Magnetic heating is bidding for recognition as a mainstream thermal technology with the introduction of the Coreflux process. The equipment generates an intense low-frequency magnetic field throughout the cross section of magnetic and paramagnetic materials.
The process is sometimes called UMH for "uniform magnetic heating," because it heats the entire cross section of a workpiece simultaneously. Temperature distribution is uniform; there is little or no thermal gradient from the surface to the center.
However, UMH should not be confused with low-frequency induction heating. Although both processes require magnetic fields, that is where the similarity ends. Induction heating is a surface process. The induction coil generates magnetic eddy currents that are stronger on the surface of the workpiece than at the center. This produces a skin heating effect, and conductivity carries heat into the interior of the part.
Low frequency induction heating systems (around 60 Hz) can penetrate components to a depth of no more than 25 mm for most materials. This type of system still uses eddy currents to generate the heat — so uniform through-heating of thicker parts is not possible.
Process Principles
UMH is based on the principle of hysteresis loss, and heats without relying on thermal conductivity to transmit heat from the surface to the center. With UMH an alternating magnetic field causes domains or crystals in the metal to align and strain in reaction to the field (in the direction of the field for ferrous metals; in the opposite direction for nonferrous metals). As field polarity is reversed, these domains realign; the lag in the process, called hysteresis, is the mechanism that creates heat. This heat is spread rapidly and uniformly through the metal component.
Process Differences
Both induction heating and UMH use coils, but in very different ways. A part to be heated by induction is placed within the loops of a single coil. The magnetic field for UMH is generated by two coils that induce magnetic fields in two cores laminated from high-permeability, low-reluctance directional steel. The cores direct magnetic flux into the workpiece, and pneumatic cylinders clamp the workpiece between the laminated cores to prevent vibration when the field is energized.
For large masses that require uniform heating to produce optimum results, UMH is the logical alternative to induction.
Generating the field for uniform magnetic heating requires less energy and equipment than induction heating and can produce a field that can be varied from 40 to 400 Hz using a standard, variable-frequency motor drive inverter. It is less costly than the power supply for induction heating.
Another advantage is that UMH has no plumbing so the installation is simplified, and one of the other biggest advantages of this power supply is that water cooling is eliminated; only 480 VAC three-phase electric service and 90 psi shop air is needed. Also, a huge amount of induction heat is lost in water-cooling the coil. With the air-cooled coils, most of the energy flows to the workpiece.
UMH also has a tooling advantage. The same laminated cores can be used for a variety of parts. Induction heating, however, requires a separate coil customized for each component.
Temperatures can be maintained to an accuracy of ±2°C, which is generally a tighter tolerance than most users can measure. A proportional integral derivative (PID) controller tapers the percentage output of the power supply as the workpiece reaches the required temperature, to avoid overshooting the set point. The programmable logic controller includes a touch-screen interface and multiple thermocouple inputs.
Processing
UMH is capable of heating to 1100°C, well beyond the Curie point, at which magnetic materials lose their magnetic properties. But its benefits compared to induction heating are said to be more tangible at sub-Curie temperatures, at which induction tends to overheat part surfaces.
UMH can through-heat to 260 to 316°C very efficiently, while induction really struggles at these low temperatures because localized surface heating is very pronounced and there is little penetration. Above the Curie point, the eddy currents of induction heating penetrate deeper into the workpiece.
Applications
UMH machines can be applied to tempering, stress-relieving, and hardening of parts as well as bonding and shrink-fitting and are all sound applications. UMH can also preheat billets for extrusion and semisolid forming, and dies for extrusion and forging. Sintering of powder metals and brazing are among its other capabilities.
