The carbon potential calculated by the controller is relative to the equilibrium carbon content of pure iron. Elements such as chromium, manganese, and molybdenum in alloy steel will increase the actual carbon content on the surface, while silicon and nickel will decrease, so the alloy factor needs to be corrected. The commonly used Gunnarson empirical formula is: lg(C_Fe / C_alloy) = 0.055Si − 0.013Mn − 0.040Cr + 0.014Ni − 0.013Mo (elements are mass percentages). The process factor (PF) is used to correct the systematic deviation between the measurement system and the actual atmosphere, and is obtained through carbon calibration or analyzer calibration.
- The definition of carbon potential is for pure iron, and alloy steel needs to be modified by the alloy factor.
- Cr, Mn, and Mo increase surface carbon; Si and Ni decrease
- The process factor corrects the systematic deviation of "controller display value vs. measured value"
- PF is determined by carbon verification foil, hot wire or three gas analysis verification
- A sudden change in PF often indicates a problem with the probe or atmosphere. You cannot just change the coefficient.
Two different "fixes"
Many sites confuse these two concepts, but in fact they solve different problems:
| Project | Alloy factor | Process Factor (PF) |
|---|---|---|
| Correction object | Materials: Effect of alloying elements on equilibrium carbon content | System: Deviation of the measurement chain from the real atmosphere |
| source | Material composition + empirical formula | Calibration data (carbon fixed sheet, hot wire, analyzer) |
| when to change | When changing materials | When the status of the probe, atmosphere, or equipment changes |
Alloy factor
The carbon potential is defined in terms of pure iron. For alloy steel, alloying elements change the activity of carbon in austenite:
- Chromium, manganese, molybdenum: Reduce the carbon activity coefficient, allowing steel to absorb more carbon under the same carbon potential;
- Silicon, Nickel: The effect is opposite.
Commonly used Gunnarson empirical formula:
lg(C_Fe / C_alloy) = 0.055·Si − 0.013·Mn − 0.040·Cr + 0.014·Ni − 0.013·Mo
Among them, C_Fe is the equilibrium carbon content (i.e., carbon potential) of pure iron in the atmosphere, C_alloy is the surface equilibrium carbon content of alloy steel, and the element content unit is mass percentage.
Example (illustration): A certain steel Cr 1.0%, Mn 0.9%, Si 0.25%, Ni 0, Mo 0: Right = 0.055×0.25 − 0.013×0.9 − 0.040×1.0 ≈ −0.038 C_Fe / C_alloy ≈ 10^(−0.038) ≈ 0.92, that is, the carbon content on the surface of the steel is approximately 1.09 times the carbon potential.
This means that to achieve 0.80% carbon on the surface of this steel, the carbon potential should be set at approximately 0.74%. The coefficients in different literatures are slightly different and should be verified with actual measurements.
Process Factor (PF)
The calculation of the oxygen probe is based on ideal conditions. In practice, there are factors such as individual differences in the probe, installation location, and deviation of the atmosphere from equilibrium, resulting in systematic deviations between the carbon potential displayed by the controller and the true carbon potential. The process factor is the coefficient used to correct this deviation (different manufacturers also call it furnace factor, gas factor, calibration factor, etc.).
How to determine:
- Put in the fixed carbon piece (or hot wire) under stable working conditions, and record the carbon potential displayed by the controller at the same time;
- Actual carbon content measured;
- Adjust PF so that the displayed value is consistent with the actual measured value;
- A three-gas analyzer can also be used to calculate the PF value in real time (Gauss Control FAA-B can calculate process values such as carbon potential and PF).
Note on usage
- PF is "calibration", not "masking": If the PF needs to be adjusted frequently and significantly, it means that the probe is aging, the furnace is leaking, or the carrier gas is unstable, and the cause should be identified;
- Record every PF adjustment The reasons and basis are the focus of the CQI-9 review;
- Different temperatures and different carbon potential segments The deviation may be different and should be verified in sections if necessary.
Frequently asked questions
Should the alloy factor be set in the controller or in the process?
There are two approaches. If the controller supports input of alloy factors by material, "surface carbon" can be directly displayed; otherwise, the carbon potential setting value is converted during process design.
What range is considered normal for PF?
Different manufacturers have different definitions and there is no unified scope. The key is to look at the stability of PF: long-term stability indicates that the system is under control, and sudden changes require investigation.
How big is the error of the fixed carbon sheet itself?
Carbon fixation is considered to be a more accurate method, but it has high operational requirements: placement, cooling, cleaning and weighing will all introduce errors, and the operating procedures need to be standardized.