Heat loss

Heat losses through radiation at high temperatures can be considerable in slow-speed induction heating. Most metals, when fully enclosed in a toroidal induction coil, typically experience minimal heat losses below approximately 850°C. Above 850°C, especially when steel is through-heated to the forging temperature range of 1250–1300°C, heat losses become significant. Most of the lost heat passes through the coil lining and is carried away by the coil's cooling water. Radiation heat losses as a function of 0s (0a = 20°C) are given. Typical values ​​of ε are given based on a normal workpiece surface.

Steel penetration heating

When applying these values, it must be noted that many surfaces will oxidize with increasing temperature, increasing the ε value and the radiation heat loss. For example, the ε value of copper can vary from 0.03 for a polished surface to 0.70 for a heavily oxidized surface. For these reasons, it can be seen that accurately calculating the radiation heat losses expected in induction heating is difficult. Details regarding radiation are discussed in detail in 813.8. For through-heating of steel and other metals below 1300°C, the power input is approximately 0.3 to 1.5 MW-m.

induction coil

Most induction coils are well insulated with a lining. The power loss from the workpiece surface through the coil lining by radiation, averaged between room temperature and 1300°C, is typically approximately 40 kW-m. This value is significant and should be calculated for the entire workpiece surface area. Induction coil linings used for through-heating are described in detail in this chapter. Most linings are between 5 and 25 mm thick and consist of a metal inner sleeve and refractory felt, or a cast or rammed solid heat-resistant sleeve with steel guides. These materials act as an excellent thermal insulator, retaining the heat within the coil (the temperature drop across the lining can be as much as 280 to 560°K).