Carbon Potential (CP) refers to the carbon content (mass fraction, %) that the iron surface can have when the furnace atmosphere and pure iron reach equilibrium at a given temperature. When the carbon potential is higher than the carbon content of the steel, the atmosphere is carburized; when it is lower, it is decarburized; when it is equal, it is neutral. During carburizing, the strong carburization stage is usually set at about 1.0–1.2%, the diffusion stage is reduced to about 0.8%, and heating before quenching sets the carbon potential to be equivalent to the carbon content of the steel.
- The unit of carbon potential is mass percentage (%C), which is defined by the equilibrium carbon content of pure iron.
- CP > carbon content of steel → carburization; CP < carbon content of steel → decarburization; equal → neutral
- The carbon potential is determined by a combination of temperature, oxygen partial pressure (or CO₂/H₂O), and CO content
- The actual surface carbon content of alloy steel needs to be corrected by the alloy factor
Definition
The carbon potential describes The "ability" of the atmosphere to transfer carbon to steel. The strict definition is: at a certain temperature, if pure iron is placed in the atmosphere for a long enough time until it reaches equilibrium, the carbon content on the surface of the pure iron is the carbon potential of the atmosphere. It is directly related to the carbon activity in the atmosphere.
Because it is the "equilibrium carbon content", the unit of carbon potential is the same as the carbon content of steel, expressed as a mass percentage, for example, CP = 0.80% or abbreviated as 0.80.
The relationship between carbon potential and steel: carburization, neutrality or decarburization
| Situation | Carbon potential in the furnace vs carbon content of the steel | Result |
|---|---|---|
| Carburization (carburization) | The carbon potential is higher than the carbon content of the steel | Carbon enters the steel surface from the atmosphere |
| neutral heating | The carbon potential is approximately equal to the carbon content of steel | Surface composition remains unchanged |
| decarbonization | The carbon potential is lower than the carbon content of steel | Carbon is lost from the steel surface |
Therefore, the same carbon potential value may be a carburizing atmosphere for 20CrMnTi and may be a decarburizing atmosphere for GCr15 bearing steel. When quenching and heating, the carbon potential should be set according to the carbon content of the material.
Common setting ranges (reference)
- Strong carburizing stage: about 1.0%–1.2%, limited by the austenite carbon saturation limit at temperature. If it is too high, carbon black will precipitate;
- Carburizing diffusion stage: approximately 0.75%–0.9% to achieve target surface carbon content;
- Quenching and heating of medium carbon steel and bearing steel: set according to the carbon content of the material (such as about 0.4%, 1.0%).
Specific values should be determined based on material, part requirements and process specifications.
What determines the carbon potential?
In an atmosphere containing CO, H₂, CO₂, and H₂O, the carbon potential is determined by the following factors:
- temperature: The higher the temperature, the lower the carbon potential of the same atmosphere;
- oxidizing components: The lower the oxygen partial pressure, CO₂ or H₂O (dew point), the higher the carbon potential;
- CO content: The higher the CO, the higher the carbon potential.
Therefore, there are three common measurement paths for controlling carbon potential: oxygen probe (measures oxygen partial pressure), infrared analyzer (measures CO₂), and dew point meter (measures H₂O). They essentially measure "oxidative components" and then combine temperature and CO% to convert carbon potential.
Why carbon potential must be controlled
- too low: Surface decarburization, insufficient hardness, reduced fatigue strength, and significantly reduced service life of gears, springs and other parts;
- too high: Excessive network carbides, excessive retained austenite, and even carbon deposits in the furnace appear on the surface;
- unstable: Layer depth and surface hardness fluctuate from batch to batch and fail to pass process audits such as CQI-9.
Frequently asked questions
What is the relationship between carbon potential and dew point?
In an endothermic atmosphere, when CO and temperature are constant, the lower the dew point (the less moisture), the higher the carbon potential. Dew point is one of the indirect measurement methods of carbon potential, but the carbon potential cannot be directly derived from dew point alone. It needs to be converted by combining temperature and atmosphere composition.
Why is the carbon potential displayed by the controller different from that measured by the carbon verification foil?
Common reasons include: oxygen probe aging or carbon deposits, furnace temperature deviation, CO% actual value different from the set value, furnace body leakage, and alloy factor not corrected. Calibration methods are needed to find deviations and correct process factors.
The higher the carbon potential, the faster carburizing?
Yes, within a certain range. The higher the carbon potential, the greater the surface carbon concentration gradient and the faster the penetration rate. However, exceeding the austenite saturation limit will produce carbon black and excessive carbides, so the "strong penetration-diffusion" segmented process is commonly used to take into account both speed and structure.