Detailed Guide to Vacuum Carburizing Process Parameter Settings

The key to vacuum carburizing lies in cyclic control of carburizing and diffusion to precisely adjust the surface carbon concentration and case depth of the workpiece. Below is a detailed explanation of the main process parameters.

Vacuum carburizing furnace

Carburizing Temperature

Temperature is the most significant factor affecting carburizing speed.

• Typical range: 900 °C to 1100 °C. The exact value depends on the material and requirements.

  • < 980 °C (low temperature): Suitable for complex‑shaped parts with strict distortion requirements.

  • ~980 °C (medium temperature): Suitable for general workpieces.

  • ~1040 °C (high temperature): Suitable for simple‑shaped parts requiring deep case depths.

• Principle: Higher temperatures greatly accelerate carbon diffusion. For example, raising the temperature from 925 °C to 1100 °C can increase the diffusion coefficient of carbon in iron by more than seven times. Vacuum carburizing has a lower activation energy for diffusion, so it is generally faster than conventional gas carburizing.

• Typical applications: Aerospace gears are often vacuum carburized at about 960 °C; 18CrNiMo7‑6 steel is carburized at approximately 925 °C.

Carburizing Pressure

A defining feature of vacuum carburizing is that it is carried out below atmospheric pressure (negative pressure).

• Typical range: Experience shows that the pressure is usually controlled between 3 and 25 mbar (i.e., 300 to 2500 Pa).

• Specific examples:

  • When using methane (CH₄) as the carburizing gas, furnace pressure is controlled at 26.6 to 45 kPa.

  • With propane (C₃H₈), the pressure is 13.3 to 23 kPa.

  • For RV reducer cycloidal gears, a pressure of 1500 Pa is used.

  • For 18CrNiMo7‑6 steel, the boost (enrichment) stage uses 1000 Pa, and the diffusion stage uses 100 Pa.

• Effects of pressure:

  • Higher pressure increases carbon supply, speeds up carburizing, and improves uniformity, but also raises the risk of soot formation.

  • Lower pressure (higher vacuum) promotes surface activation and enhances the adsorption of active carbon atoms.

• Principle: While ensuring uniform case depth, use as low a carburizing pressure as possible to minimise soot.

Carburizing and Diffusion Times

The critical time parameter is the ratio between the boost (enrichment) stage and the diffusion stage.

• Boost‑to‑diffusion ratio: This determines the final carbon concentration gradient. A typical ratio ranges from 1:2 to 1:7. For example, RV reducer cycloidal gears use a ratio of 1:5.

• Total time estimation: The total carburizing time (*t*) is related to the target case depth (*d*) and the carburizing temperature (T). It can be estimated using the Harris relationship:

  Estimation formula: *d* = φ(T) · √*t*
  where φ(T) is the carburizing rate coefficient, which increases with temperature.

• Pulse carburizing: In pulse processes, one “pulse” consists of a cycle of “gas‑injection carburizing – holding – vacuum diffusion”. The boost and diffusion times within each sub‑cycle must be set precisely.

Carburizing Medium (Carburizing Agent)

Choosing the right carburizing gas is vital.

• Common gases: Methane (CH₄), propane (C₃H₈), and acetylene (C₂H₂) are the main choices, with purity typically above 96%. Natural gas can also be used, often with nitrogen (N₂), hydrogen (H₂), or other diluent gases.

• Advantages of acetylene: Acetylene is currently recognised as an excellent carburizing medium.

  • High carburising capacity: One acetylene molecule provides two carbon atoms, whereas propane provides only one.

  • Reduced soot: Acetylene decomposes mainly upon contact with the workpiece surface, virtually eliminating soot and tar problems.

  • Wide adaptability: Especially suitable for uniform carburizing of complex shapes with deep, narrow blind holes.

• Gas flow control: The flow rate of carburising gas is typically set so that the furnace pressure rises at a rate of about 133.33 Pa/s.

Other Key Process Points

• Process mode selection: Choose according to workpiece shape and requirements.

  • Single‑stage: One boost stage followed by one diffusion stage; suitable for simple shapes.

  • Pulse‑type: Multiple alternating boost and diffusion cycles; particularly suitable for complex shapes with fine holes or blind holes.

• Carbon potential control: Vacuum carburising uses the “austenite carbon saturation control method” rather than the traditional atmosphere carbon potential. The control target is typically the surface carbon concentration of the workpiece. For example, the target in the boost stage may be as high as 1.8%, and the final surface carbon concentration is often set between 0.7% and 1.0%.

• Quenching and tempering: After carburising, quenching and tempering are usually performed to obtain the final properties – e.g., vacuum oil quenching or high‑pressure gas quenching. The tempering temperature is set according to hardness requirements, for example 180 °C.

Summary

Vacuum carburizing is a precision process with multiple coupled parameters. In production, the optimal process parameters are usually determined by combining theoretical calculations, computer simulations, and process trials specific to the material and component.

It is recommended to refer to relevant national standards, such as “Vacuum Carburizing and Quenching of Gears”, for more formal process guidance.