The hot wire resistance method uses filaments of special materials that are put into a furnace for carburization. The resistivity of the wire changes with the carbon content. After taking it out, the resistance can be directly measured to convert the carbon potential. It is faster than the carbon-fixed plate weighing method and requires less operators. The accuracy can reach about ±0.03 in the carbon potential range of about 0.7–1.2, and can be compared with the fixed-carbon plate placed in the same furnace. The key is consistency in placement, equilibration time, and pre-measurement processing.
- Principle: The resistivity of the wire after carburization changes with the carbon content
- Process: Unwind wire → Carburize balance → Take out and cool → Measure resistance → Convert carbon potential
- Accuracy: 0.7–1.2 CP range approximately ±0.03
- Advantages: fast, low requirements on operators, little impact on cleaning and drying
- It can be compared with the fixed carbon sheet in the same furnace as a daily verification method.
Why rapid carbon determination is needed
CQI-9 requires the carbon potential to be calibrated every day, while traditional carbon verification foil require more time and skilled operation. Hot wire resistance and three gas analysis are the two main fast alternatives.
Principle of hot wire resistance method
The resistivity of iron-based alloys changes with increasing dissolved carbon content. Put a filament (with a small diameter that can reach equilibrium with the atmosphere in a short time) into the furnace, take it out after carburization and equilibrium, measure its resistance (or resistance ratio), and then convert it into carbon content through the calibration curve, that is, the carbon potential of the atmosphere.
Operation process
- Prepare: Check the batch and appearance of the silk, and record the number;
- place: Use a special device to put the wire into the workpiece area or sampling port in the furnace, and record the time and furnace conditions;
- balance: Maintain the specified time to allow the yarn and atmosphere to reach balance;
- take out: Take out and cool under protective conditions to prevent oxidation and decarburization;
- Measurement: Use an instrument to measure resistance and convert carbon potential;
- Record: Compare with the carbon potential displayed by the controller to calculate the deviation.
Features
According to the introduction of "Carbon Potential Management Method Based on Heat Treatment CQI-9" by Gos Control:
- It can be measured directly after carburization and is easy to use;
- In the carbon potential range 0.7–1.2, the measurement accuracy is up to approximately ±0.03;
- Processing processes such as cleaning and drying have little impact on the results and have low requirements on operators;
- It can be put into the furnace together with the fixed carbon sheet and the carbon potential can be compared at the same time.
This method has been widely used in carburizing automobile parts in recent years, and key components and core algorithms have been domestically produced.
Compare with other methods
| Method | Speed | Accuracy | Operational requirements |
|---|---|---|---|
| hot wire resistance method | faster | 0.7–1.2 CP approximately ±0.03 | Low |
| Calibration plate weighing method | slow | high | high |
| Combustion analysis of fixed carbon tablets | Slow (needs inspection or special equipment) | high | middle |
| Three gas analysis | Quick (about 5 minutes) | good | Low |
Things to note
- The accuracy decreases when exceeding the optimal measurement range (too low or too high carbon potential);
- The placement location should represent the workpiece area;
- Insufficient equilibration time can lead to low results;
- Oxidation or decarburization during removal will result in lower results;
- Regularly compare with fixed carbon sheets to confirm that the method is effective.
Frequently asked questions
What should I do if the results of the hot wire and the fixed carbon sheet are inconsistent?
Check whether the placement position, balancing time and measurement procedure of both are standardized, and repeat as many times as necessary. If the deviation is stable, the method that is recognized as more accurate (usually a standardized carbon verification foil or a third-party laboratory) shall prevail.
Can hot wire be reused?
Generally for one-time use, please refer to the instrument instructions for details.
Is the hot wire method suitable for carbonitriding?
The presence of nitrogen affects resistivity and results need to be interpreted with caution. The instrument manufacturer's instructions regarding the suitability of the carbonitriding atmosphere should be consulted.