Induction heating is one of a wide range of electrical heat used in industry and household today. The main applications of the process are in the steel and metalworking industries. Clean and fast heat being supplied to the heated workpiece meets the considerably increased requirements with regard to environmental protection. The surroundings is not exposed to any thermal and atmospheric pollution. The particular advantage of this process is to produce the heat inside the workpiece without the need for any external heat source.

According to the physical law of induction an alternating magnetic field is generated around each electrical conductor through which an alternating current is flowing. By considerably increasing these magnetic fields, metals brought into close proximity will be heated by eddy currents produced within the metal. Heating by induction makes use of the capability of the magnetic field to transmit energy without direct contact. This means heating is not done by contact transmission such as known in resistance heating in light bulbs, heating plates or electrical furnaces where the direct current flow causes resistance wires to glow.

A basic problem of induction heating is to create a sufficiently intense electro-magnetic field and to position the component to be heated within the center of the field in such a way as to obtain optimum transmission of energy from the electrical conductor to the workpiece. Normally this is achieved by forming the electrical conductor also referred to as inductor or coil with one or more turns. The workpiece is positioned in the centre of the coil, thus concentrating the magnetic field onto the component. The field will then force the electrical current to flow within the workpiece. According to the law of transformation, the strength of the current flow in the component is equal to that in the coil. To create a sufficiently strong magnetic field, th current flow in the coil must be very high (1000 – 10.000 A), normally a current of this intensity would cause the coil to melt; by comparison, 10 A is the current flow within a 2000 W heating furnace. In order to avoid this problem, the coils are made of water cooled copper tubing. Another method of creating a strong alternating magnetic field is to increase the frequency of the current. Normally the electrical mains supply to both household and industry operates at a frequency of 50 Hz, i.e. the current will change direction 50 times per second. Depending upon the application, an induction heating equipment will operate at a frequency of between 50 and 1 million Hz.

These high frequencies, which are not available from the normal mains electrical supply, are obtained by means of generators: medium frequency generators in the range up to 10.000 Hz and high frequency generators above this level. It may be asked why such a large frequency range is necessary and why not all induction heating processes cannot be carried out at the same frequency. This is due to a physical reason as well, i.e. the so called skin effect. The electrical current flows into the outer skin of the workpiece only, this means the center of the workpiece remains theoretically cold.

The thickness of the layer in which the current flows in turn is dependent on the frequency. At low frequencies, the layer is thick, i.e. the workpiece is penetrated by the current almost to the centre, and consequently heated through. At very high frequencies, the current flows at the surface only and the penetration depth is in the range of 0 to 1 mm. This effect is made use of in order to use the frequency appropriate for the application.

The most common applications utilising induction heating technology are:

․Melting of steel and non ferrous metals at temperatures up to 1500 °C.

• Heating for forging to temperatures up to 1250 °C.

• Annealing and normalising of metals after cold forming using temperatures in the range of 750 – 950 °C.

• Surface hardening of steel and cast iron workpieces at temperatures from 850 – 930 °C (tempering 200-300 °C) and soft and hard soldering at temperatures up to 1100 °C, moreover, special applications such as heating for sticking, sintering.

 While for melting , forging and annealing mostly medium frequency is used as energy source, for hardening and soldering applications it depends on the requirements whether high or medium frequency can or is to be used.

Summary:

Induction heating provides a heat source which is very easily controllable, can be limited to partial heating zones and creates reproducible heat-up processes. This provides the opportunity to build heating equipment with a high level of automation which allows to be integrated in a production line, such as machine tools.

The martensite being a result of quench hardening is hard, but also very brittle. Its specific volume is larger than that of the original structure. This causes unavoidable changes in the dimensions of the hardened part and internal tensions when the workpiece is only locally martensitic due to surface hardening. These tensions are overlapped by tensions which are caused by the considerable differences of temperature in the workpiece in the heating and quenching process. The totality of tensions causes the hardening distortion and possibly hardening cracks.

Tempering at temperatures of 150 – 200 ° C will change the martensite structure. The martensite experiences a considerable stress relief without any substantial hardening reduction. This has a very positive effect on the mechanical features (stretch and toughness). The workpiece is less sensitive to shock and cracks are hardly to be expected.

Although in induction hardening the same process is done in the workpiece as in the other transformation hardening processes, the necessarily preceding austenitizing process is very limited in time as a result of the fast heating. When a workpiece is heated in the furnace to hardening temperature, the time required for through hardening is in general sufficient to austenize the structure completely. On the basis of the usual ferrite-perlite structure of the steel, this means that with increasing temperature and dwell time beyond the transformation point first the perlite is transformed into austenite and then increasingly the ferrite. Since both structure components have a very different carbon content (perlite ≈ 0,9 and ferrite < 0,01) this difference of concentration of carbon must compensate by diffusion in the austenite come into being. The compensation process depends on time and temperature. It goes slow closely above the transformation temperature and faster at increased temperatures. Are in the steel besides the iron carbide (cementite) any carbides from alloy elements (e.g. chrome) present, the austenitizing process will take longer due to the dissolution of the carbides either starting with delay or going slower.

Steel provides the optimal requirements for the hardenability, provided the austenitizing process

1. dissolves and transforms the perlite and ferrite 2. largely dissolves the alloy carbides

3. all differences in concentration (carbon and alloy elements) are compensated.

Both, a dwell time longer than required (overtimes) and a too high austenitizing temperature cause a coarse austenite grain unless the dwell time is reduced at the same time (overheating). The risk of forming a coarse grain as a result of increased hardening temperatures, as applied for a faster austenitizing in induction hardening, however does not exist as long as there are undissolved rests of carbide present.

Induction hardening cannot and is not to replace those surface hardening processes being generally in use. It is an additional hardening process which is used for those applications where there is a benefit, both in technical and economic respect. The advantage becomes the more obvious the smaller the surface to be hardened on a workpiece is, compared with its total surface. The following is a summary of the advantages and disadvantages of the different surface hardening processes. The decision which hardening process is advantageous for a specific workpiece can be taken by the processing company only and, in case of doubt, after having consulted experts for such processes.

Disadvantages