Hardening with high frequency current
(induction hardening)

Induction heating occurs due to the thermal effect of the current induced in the product placed in an alternating magnetic field. The choice of the optimal thickness of the hardened layer is determined by the operating conditions of the workpiece. When the product operates only under wear or fatigue conditions, the thickness of the hardened layer is most often taken as 1.5-3.0 mm; under conditions of high contact loads and possible regrinding 4.0-5.0 mm. In the case of particularly high contact loads, the thickness of the hardened layer can reach 10-15 mm or more.

During surface hardening with high-frequency current, heating is carried out to a higher temperature than during conventional bulk hardening. This is due to two reasons. Firstly, at a very high heating rate, the temperatures of the critical points at which the transition of pearlite to austenite occurs increase, and secondly, this transformation must have time to be completed in a very short heating time, and the higher the temperature, the faster it occurs.

Despite the fact that high-frequency hardening is heated to a higher temperature than conventional hardening, the metal does not overheat. This is explained by the fact that the high-frequency heating time is very short, and the grain in the steel does not have time to grow. On the other hand, due to the higher heating temperature and more intensive cooling, the hardness after high-frequency hardening is approximately 2-3 Rockwell units higher. This ensures higher strength and wear resistance of the part surface.

In many cases, high-frequency hardening allows replacing alloy steels with cheaper carbon steels. This is explained by the fact that such important advantages of alloy steels as deep hardenability and less deformation lose their significance for a number of parts. For example, when hardening gears with fine teeth, deep hardenability is even undesirable, since this can lead to through hardening of the teeth, which will cause their brittleness.

After surface high-frequency hardening, the parts are subjected to low tempering at a temperature of 160-200°C. This helps to reduce the brittleness of the hardened layer. Tempering is carried out in electric furnaces. Self-tempering can also be carried out.
Induction heating allows to reduce the duration of heat treatment, to obtain products without scale, which reduces the amount of allowance for further mechanical processing and reduces deformation and warping of products during heat treatment.

Purposes of surface hardening:

• increase in hardness;
• increase in wear resistance;
• increase in the endurance limit.