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Key points in the induction heat treatment process

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Update time : 2019-06-08 21:04:00
During the induction heat treatment process, some process parameters such as heating, cooling, and self-tempering have an impact on the quality of the hardened parts, so they must be set strictly in accordance with the requirements of the process.



1. Purpose of heating

  • After the heating is completed, the temperature of the surface layer of the part should be equal to or slightly higher than the hardening temperature.
  • Get the proper heating depth.

2. The heating temperature and the depth of the heating layer depend on the following factors:

  • The average effective power delivered to the parts within the heating time.
  • Heating time.
  • Current frequency.
When the other conditions are the same, the greater the power and the longer the heating time, the greater the depth of the heating layer and hardened layer of the part. If the heating power or time is insufficient, it will result in incomplete hardening or no hardening at all. When using a time relay to adjust and control the heating time, check the time relay at least twice a month with a stopwatch. After the time relay is adjusted, it should be checked immediately with a mechanical stopwatch. The error of the time relay should be within ± 0.1s, and those using energy monitors should follow the requirements of the device.




The parts should be cooled immediately after heating or after a certain pre-cooling time to complete hardening.

1. The quality of the workpiece after hardening is based on the following three aspects:

  • The hardness value measured directly after cooling.
  • The magnitude of internal stress in the part.
  • The depth, area and microstructure of the hardened layer.

2. The hardening result is determined by the following parameters:

  • Cooling time.
  • Temperature of hardening cooling medium (water, oil, polymer aqueous solution, etc.).
  • Pressure (or flow rate) of hardening cooling medium when it is sprayed out.
The longer the cooling time, the lower the hardening cooling medium temperature, the higher the injection pressure, the stronger the hardening, the higher the hardness of the part surface, the greater the hardening stress, the greater the risk of crack formation.
In order to avoid the generation of waste products, the heating process must be strictly followed, the pre-cooling and cooling time should be adjusted according to the specified process parameter range, and checked with a stopwatch.



1. The results of self-tempering are:

  • The decrease of hardening hardness.
  • The degree of internal stress relief.

2. The result of self-tempering depends on:

  • The highest value of tempering temperature.
  • Self-tempering time.
The part stays in the air from the completion of cooling to re-wetting (if the post-process needs to be processed in time), the time enough to complete the tempering is called the self-tempering time. The self-tempering time shall be carried out in accordance with the process regulations. When the other conditions are the same, the shorter the cooling time of the part, the more residual heat in the core of the part, the higher the self-tempering temperature, the more complete the internal stress relief, and the more the hardening hardness is reduced.

3. Self-tempering quality inspection:

  • Measure the reduction value of hardening hardness, and compare the self-tempered parts after hardening with the parts without self-tempering (complete cooling after hardening) to obtain the value of the decrease of the hardness of the parts by self-tempering.
  • Check for hardening cracks.
  • Use a file to file the surface of the part that has just been sprayed, and observe the tempering color (oxidation color) of the surface to determine the self-tempering temperature.
  • It is the most reliable to directly measure the temperature of self-tempering with infrared thermometer.
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