Numerical simulation of induction heating

Numerical simulation of induction heating

Design of induction heating device for forging lines is based on parameters that are important for our customers. These parameters include in particular:

  1. Heating speed – minimizes cycle time and increases productivity
  2. Heating efficiency - directly reflected in the costs
  3. Temperature uniformity of heated billet - is essential for product quality and rejects reduction
  4. Range of heated material - the versatility of the device allows production of a wide products range

In addition to these requirements, it is necessary to respect various constraints, such as dimensional, physical, technological and safety. Obviously, many demands are contradictory, and each may have a different importance. That's why the device is usually tailor-made. Designing induction heating is often a matter of seeking compromises, requiring expertise and rich experience. Recently, also the numerical simulations help us to design the heater. They allow us to better understand the physical processes of induction heating and to calculate the values (e.g. temperature distribution inside the billet) that are experimentally either not at all or very difficult measurable. Numerical simulation does not replace the experiment because it does not accurately calculate real values. The veracity of the result corresponds to the used mathematical model and the accuracy of the initial and boundary conditions. However, it can detect dependencies and thus determine the direction in which optimization of heating process should proceed.

To simulate induction heating process, we use computational software that solves the electromagnetic and thermal fields of two-dimensional physical problems by the finite element method. The result of the simulation is the time-dependent solution of these fields. The simulation calculates heat losses through radiation and convection. This is important, because especially the radiation losses are very high when heating steel at a forging temperature of about 1200°C.

A typical task of induction heating process prior to forging is to determine the sufficient time for a uniform temperature distribution in the billet. Due to the skin effect, the heat is generated more at the surface of the billet and gets into the core through the conductivity. A long time is needed to warm up the core properly. On the contrary, too long time leads to lower efficiency (higher heat loss) and lower productivity or increasing the heating line length. The desired shortening of the heating time can be achieved by dividing the process into two or more stages where the billet is in the first stage heated by increased power (progressive multi-stage heating). The simulations give us valuable information about the time development of surface-to-core temperature profile. At the beginning of the heating process, the surface is rapidly heated and the surface-to-core temperature difference increases. After some time the heat penetrates into the core, the temperature in the core begins to rise, and the surface-to-core temperature difference does not change much. Towards the end of the heating process there is a significant loss through radiation. The surface heats up more slowly and the surface-to-core temperature difference decreases, the temperature maximum moves below the billet's surface. After the heating is finished, the surface is naturally cooled and the temperature maximum moves to the core.

Simulace indukčního ohřevu

Temperature development in case of progressive two-stage heating

Another field of frequent use of numerical simulations is induction heating for surface hardening where local surface heating only to a certain depth is required. It is advantageous to use a simulation in the case of complex shaped parts where for example external edges may overheat due to electromagnetic effects. With the help of simulation the shape of the coil can be optimized in order to warm up the part where it is needed. In this field we cooperate with external simulation engineers.

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