Induction Heating Service Case Study
Induction heating services now complement the existing electrical resistance and gas fired techniques that are offered by Thermal Hire. This service is available for a variety of applications in preheating and PWHT of pipework weldments and, in particular, for pipeline applications. The high rates of heating associated with the induction process make it especially attractive for preheat work as the delay before welding is minimised. For PWHT applications the process permits rates of heating that can be controlled to the permitted maximum values and offers a relatively simple but efficient preparation. The suitability of the process does vary from application to application - it does, however provide a useful optional technique. [See Figure for typical preheat application]. The system offered is also suitable for the removal of shrink-fit parts and for curing application where direct contact with the base metal must be avoided.
Induction Heat Treatment
Induction heat treatment is hardly a new concept: it has been around for over 40 years. Heating is achieved without any direct physical contact and finds use in almost every industry where metals are used, be it in alloying, metal forming, tempering, hot working or shrink fitting. The technique is capable of heating materials at rates of temperature rise not normally associated with more other methods, such as electrical resistance or gas heating, with a high degree of efficiency.
As there is no or limited physical contact, there are potential advantages in production. For example, there is less risk of contamination, a simpler heating arrangement and less insulation may be required.
The induction heat treatment process relies on induced electrical currents within the material to produce heat. The process involves three basic factors - electromagnetic induction, the skin effect, and then heat transfer. The basic components of an induction heating system are an AC power supply, induction coil, and the material to be heated. The power supply provides an alternating current through the coil, generating a magnetic field. When the work piece is placed in the coil, the magnetic field induces eddy currents in the work piece and thus generates localized heat without any physical contact between the coil and the work piece.
There is a relationship between the frequency of the alternating current and the depth to which there is penetration of the work piece Low frequencies (up to 30kHz) are effective for thicker materials requiring deep heat penetration, while higher frequencies (100 to 400kHz) are effective for smaller parts or shallow penetration. In addition, the higher the frequency used, the higher the rate of heating. The induced current flow within the part is most intense on the surface, and decays rapidly below the surface. Hence the outer surface will heat more quickly than the inside surface. 80% of the heat produced in the part is produced in the outer "skin". This is described as the "skin depth" of the part. The skin depth decreases when resistivity decreases, permeability increases or frequency increases.
Owing to the effect of hysteresis, magnetic materials are more readily heated compared to non-magnetic materials. Magnetic materials 'resist' the rapidly changing magnetic fields within the induction coil. As a result heat is produced by hysteresis in addition to eddy current heating. A metal which offers high resistance is said to have high magnetic permeability which can vary from 100 to 500 for magnetic material. Non-magnetic materials have a permeability of 1. Heating associated with hysteresis occurs at temperatures below the "Curie" point - the temperature at which a magnetic material loses its magnetic properties.