Induction Furnace Stirring Effects: How Electromagnetic Forces Drive Melt Circulation
Turn on an induction furnace and the melt does not just sit there. The same electromagnetic field that heats the metal also pushes it. The push comes from the Lorentz force - the force on a current-carrying conductor in a magnetic field. The result is a circulating flow in the melt that is essential for uniform temperature, uniform chemistry, and clean steel. Get the stirring right and the melt is homogeneous. Get it wrong and the melt is hot at the edges and cold in the center, with a meniscus that damages the crucible wall.
Here is how the stirring actually works.
The physics starts with the magnetic field.
A coil carrying alternating current creates a magnetic field along the axis of the coil. The field is strongest at the center of the coil and falls off toward the ends. The field alternates at the same frequency as the coil current, so the field oscillates in strength and reverses direction every half cycle.
The magnetic field induces eddy currents in the metal charge. The eddy currents flow in the opposite direction to the coil current (Lenz's law), and they are concentrated near the surface of the charge (skin effect). The depth of penetration depends on the frequency and the metal resistivity - higher frequency means shallower penetration, lower frequency means deeper penetration.
The eddy currents interact with the magnetic field. The force on a current-carrying conductor in a magnetic field is the cross product of the current and the field. In the molten metal, the eddy currents flow in loops that are perpendicular to the magnetic field. The force on each segment of the loop is perpendicular to both the current and the field, which means the force is radial - either pushing the melt toward the center of the furnace or pushing it toward the wall.
The net result is a flow pattern that looks like a figure-eight.
In the upper part of the melt, the force pushes the melt downward and toward the center. In the lower part of the melt, the force pushes the melt upward and toward the wall. The result is two toroidal vortices - one in the upper half of the melt, one in the lower half - that circulate the melt in opposite directions.
The flow velocity depends on the power input, the frequency, the crucible size, and the melt level. A typical flow velocity in a 1-ton induction furnace is 0.1 to 0.5 m/s. In a 10-ton furnace, the velocity is similar or slightly lower. Higher power input means higher flow velocity. Higher frequency means shallower penetration and less effective stirring in the center of the melt.
The stirring has three important effects on the melt.
Effect one: temperature uniformity. The melt is heated from the outside in, but the stirring circulates the hot outer melt to the center and the cooler center melt to the outside. The result is a melt that is uniform in temperature from edge to center. Without stirring, the temperature difference between the edge and the center of the melt could be 100 to 200 degrees C. With stirring, the difference is 10 to 20 degrees C.
Effect two: chemistry uniformity. Alloy additions dissolve at the surface of the melt. The stirring carries the dissolved alloy to the center of the melt and brings fresh melt to the surface to dissolve more. The result is a melt that is uniform in chemistry within minutes of the addition. Without stirring, the alloy additions take much longer to distribute.
Effect three: inclusion floatation. Non-metallic inclusions (oxides, silicates, sulfides) are less dense than the steel melt. The stirring carries the inclusions to the surface, where they can be absorbed by the slag. Without stirring, the inclusions stay in the melt and end up in the casting. The stirring is not as effective as gas bubbling for inclusion removal, but it helps.
The meniscus is the visible signature of the stirring.
The force that pushes the melt toward the center of the furnace also pushes the surface of the melt upward at the center. The surface forms a dome shape - the "meniscus" - that is high in the center and low at the edges. The height of the meniscus depends on the power input and the melt level. A high-power furnace with a shallow melt can have a meniscus 50 to 100 mm high. A low-power furnace with a deep melt might have a 10 to 20 mm meniscus.
The meniscus is not a problem by itself, but it interacts with the slag layer. The slag sits on top of the melt, and the meniscus pushes the slag toward the wall. If the slag layer is thin, the slag gets pushed to the wall and the center of the melt is exposed to the atmosphere. If the slag layer is thick, the slag gets pushed to the wall and piled up at the slag line, where it can attack the crucible.
A high meniscus also exposes more of the melt surface to the atmosphere, which means more oxidation and more hydrogen pickup from moisture in the air. For steels that are sensitive to hydrogen (forgings, high-strength steels), a high meniscus is a quality risk.
The meniscus is controlled by the power input and the frequency. Lower power means lower meniscus. Higher frequency means shallower penetration and lower meniscus. Some modern power supplies have a "soft start" or "low power" mode for charging, when the melt is shallow and the meniscus is most pronounced.
The coupling gap is the design variable that controls the stirring intensity.
The coupling gap is the distance between the inside of the coil and the outside of the crucible. A small gap means tight coupling, high power transfer, and high stirring. A large gap means loose coupling, lower power transfer, and lower stirring.
A typical coupling gap for a 1-ton furnace is 30 to 50 mm. For a 10-ton furnace, the gap is 50 to 80 mm. The gap is filled with the backup refractory (sand or fiberboard) and the crucible wall.
A small gap gives high efficiency but increases the risk of crucible failure (the crucible is closer to the coil, the crucible wall sees more stress, and the consequences of a crack are more severe). A large gap gives lower efficiency but extends crucible life. The design choice is a trade-off.
The stir pattern can be modified by the coil design.
A simple solenoid coil (uniform pitch, uniform diameter) gives the standard figure-eight flow pattern. A coil with a varying pitch (tighter at the bottom, looser at the top) can shift the flow pattern. A coil with a magnetic flux concentrator (a laminated silicon steel collar at the top or bottom of the coil) can intensify the field in one region and reduce it in another. These modifications are used in specialty applications where the standard flow pattern is not optimal.
For example, in a vacuum induction melting (VIM) furnace, the stir pattern can push the melt against the crucible wall, increasing the heat loss and the crucible attack. A modified coil design with a flux concentrator at the top of the coil reduces the upward flow and the wall contact, extending crucible life in VIM service.
The stirring is also affected by the charge material.
A solid charge in the crucible does not stir - it sits there while the induced currents heat it. As the charge melts from the outside in, the liquid metal starts to stir, but the solid pieces constrain the flow. The full stirring pattern only develops when the charge is fully molten.
Cold charge additions during a melt also disrupt the stirring. The cold pieces sink, melt, and create a thermal and flow disturbance. The disturbance settles out within 30 to 60 seconds, but it can cause splashing and crucible attack if the addition is too cold or too heavy.
Author: MONTE INTELLIGENCE induction melting engineering team. For stirring analysis and coil design optimization, contact helenxu@cnlymonte.com.
