According to the carbon potential calculation formula, the three major factors that affect accuracy are the output status of the oxygen probe, furnace temperature accuracy and atmosphere CO stability. The analysis of Gauss Control shows that: at 930 °C, CO 20%, and CP 1.20, a furnace temperature deviation of ±5 °C will bring about a carbon potential error of ±0.03, and ±10 °C will bring about a carbon potential error of about ±0.05; the oxygen probe attenuation of 5 mV can cause a carbon potential deviation of about ±0.10 during carburizing at 930 °C; nitrogen-methanol atmosphere CO at 19.3%–22.2% Fluctuations can cause deviations of approximately ±0.12. CQI-9 requires that the carbon potential be controlled within ±0.05 of the set value.
- Furnace temperature ±5 °C → approximately ±0.02–0.03 CP; ±10 °C → approximately ±0.05 CP
- Oxygen probe 5 mV deviation → approximately ±0.10 CP at 930 °C
- CO 19.3%–22.2% fluctuation → approximately ±0.12 CP
- Any loss of control may exceed the ±0.05 requirement of CQI-9
- Control methods: TUS/SAT, probe management, stable carrier gas
Why are these three factors
There are only three inputs for calculating the carbon potential from the oxygen probe: mV, temperature, and CO%. If any input has an error, the output carbon potential will have an error. Therefore, the problem of carbon potential accuracy is essentially a problem of the quality of these three inputs.
Factor 1: Furnace temperature
The furnace temperature uniformity of conventional controlled atmosphere equipment can generally be within ±5 °C. But even within this range, the effect of temperature deviation on the carbon potential cannot be ignored.
| Working conditions | Furnace temperature deviation | Carbon potential deviation (approx.) |
|---|---|---|
| 930 °C，CO 20%，CP 1.20 | ±5 °C | ±0.03 |
| 930 °C，CO 20%，CP 1.20 | ±10 °C | ±0.05 |
| 860 °C，CO 30%，CP 1.00 | ±5 °C | ±0.02 |
| 860 °C，CO 30%，CP 1.00 | ±10 °C | ±0.05 |
Control means: Regularly conduct furnace temperature uniformity test (TUS) and thermocouple system accuracy test (SAT); probe temperature and furnace control temperature are monitored separately.
Factor 2: Oxygen probe
The output error of a new oxygen probe usually needs to be within ±5 mV. However, in a typical carburizing process, this seemingly small deviation has a significant impact: when carburizing at 930 °C, an oxygen probe attenuation of 5 mV has an impact on the carbon potential of up to approximately ±0.10.
Control means：
- Establish probe entry and exit records and daily inspection records;
- Burn charcoal regularly and check the reference air;
- Use an analyzer or fixed carbon sheet to compare and replace it promptly if the attenuation trend is found.
Factor three: CO stability (carrier gas)
CO% is usually the set value in calculations. If actual CO fluctuates, carbon potential calculations will be systematically biased. One month of continuous monitoring of a nitrogen-methanol integral-quench furnace (920 °C, CP 1.1) by Gauss Control showed that CO varied between 19.3%–22.2%, causing a carbon potential deviation of approximately ±0.12. In contrast, after the gas generated by the RX generator is introduced into the carburizing furnace, CO and CO₂ remain stable, and CH₄ remains at a low level.
Control means: Use a carrier gas with stable composition (such as RX gas); measure CO online and correct the calculation; stabilize methanol drip and nitrogen flow.
Comparison with CQI-9 requirements
CQI-9 (Sections 3.7 and 3.8 of the third edition) requires that the atmospheric carbon potential be continuously monitored, automatically controlled and recorded, and the recorded carbon potential should be controlled within ±0.05 of the set value. It can be seen from the above data that if any one of the three factors is out of control, it may alone exceed this range. Of the 45 heat treatment companies in the automotive industry analyzed by Gauss Control Sampling, 55% failed to meet the ±0.05 requirement.
Frequently asked questions
Can the carbon potential problem be solved by just replacing the oxygen probe with a new one?
uncertain. If the problem comes from oven temperature deviations or CO fluctuations, changing the probe will not solve it. The main sources of error should be identified first using analyzers and temperature tests.
Why are the carbon potentials different at different locations in the same furnace?
Uneven temperature fields and atmosphere flows in the furnace, as well as furnace loading methods, will lead to local differences. The probe position should be representative of the area where the workpiece is located.
Can furnace temperature deviation be corrected using process factors?
Process factors can correct stable system deviations, but cannot correct fluctuating temperature errors. The fundamental solution is to ensure temperature system accuracy and uniformity.