SURFACE HARDENING BY INDUCTION

Main Principles

When electric current transmitted on a conductor, it creates electromagnetic field around the conductor. If conductor is a coil, this magnetic field flows through this coil. Even a matter (forexample a rod) is put into the coil, still there is a magnetic field. If magnetic current is created by high frequency alternate current, eddy current is increased and then rod is heated. But the steel is not magnetic over 768°C.

Induction Machines

During the frequency controls depth of hardness, it must been choose the machine according to depth. Because of the depth of hardness is increased by increasing amount of hardened area, it is choosen the machines with low frequency and voltage for the big parts. The best results for the small parts are got with using the machines that have high frequency and voltage.

Coils and Their Shapes

The coils that known conductors, are heated by both electric current and thermal radiation of steel. Coils are mostly made by copper tube and they are cooled by water that passed inside the tube, during the hardening. The coils are twisted with the distance will be 2-5 mm between spirals. It is allowed approximately same distance between coil and matter. Changing this distance affects the heating speed.

Hardening Method

Steel heated in the coil, is hardened by water or oil. The parts can be rotated during heating and hardening. This method is applied for round parts. Thus it is obtained uniform hardness depth. Because untidiness on the coil will be tolerated by rotating.

Effects of Some Factors to Hardness and Depth

Most important factor is amount of carbon in hardening. With increasing this amount to %0,80, in the case of  carbon will be present with other alloy elements, hardenableness will be increased, too. Heating temperature  and holding time in this temperature are most important factors that controlled hardness and depth. In the induction hardening when the conditions will be same, hardness depth will be controlled by frequency of inductive current.
In induction hardening heating temperatures are higher 50 °C than other usual hardening methods. Holding time in this temperature is very short, even usually holding is not applied in hardening temperature. It means when it is reached to required heat temperature, the part is cooled immediately.

Advantages and Disadvantages of Induction Hardening

Advantages:

  1. Limited partial hardening
  2. Short heating time,
  3. Minimum surface decarburizing and oxidation
  4. Deformation a bit
  5. Increasing tiring strength
  6. It can be applied needed straighten the part.
  7. This process can be combined with a production line
  8. Low process cost

Disadvantages:

    • Because of requirements of using high quality machine, high capital cost,
    • This method is limited for suitable shaped parts for induction hardening,
    • Limited steel types can be hardened by induction.