the crucible is scrapped out, and the furnace is readied for charging again.
As the furnace is equipped with a higher cover over the crucible, very little oxidation occurs during melting. Such a cover also prevents cooling by radiation from the surface heat loss, and protecting the metal is unnecessary, though slags are used in special cases. Another advantage of the induction furnace is that there is hardly any melting loss compared to the arc furnace.