Coreless Type Induction Furnace

Coreless type induction furnace of Two-furnace system refers to melting the charge into molten iron in one furnace and then converting the molten iron into a pourable state in a second furnace. In this second furnace, the role of the coreless furnace is usually to recarburize the molten iron coming from the cupola and then adjust it to the required chemical composition and tapping temperature. Similar to single melting, a two-furnace series arrangement is also commonly used, especially if the molten iron is replenished directly from the cupola through a launder and cover opening. In this case, one furnace is used for replenishing and treating the molten iron, while the other is used for pouring operations.

Coreless type induction furnace, on the one hand, can use relatively low power, and on the other hand, also provides a strong stirring process. Therefore, the coreless furnace here is designed with a power density of 200–400 kW/t and a frequency of 60–100 Hz. Its purpose is short recarburization and alloying time with high output. These goals are achieved through the condition shown in Figure 2.12, namely that the molten iron does not exceed the height of the coil. By increasing the power density and frequency, an increase of 0.5% C and a temperature rise of 40–50° can be completed within 5–10 minutes.

Following the trend toward greater flexibility in the melting shop, Coreless type induction furnace are required to be usable in single melting mode at one time and in duplex melting at another. Operating parameters for 5 t and 10 t electric furnaces show that these furnaces can be used for melting at higher power and frequency (single melting mode), while using low power and frequency (duplex melting) is very helpful for treating molten iron. With comparable stirring capability, that is, characterized by a meniscus height hᵤ and flow velocity v (with the molten iron surface reaching near the top of the coil), if the power and frequency of the two furnaces are each reduced by half, then under the same condition of a 100° temperature rise, the carburization time is doubled.

In 5 t and 10 t Coreless type induction furnace, the relationship between carburization time and frequency f. Calculation of the relationship among meniscus height hᵤ, flow velocity v, and power P, each with a 100° temperature rise.

Under special circumstances, coreless furnaces are used for holding. This takes advantage of their advantages compared with channel-type induction furnaces: even very large-capacity coreless furnaces have an acid lining and can be completely emptied and cooled to room temperature. This mode of operation is particularly useful when molten iron only needs to be stored for one to two days per week, for example, for producing single large castings. For this application, the coreless furnace is selected with low power density, low frequency, and a short working coil configuration.

It should still be mentioned here that when a Coreless type induction furnace is used purely as a holding furnace, its open structure has disadvantages compared with the siphon type of channel-type induction furnace. With longer holding times and a lower replenishment level, unfavorable losses of carbon and silicon will occur. During tapping, the slag on the surface of the molten iron enters the ladle together with the molten iron. Finally, simultaneous tapping and replenishment of molten iron cannot be achieved in a coreless furnace.

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