The nitriding layer consists of a surface compound layer (white layer) and a diffusion layer below. The compound layer provides wear resistance, corrosion resistance and anti-seizure properties, but is prone to peeling off when too thick or loose; the diffusion layer provides hardness gradient and fatigue resistance properties. The ε phase is wear-resistant and corrosion-resistant but brittle, while the γ′ phase has better toughness. The control method is to use nitriding potential to determine the phase composition, time and temperature to determine the thickness, and reduce the porous layer through a two-stage process.
- Compound layer: wear-resistant, corrosion-resistant, anti-seize; too thick or porous and easy to peel off
- Diffusion layer: provides hardness gradient and fatigue strength
- The ε phase is wear-resistant and corrosion-resistant but brittle, while the γ′ phase is relatively tough.
- The porous layer is mostly caused by too high nitriding potential or too long time
- Control: Kn phasing, temperature/time thickness
The structure of the nitrided layer
From the surface inward:
- Compound layer (white bright layer): White after metallographic corrosion, mainly ε-Fe₂₋₃N and/or γ′-Fe₄N, generally a few microns to more than 20 microns;
- diffusion layer: Nitrogen is solidly dissolved in ferrite and precipitates fine alloy nitrides, and the hardness gradually decreases from the surface inwards;
- matrix。
Performance of two types of layers
| Layer | Main function | Risk |
|---|---|---|
| compound layer | Wear-resistant, anti-seize, corrosion-resistant | If it is too thick or porous, it will be brittle and easy to peel off. |
| diffusion layer | Hardness gradient, fatigue resistance, contact fatigue resistance | The bearing capacity is low when the layer depth is insufficient. |
Choice of ε and γ′
| Mutually | Features | Suitable |
|---|---|---|
| ε-Fe₂₋₃N | High nitrogen content, high hardness, wear and corrosion resistance, but brittle | Wear-resistant parts, nitrocarburized parts, occasions requiring corrosion resistance |
| γ′-Fe₄N | Dense structure and good toughness | Parts that bear certain impact or alternating loads |
| No compound layer | Only diffusion layer, avoid brittle layer | Gears with high fatigue requirements and parts that require subsequent finishing |
Where does the porous layer come from?
Outside the compound layer, a loose layer containing micropores often appears. The main reason is that the nitrogen in the ε phase recombines into N₂ molecules and precipitates under high nitriding potential for a long time, leaving holes.
Ways to reduce the porous layer:
- Avoid maintaining Kn too high for a long time;
- Adopting a two-stage process, Kn is reduced in the latter stage;
- Control total time and temperature.
Control method
- The phase composition is determined by Kn: Refer to the Lehrer diagram to select the phase area;
- Thickness is determined by time and temperature: When the temperature is high and the time is long, the compound layer and diffusion layer will thicken;
- Uniformity is determined by atmosphere circulation and furnace charging: Ensure that Kn of each position is consistent;
- surface preparation: Clean and remove the passivation film, and pre-oxidize if necessary to ensure an even start of nitriding.
Inspection method
- Metallographic: Measure the thickness of the compound layer and observe the proportion and phase of the porous layer (can be distinguished by specific corrosive agents);
- microhardness gradient: Measure the effective depth of the diffusion layer;
- X-ray diffraction: Accurate determination of phase composition.
Frequently asked questions
Can the white glossy layer be removed?
It can be removed by grinding or grinding, but this adds cost and may affect size. A better approach is to directly obtain the required thickness or compound-free layer via Kn control.
Why does the surface turn black or have color difference after nitriding?
It may be related to pre-oxidation, residual oxygen in the furnace, cooling atmosphere or surface contamination. It usually does not affect the performance of the infiltration layer, but the atmosphere and cleaning process should be checked.
What is the appropriate thickness requirement for the compound layer?
Depending on part service conditions and drawing specifications, common requirements range from a few microns to around 20 microns, or explicitly require "no compound layer."