It is, however, fair to say that as a general rule the induction heat treatment will not display as much total distortion as might occur with conventional thermal heat treatment. The reason for the lower distortion is the material supporting the newly induction-transformed martensitic case while the part is being scanned for heat treatment.
It is also fair to state that the maximum amount of distortion will occur as a direct result of through-heating by induction.
One can say that the higher the austenitizing temperature and the deeper the formed case, distortion will be proportionate. So, it follows that shallow case depth will encourage less potential distortion. In addition to this, when the part is being induction scanned, it means that only a small part of the workpiece surface is heated at any one time, which will also contribute to a lower distortion factor. As long as the part is symmetrical and in rotation, this will enhance the uniformity of heating.
When distortion is seen directly after quenching, it will usually have been caused by too high an austenitizing temperature and uniformity of the quench impingement on the induction heated surface.
The principles of induction heat treatment and transformation of the steel surface to martensite will necessitate the following conditions to be present:
- Sufficient surface carbon
- Appropriate austenitizing temperature
- Appropriate cooling rate to transform to martensite without, of course, any crack risk
- Prior microstructure
Case depth of the surface will depend on power selection and residence time at the appropriate austenitizing temperature. As previously stated, the surface of the induction hardened workpiece is likely to have almost 100% freshly formed untempered martensite.
It then becomes necessary to temper the workpiece as one would temper a conventionally heat-treated component. In other words, place the induction hardened component into temper as soon as is possible after induction hardening so as to reduce the risk of crack initiation.