Induction hardening and self-tempering processes are important processes in heat treatment. In industrial applications, many steel workpieces need to achieve a certain hardness, but also must have a certain toughness to achieve the best mechanical properties. Induction hardening can provide the hardness of the workpiece, but there is a large internal stress inside the workpiece after hardening. The self tempering process can eliminate the internal stress and increase the toughness and plasticity of the workpiece. Therefore, we often use induction hardening and self-tempering processes in the heat treatment process of workpieces.
How to perform induction hardening and self-tempering to improve the quality of steel parts? This is a question that concerns many engineers, and this article will introduce the reader to the important influencing factors in the process.
Induction hardening and self-tempering processes
In the induction heat treatment process, some process parameters such as heating, cooling and self-tempering parameters have an impact on the quality of hardened parts. Therefore, these parameters must be set in strict accordance with the process requirements.
Induction heating
1. Purpose of heating
- At the completion of induction heating, the temperature of the surface layer of the parts shall be equal to or slightly higher than the induction hardening temperature.
- A suitable heating depth shall be obtained.
2. The heating temperature and the depth of the heating layer depend on the following factors:
- Average effective power transmitted to the parts during the heating.
- Heating time.
- Current frequency.
When other factors remain unchanged, the greater the power and the longer the heating time, the larger the depth of the heating layer and the hardening layer of the parts. Insufficient heating power or time will result in incomplete hardening of the workpiece or no hardening at all. If a time relay is used to adjust and control the heating time, it shall be checked with a stopwatch at least twice a month. The relay shall be checked with a mechanical stopwatch immediately after adjustment. The error of the relay shall be kept within ± 0.1s, and any energy monitor shall be used in accordance with the requirements of the device.
Cooling
After the heating is completed, the parts shall be cooled immediately or after a certain pre-cooling time to complete induction hardening.
1. The quality of induction hardening results is reflected in the following three aspects:
- Hardness value measured directly after cooling.
- Magnitude of internal stress in the parts.
- Depth, area and microstructure of the hardening layer.
2. Hardening results depend on the following parameters:
- Cooling time.
- Temperature of the hardening and cooling medium (water, oil, polymer aqueous solution, etc.).
- Pressure (or flow rate) of the ejected hardening and cooling medium.
Longer cooling time, lower temperature of the hardening and cooling medium, and higher injection pressure mean more intensive hardening and lead to higher surface hardness of parts, greater hardening stress, and greater risk of crack formation.
In order to avoid rejected products, the process must be strictly followed, and the pre-cooling and cooling time must be adjusted according to the specified range of process parameters and checked with a stopwatch.
Self-tempering
1. The self-tempering results are reflected in the following aspects:
- Reduction of hardening hardness.
- Degree of internal stress relief.
2. The self-tempering results depend on:
- Maximum tempering temperature.
- Self-tempering time.
Self-tempering time is defined as the period in which the parts stay in the air from the completion of cooling until they are re-wetted (if the subsequent process requires timely processing), which is sufficient to complete the tempering effect. Self-tempering time shall comply with the process regulations. When other factors remain unchanged, the shorter the cooling time of parts, the more residual heat in the center of parts, the higher the self-tempering temperature, the more complete the internal stress relief, and the more the hardening hardness reduction.
3. Inspection of self-tempering quality
- Measure the reduction of hardening hardness, and compare the parts that are self-tempered after hardening with those that are not (completely cooled during hardening) to obtain the reduction of hardness of parts due to self-tempering.
- Check for hardening cracks.
- Use a file to file the surface of parts that has just been sprayed with liquid and observe the tempering color (oxidation color) of the surface to roughly determine the self-tempering temperature.
- Measure the self-tempering temperature directly with an infrared thermometer. It is the most reliable method.
Conclusion
Induction hardening and self-tempering require attention to parameters during heating, cooling and self-tempering. Temperature, time, power and frequency can have a major impact on the quality of steel workpieces. Generally it is often used for hardening steel parts such as gears, bolts, shafts, guide rails, etc.
The heat treatment plant should set various parameters according to the hardness requirements and hardened layer depth requirements of carbon steel workpieces. The induction heat treatment process is then continuously optimized based on the quality check results.
Related Induction hardening and tempering machines
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