What is Induction Heating?
Fig. 1 Induction heating is a noncontact heating method Fig 2 Heat energy (E) produced in an electric circuit is equal to I2 R.
Induction heating (Fig. 1) is a noncontact heating method; one in which an electrically conductive material (typically a metal) is heated by an alternating magnetic field. Invisible lines of force are created by a work coil when a current flows through it, the result of which is an induced current in the conductive workpiece. Heating results due to the Joule effect and, to a lesser degree, magnetic hysteresis (i.e., power loss other than by eddy currents in a magnetic material caused by reversals of the magnetic field). Joule’s Law (Fig. 2) states that the rate at which heat energy is produced in any part of an electric circuit is measured by the product of the square of the current (I) times the resistance (R) of that part of the circuit.
What are the scientific principals involved in induction heating?
The principle of induction heating is a consequence of the Faraday effect, named for the physicist Michael Faraday who was the first to produce an electric current from a moving magnetic field.
How does induction heating work?
Induction heating is based on the principle of resistance to induced currents. These currents, called eddy currents, are similar in magnitude and opposite in direction to the current produced by the induction coil (also known as the inductor). 
A number of other independent variables, including the workpiece’s magnet
ic permeability (a measure of how magnetic the material is), the air gap (coupling distance between the inductor coil and workpiece) and the frequency influence the induction process and its efficiency.
How deep can I penetrate a part using induction?
The depth of (current) penetration during heating depends on the choice of frequency (for an alternating current, frequency is the number of complete cycles per second. The standard unit of frequency is the hertz, abbreviated Hz. For example, if an electric current completes one cycle per second, then the frequency is 1 Hz). The penetration depth is inversely proportional to the square root of frequency. In other words, for applications requiring deep case depth hardening, a low frequency is used.
For surface heating or shallow case depth hardening, a high frequency is needed. Depth of heating is also influenced by such variables as the magnetic permeability and conductivity 
of a part. Finally, it is important to remember that for the same applied frequency, different metals have different penetration depths. For example, at 1 kHz (at ambient temperature), current penetration depth in stainless steel is about 6 times deeper than in copper.
What are induction coils?
Depending on application and workpiece geometry, the shape of an induction coil can be noticeably different. In some cases, induction coils are simple single-turn or multiturn geometries, where the workpiece is placed inside of the coil. In other cases, induction coil geometries are more complex, such as butterfly-shape, split-return, pancake, etc. Although any current carrying conductor can serve as an induction coil, coils usually are made of copper, which has good electrical conductivity, economy and availability.
Are all parts heated the same?
Different types of parts require different heating modes. For example, gears, camforms, camshafts and axle shafts are typically treated using a single-shot process (where the part is rotated in an inductor for uniform heating, followed by the appropriate quenching process). Longer parts such as shafts, rolls, ballscrews and bar stock are treated progressively, or scanned, where a minimal area is heated to achieve proper temperature, then either the part or inductor moves over the desired heat treat length.
What makes induction heating different from other methods of heating?
In induction heating, heat is generated within the work itself; it does not rely on transmission heat energy by radiation or convective as in a furnace or oven. Therefore, hardening to a specified depth below the surface is possible without excessively long process times or excessively high surface temperatures.
In addition, the heating can be applied to a specified area. Induction processes are accurate, consistent and repeatable. Because heating occurs in the part itself, induction heating is considered a very high (energy) efficiency process (and more efficient than the majority of alternative methods).
What makes up an induction heating system?
An induction heating system comprises a basic induction power source, which provides the required power output at the required power frequency, complete with matching 
components, an induction coil assembly, a method of material handling and some method of water cooling and quenching. Most induction heating systems are water cooled with the exception of small, low-power units. The methods of material handling and the induction coil arrangement depend entirely on the application. The choice of induction power source is related to the application requirements and to production rate.
What are the most common applications for induction heating?
Typical induction heating applications can be divided into the following general categories:
- Heat treating including hardening, tempering, stress relief,
- Annealing and normalizing
- Mass heating of billet and bar, slab and bloom, strip and
- Plate, wire and cable, tube and pipe and slug heating for
- Semisolid forming
- Joining, brazing, bonding and soldering
- Preheating
- Shrink fitting
- Heating for deformation shaping including forging, swaging,
- Upsetting, bending and piercing
- Melting
- Special applications such as crystal growing, cap sealing,
- Sintering,
- Spheroidizing, carbon vapor deposition, welding, levitation,
- Epitaxial deposition
- And plasma generation
