The Lehrer diagram uses temperature as the vertical axis and nitriding potential Kn (logarithmic coordinate) as the horizontal axis, indicating the phases that will be formed on the surface of pure iron under equilibrium conditions: when Kn is low, there is only α-Fe (nitrogen-containing ferrite), when it increases to a certain value, γ′-Fe₄N appears, and when it is higher, ε-Fe₂₋₃N appears. In the process, it is decided whether to form a compound layer and which phase to form by selecting which phase region Kn falls in. The phase boundaries of actual alloy steel will shift and need to be corrected based on experience.
- Temperature on the vertical axis, Kn on the horizontal axis (logarithmic coordinate)
- Three main phase regions: α (diffusion), γ′, ε
- Kn is lower than α/γ′ boundary → no compound layer is formed
- Want γ′ layer → Kn falls in γ′ zone; want ε layer → higher Kn
- The diagram is based on pure iron balance, alloy steel needs to be corrected
Coordinates and phase areas
The Lehrer diagram was established by E. Lehrer in the 1930s based on the pure iron-ammonia-hydrogen equilibrium:
- vertical axis: Temperature (°C);
- horizontal axis: nitriding potential Kn (logarithmic coordinate), or the equivalent NH₃/H₂ relationship;
- phase area: From left to right are α-Fe (nitrogen is solidly dissolved in ferrite), γ′-Fe₄N, and ε-Fe₂₋₃N. There is also γ (nitrogen-containing austenite) in the high temperature zone.
At a given temperature, the higher the Kn, the further to the right into the phase region with higher nitrogen content.
How to use
Example 1: Only diffusion layer, no brightening layer
Choose Kn that is lower than the α/γ′ boundary at the process temperature, so that only nitrogen-containing ferrite and alloy nitride precipitation are formed on the surface, and no compound layer is formed. Commonly used on parts that are sensitive to brittleness.
Example 2: A dense γ′ layer is required
Kn is selected in the γ′ region and the time is controlled to obtain a compound layer dominated by γ′ and with good toughness.
Example 3: Need a thicker ε layer (wear-resistant, corrosion-resistant)
Increasing Kn into the ε zone is common in nitrocarburizing or parts requiring corrosion resistance.
Two-stage nitriding
In order to balance process speed with microstructure, a two-stage process is commonly used:
- Use higher Kn in the first stage to quickly establish nitrogen concentration on the surface;
- The second stage reduces Kn, allowing nitrogen to diffuse inward, while controlling the thickness and phase composition of the compound layer to reduce the porous layer.
The Lehrer diagram helps determine in which region each segment Kn should fall.
Note on usage
- Diagram based on pure iron equilibrium: Alloying elements (Cr, Al, Mo, etc.) form alloy nitrides, which will change the actual nitrogen absorption and phase boundary position;
- dynamics: Lehrer diagram reflects the equilibrium state, the actual compound layer formation, incubation period and growth kinetics;
- surface state: Passivation film and oil stains will significantly affect the initial stage of nitriding, requiring cleaning and pre-oxidation/activation treatment;
- Measurement accuracy: Kn calculation relies on the accuracy of the hydrogen probe or analyzer.
Frequently asked questions
Why do the same Kn have different results on different steel types?
Alloying elements form nitrides with nitrogen, consuming surface nitrogen and changing the diffusion behavior; the initial structure and surface state of the steel also have an impact. The process needs to be verified by steel type.
Does the Lehrer diagram apply to nitrocarburizing?
Carbon is involved in nitrocarburizing, carbon-nitrogen compounds will appear, and the phase boundary is different from the Lehrer diagram of pure nitrogen. It can be used as a reference, but phase diagrams or empirical data of carbon-containing systems are required.
Can the controller display its current position in the Lehrer diagram?
Many modern nitriding control systems will display real-time Kn and temperature points with the Lehrer diagram as the background to facilitate intuitive judgment by the operator.