The process works on the simple principle that when an electrical current is passed through a conductor, an electro-magnetic field is created around the conductor. The conductor is generally (not in all cases) a coiled copper conductor through which a high-frequency magnetic field is induced to flow through the coil. This sets up a magnetic field around the coil and within the coil. If a steel bar is inserted into the coil, the magnetic flux that is generated will create eddy currents within the surface of the steel bar, which creates heat within the immediate surface of the inserted bar within the conductance coil.
The depth of the heated and hardened surface will be dependent on the carbon content of the steel bar, induction frequency, induction power, residence time within the coil and quench medium.
The steels that can be used for an induction heat-treatment procedure will generally contain approximately 0.3-0.5% carbon. Care needs to be taken with the higher carbon grades for the potential risk of cracking. Chromium can be added to the steel (generally 0.25-0.35%) to interact with the carbon content of the steel and produce surface chromium carbides.
It is at this point that the decision should be made if the system will quench with water or a poly-alkaline glycol mixture to reduce the risk of cracking. The induction coil can be designed to accommodate any geometric shape that will allow access to the contour to be heat treated and quenched accordingly.
The following will show some of the advantages and disadvantages of induction heat treatment.
- Localized areas can be heat treated
- Very short surface heat-up times
- Steel can be pre-heat treated to obtain prior core hardness values
- Very minimal surface decarburization
- Very minimal surface oxidation
- Slight deformation (bending); this can occur due to internal residual machining stresses
- Straightening can be carried out on a deformed bar/shaft; however, care must be exercised
- Increased fatigue strength
- Can be incorporated into cell manufacture
- Low operating costs
- High capital investment (however, the investment will be dependent on the degree of automation built into the equipment)
- Only certain steels can be induction hardened
- The method is restricted to components having a shape that is suitable for induction hardening.