The strong penetration-diffusion process first uses a higher carbon potential (close to the austenite saturation limit, such as 1.1%–1.2%) to quickly input carbon to the surface, and then reduces it to the target surface carbon content (such as 0.8%) to allow the carbon to diffuse inward, thereby obtaining appropriate surface carbon and a gentle carbon concentration gradient while shortening the total time. The total carburized layer depth can be roughly estimated using the Harris empirical formula: approximately 0.635√t (mm, t is hours) at 927 °C. Accurate design requires diffusion simulation.
- Strong penetration: high carbon potential and rapid carbon supply, but does not exceed the saturation limit
- Diffusion: Reduce the carbon potential to the target surface carbon, and the carbon migrates inward
- Layer depth ∝ √ time, the higher the temperature, the faster the diffusion
- The Harris empirical formula is suitable for rough estimation, and diffusion simulation is used for accurate design.
- Common misunderstandings: excessive penetration leads to carbides, insufficient diffusion leads to excessive carbon on the surface
Why segmentation
If the entire carburizing process uses the carbon potential corresponding to the target surface carbon content (such as 0.8%), the carbon concentration difference between the surface and the core will be small, the diffusion driving force will be limited, and it will take a long time. If a high carbon potential is used throughout the process, the surface carbon will be too high, forming network carbides and too much retained austenite.
Strong penetration-diffusion takes into account both:
- Strong penetration stage: The carbon potential is close to the carbon saturation limit of austenite at this temperature, and a large amount of carbon is quickly obtained on the surface;
- diffusion stage: The carbon potential is reduced to the target surface carbon content, the excess carbon on the surface diffuses inward, and the surface carbon is reduced to the target value.
Carbon potential setting reference
| Stage | Carbon potential (reference) | Effect |
|---|---|---|
| Strong penetration | 1.0%–1.2% | Quickly supply carbon without exceeding the saturation limit to avoid carbon black and carbide |
| diffusion | 0.75%–0.9% | Achieve target surface carbon content and smooth carbon gradient |
| Before cooling/quenching | According to material and quenching requirements | Cool down to quenching temperature and homogenize |
Specific values are determined by material, temperature and product requirements.
Stratum Depth Estimation: Harris Empirical Formula
Carbon diffusion depth is approximately proportional to the square root of time. The commonly used Harris empirical formula at 927 °C (1700 °F):
Total carburized layer depth d ≈ 0.635 × √t (mm, t is the number of carburizing hours)
For example, carburizing at 927 °C for 4 h results in a total layer depth of approximately 1.27 mm. When the temperature decreases, the diffusion coefficient decreases, and the layer depth becomes shallower at the same time.
Notice:
- This is total carburized layer A rough estimate of is not equal to Effective hardened layer depth(e.g. to 550 HV), the latter is usually shallower;
- The ratio of strong penetration to diffusion time needs to be calculated separately;
- For alloy steels, complex cross-sections and precise processes, numerical simulations based on Fick's second law should be used.
Time allocation ideas
The diffusion stage requires sufficient time for the surface carbon to drop from the strong penetration level to the target value. The general approach is:
- Estimate the total time based on target layer depth and temperature;
- Estimate the proportion of strong carburization based on the strong carburizing potential, target surface carbon and matrix carbon content;
- Use simulation software or sample verification to determine the final process.
Common misunderstandings
- Strong carburizing potential exceeds saturation limit: Carbon deposits in the furnace and carbide formation on the surface;
- Insufficient diffusion time: The surface carbon is relatively high and there is a lot of retained austenite;
- Ignore alloying elements: Chromium, molybdenum and other elements increase surface carbon absorption and require alloy factor correction;
- Only look at the total time: The heating, equalizing and cooling stages also contribute to diffusion and should be included.
Frequently asked questions
What is the typical time ratio between strong penetration and diffusion?
There is no fixed ratio and depends on target layer depth, surface carbon, temperature and material. The deeper the layer depth, the greater the proportion of diffusion usually. It is recommended to use simulation software to calculate and verify.
How much time can be shortened by increasing the temperature?
The diffusion coefficient increases approximately exponentially with temperature. An increase of several dozen degrees Celsius can significantly shorten the time, but attention must be paid to grain growth, deformation and equipment life.
How to convert effective hardened layer and total carburized layer?
There is no uniform coefficient and depends on the material hardenability, surface carbon and quenching conditions. The enterprise's own correspondence should be established through metallographic or hardness gradient measurements.