Question:
Can you provide some general guidelines on how to purchase induction heating equipment?

Answer:
The following factors typically influence equipment design:

1. Material
2. Prior microstructure
3. Part geometry
4. (Austenitizing) temperature
5. Production rate
6. Power requirements*
7. Frequency selection*
8. Pattern/profile (i.e. shape of heating area)
9. Coil design*
10. Process development requirements
11. Application-specific criteria (e.g., oil vs. polymer)
* Typically selected by vendor based on information provided.

Key process parameters for induction heating include:

1. Heating time (scan rate)
2. Power level
3. Power frequency
4. Part Position (e.g., rotation)
5. Quench flow
6. Quench temperature
7. Quench time
8. Quench concentration (if polymer) or type and speed (if oil)

Here are some other things to look for as well.

A. Power

Power, expressed in kilowatts (KW), refers to the induction power-supply size. A power supply must be sized to heat a given mass or surface area to a specific temperature within a specified time. A general rule of thumb is that the surface area (exposed to the coil) used to determine the power level is 6-12 KW/in2. The prior part microstructure (annealed, normalized, quenched & tempered) will influence the power density (KW/in2) required. For example, a quench-and-tempered microstructure is optimum for most induction applications.

More power is not necessarily better. Matching the power and frequency is the key. While more power lets you heat faster, it also produces a deeper pattern (there is also more danger of through hardening) and it is harder to control.

B. Frequency

High frequency in the form of alternating current is passed through the coil to create a magnetic field producing eddy currents (that are generated within the metal). The resistance to this current flow is one of the principal sources for heating of the metal.

The following guideline for “relative” depth of penetration (depth of hardening or case depth) as a function of frequency might be useful. This information is application-specific and dependent on both power density and heat time but is considered typical of what is found in the industry.

  • @ 450 kHz the case depth developed is (approximately): 0.030–0.040”
  • @ 100 kHz the case depth developed is (approximately): 0.050–0.080”
  • @ 30 kHz the case depth developed is (approximately): 0.080–0.120”
  • @ 10 kHz the case depth developed is (approximately): 0.090–0.200”
The depth of current penetration (hardened depth) is a function of part diameter and the resistivity of the material. There is an optimum depth of current penetration range, which is what each manufacturer strives to provide.

Higher frequency has a distinct advantage when you have marginal prior part microstructures (annealed or normalized). Higher frequency allows the concentration of more energy on the surface of the part avoiding long heating times or requirements for higher power density, which is more costly in terms of $/KW.

C. Tuning

The goal is to have a system that requires a minimal amount of tuning. Tuning (power-supply matching) is a very important consideration to achieve the desired case profiles and for overall system efficiency. Tuning is MANDATORY when changing frequency, and a learning curve is required to make the necessary adjustments.