Coreless Induction Furnaces and Metal Bath
Coreless induction furnaces are commonly used in foundries for melting recycled scrap metal to provide molten metal for making castings (Figure 1).
In a coreless induction furnace, electromagnetic stirring of the molten metal bath occurs when power is applied to the furnace. Bath motion is present throughout the bath, ensuring good mixing action and uniform dispersion of alloys. An induction furnace is normally designed so the amount of stirring action is appropriate for the metal type being melted. Factors that influence stirring include the bath metal density, the electrical conductivity, the crucible size, the bath height, the power rating, and the frequency. In general, the stirring action increases directly in proportion to applied power and decreases with the square root of increasing frequency. Heavier metals such as iron and copper will exhibit less stirring than lighter metals such as aluminum for the same applied power and frequency.
Typical stirring patterns for a single-phase induction furnace are shown in Figures 2a and 2b.
The induction coil creates an electromagnetic field that produces a force between the coil and the molten metal. This pushes the metal away from the crucible sidewalls in the upper portion of the crucible. The resulting gap is called the meniscus. The meniscus is caused by the applied alternating magnetic field created by the induction furnace coil, which induces a high current in the molten metal bath. These two forces repel each other, resulting in a visible gap between the crucible wall and the molten metal.
In addition to creating heat due to the Joule effect (also known as losses), the induced magnetic field reacts against the applied magnetic field to produce a force that repels the molten metal away from the crucible wall. This force is represented by the red arrows in Figure 3.
Electromagnetically induced eddy currents cause a swirling action that moves the molten metal in a defined flow pattern as shown in Figure 3.
The metal velocity can be up to 2.5 m/sec.
In order to efficiently melt scrap containing chips and small, light pieces, higher stirring velocities approaching the maximum limit along with a large meniscus are necessary to draw the scrap quickly under the surface of the molten metal. For heavier scrap lower velocities are normally sufficient. By contrast, certain alloys, such as steel, require the stirring to be minimized in order to reduce atmospheric contamination of the melt. Modern induction furnaces are powered by converters, which perform the function of converting three-phase alternating current 50 or 60 hertz power available from the public electric utility network into a single phase power source of the appropriate frequency and voltage level for a particular furnace.
Polyphase Coreless Induction Furnaces
When higher stirring velocities than what can be achieved by a single-phase furnace are required, the induction furnace can be constructed with a multi-section coil, typically either two or three sections powered by a specialized stirring converter that produces multiple phase shifted output voltages. The relationship between stirring action and induced heat depends on the phase shift of the voltage applied between the coil sections with more heating and less stirring at low phase shifts and more stirring and less heating at higher phase shifts. This allows for unique flexibility with processes that require controllable stirring and heating.
An electrical block diagram for a typical stirring converter and switchable melting converter arrangement is shown in Figure 4.
Polyphase stirring requires the induction furnace to have a multiple section coil. There is one coil section per phase. For example, a three-phase stirring coil will have three independently powered coil sections. The power supplied to the coil may be from a transformer, in which case the phase shift between applied voltages will be fixed, typically 60 degrees. It also can be generated by a converter, which allows the phase shift to be infinitely variable. By varying the phase shift, the ratio of stirring action to induced heating power can be optimized for a particular process. By switching the phase rotation, the direction of the stirring motion can be up or down.
In the stirring mode, phase shifted voltages are applied sequentially to each coil section, providing a magnified stirring effect up to five times greater than a single-phase furnace of the same applied power.
In a melting operation, the coil sections can be powered from a single-phase source. With the single-phase power applied, the furnace can couple more energy into the charge for efficient melting. After the bath is molten, the three-phase stirring power is applied to efficiently mix the alloying elements into the molten bath, while at the same time reducing the absorbed power. In this way, the polyphase stirring induction furnace can optimize the combination of melting and stirring performance to an ideal balance for a particular process. This feature facilitates the production of special alloys such as metal matrix composites containing difficult to mix additives. An example of a polyphase furnace up-flow stirring pattern is shown in Figure 5. The up-flow stirring pattern can create a concave meniscus that can in some cases enhance recovery of light metallurgical inoculants.
A third type of stirring method is amplitude modulated stirring. This method applies to a standard single-phase induction furnace. The single-phase power is modulated by a lower frequency, raising and lowering the power periodically at a controllable rate. Raising and lowering of the power causes a “wave action” on the surface of the bath. The bath is squeezed toward the center by the magnetic force from the furnace coil and then the squeezing force is released, allowing the metal to flow back toward the crucible wall. The wave action on the surface of the bath can help light scrap, such as chips that would otherwise tend to float on the bath surface and become oxidized into dross or slag, to be subsumed more quickly below the bath surface. The usual single-phase induction furnace bath movement continues below the surface of the molten bath. This method of stirring can provide some improvement in the ability to wet lightweight inoculants such as carbon or silicon, particularly in underpowered furnaces that would otherwise be under-stirred.
Design guidelines for good stirring performance
To design furnace systems that consistently have the right level of stirring for a particular application, the concept of stirring factor was developed. A stirring factor of 100% represents the maximum practical amount of stirring (Velocity + Meniscus) that can be safely achieved without excessive splashing or ejection of the molten metal bath.
For typical melting applications the following range guidelines for stirring factor have been proven to achieve good practical results:
For iron, a high stirring factor is normally desired to quickly homogenize carbon, silicon, and other inoculants into the molten iron during chemistry adjustments near the end of a melt cycle.
For brass, copper, and aluminum, a moderate stirring factor is usually preferred. An exception occurs where chips are to be melted. In such cases a high stirring factor, usually close to 100%, is specified to quickly absorb the chips into the molten metal bath. Due to their small size, chips cannot couple with the magnetic field of the induction furnace, so they must be melted by conduction of heat from the molten metal bath, which is heated by electromagnetic induction.
For steel, a very low stirring factor is usually desired. Low stirring activity reduces gas pickup and slag inclusions in molten steel. This is critical for making good quality steel castings in foundries, particularly where no further post processing of the melt will be done to remove the gasses and slag inclusions.
In steel mill applications where the melt is refined in a secondary operation such as an AOD or other processing stage, a higher stirring factor can be tolerated in the melting furnace.
Good performing combinations of furnace size, power rating, and frequency for iron, copper/brass, aluminum, and steel can be calculated from the equations given in this article. These guidelines enable a furnace system to be specified that will provide desirable stirring performance in most applications. A detailed analysis should still be performed in each case to make sure the optimum performance is obtained.
Click here to see this story as it appears in the November 2018 issue of Modern Casting