Gas carburizing includes two series processes: one is that carbon in the atmosphere is transferred to the steel surface through a chemical reaction (controlled by the carbon potential and carbon transfer coefficient), and the other is that carbon diffuses from the surface to the interior (controlled by temperature, time and the diffusion coefficient of the steel). The main reactions are the methane reaction CH₄ → [C] + 2H₂, the boundary reaction 2CO → [C] + CO₂, and the water gas reaction CO + H₂ → [C] + H₂O. When the residual methane in the carrier gas is very low and the atmosphere is close to equilibrium, the carbon potential can be characterized by boundary reactions or water-gas reactions.
- Two-step process: surface carbon transfer + internal diffusion
- Three main reactions: methane reaction, boundary reaction, water gas reaction
- When the carrier gas is close to equilibrium, use the CO/CO₂ or CO/O₂ relationship to calculate the carbon potential
- The penetration rate is affected by temperature, carbon potential, carbon transfer coefficient and steel type.
Two series processes
- Transfer of carbon (surface reaction): The carbon-containing components in the atmosphere decompose on the surface of the steel and "give" carbon to the steel. The velocity depends on the difference between the carbon potential of the atmosphere and the carbon content on the surface, and the carbon transfer coefficient β.
- Diffusion of carbon (solid state diffusion): Carbon diffuses from the surface of high concentration to the center of low concentration, following Fick's law. The speed depends mainly on temperature (diffusion coefficient increases exponentially with temperature) and time.
The design of the carburizing process is to find a balance between the two: sufficient carbon supply on the surface and avoid excessive surface carbon.
Three main reactions
| Reaction | Equation | Description |
|---|---|---|
| Methane reaction | CH₄ → [C] + 2H₂ | Enriched gas (natural gas, propane) provides carbon and reacts slowly |
| Border reaction (Boudouard) | 2CO → [C] + CO₂ | CO is the main carbon supply component |
| water gas reaction | CO + H₂ → [C] + H₂O | It is fast and is one of the main ways of carburizing. |
In addition, there is a continuous water-gas shift reaction CO + H₂O ⇌ CO₂ + H₂ in the atmosphere, linking H₂O and CO₂.
When designing the atmosphere, the residual methane after the carrier gas enters is generally controlled to be very low, so that the atmosphere is close to chemical equilibrium and remains stable. At this time, the transfer of carbon can be characterized by water gas reaction or boundary reaction. This is the theoretical basis for the oxygen probe and CO₂ analyzer to calculate the carbon potential.
Carbon transfer coefficient β
Even if the carbon potential is the same, different atmospheres transfer carbon to the steel surface at different rates. β is roughly related to the product of the partial pressures of CO and H₂, so:
- Pure methanol cracked gas (both CO and H₂ are high) delivers the fastest;
- Endothermic gas centered with nitrogen-methanol (approximately 20% CO, 40% H₂);
- A diluted atmosphere (high N₂ ratio) travels more slowly.
Factors affecting penetration rate
- temperature: The most significant. Increasing the temperature can greatly shorten the carburizing time, but it will cause grain growth and increased deformation;
- Carbon potential: The higher the carbon potential, the higher the surface carbon concentration, and the greater the diffusion driving force;
- atmosphere type: Determines the carbon transfer coefficient;
- Steel type: Alloying elements affect the solubility and diffusion of carbon in austenite;
- Furnace loading and atmosphere circulation: Affects the uniformity of atmosphere throughout the parts.
Frequently asked questions
What is the general carburizing temperature?
Conventional controlled atmosphere carburizing is usually between 900–950 °C, with 920–930 °C being common. High-temperature carburizing can shorten the time, but attention must be paid to grain size and deformation.
Natural gas or propane for enrichment gas?
Both are fine. Propane contains more carbon, has strong enrichment ability, and uses less amount; natural gas is easier to obtain and the combustion products are cleaner. The choice depends on local supply and control system settings.
Why does it need to be quenched after carburizing?
Carburizing only increases the carbon content of the surface layer, which must be quenched to form high-carbon martensite before the surface can obtain high hardness; then tempered at low temperature to reduce brittleness.