Gas Nitriding: Nitriding Potential (Kn) and Ammonia Dissociation

• The nitriding temperature is about 500–580 °C, and the steel is in a ferrite state with no phase change and small deformation.
• Reaction: 2NH₃ → 2[N] + 3H₂
• Kn = p(NH₃) / p(H₂)^1.5, unit bar^-0.5
• The decomposition rate is only an indirect indicator, and Kn directly determines the surface nitrogen activity.
• Controlled nitriding can precisely control the thickness and phase composition of the white layer

The basic process of nitriding

After ammonia enters the furnace, it decomposes on the surface of the steel (iron acts as a catalyst):

2NH₃ → 2[N] + 3H₂

Part of the active nitrogen atoms produced by decomposition penetrates into the steel, and part of it combines to form N₂ and leaves. Infiltrated nitrogen:

• Formed after reaching a certain concentration on the surface compound layer(ε-Fe₂₋₃N and/or γ′-Fe₄N, commonly known as white layer);
• Formed in the form of solid solution and alloy nitride precipitation below the compound layer diffusion layer。

Since the temperature is lower than the phase transformation temperature of steel, there is no phase transformation during the nitriding process, the parts deformation is small, and quenching is usually not required after nitriding.

Definition of nitriding potential Kn

Kn = p(NH₃) / p(H₂)^1.5

p is the partial pressure of ammonia and hydrogen in the furnace gas (bar or atm), so the unit of Kn is bar^-0.5.

The higher the Kn, the stronger the nitriding ability of the atmosphere, and the easier it is to form a compound layer or even the ε phase; the lower the Kn, the more likely it is that only the diffusion layer or the γ′ phase will be formed.

Why not just look at the decomposition rate

Traditional nitriding controls the process by measuring the "ammonia decomposition rate": low decomposition rate means more undecomposed ammonia and strong nitriding ability. But the relationship between decomposition rate and Kn is not fixed:

• When diluting with N₂ or pre-decomposed ammonia, the Kn corresponding to the same decomposition rate is different;
• Furnace temperature, furnace loading area (catalytic surface), and gas flow will all affect the decomposition rate;
• Decomposition rate measurement (traditional water absorption method decomposition rate meter) has limited accuracy and automation.

therefore,Taking Kn as the control variable, which calculates Kn in real time by measuring the hydrogen (or ammonia) content and adjusts the ratio of ammonia gas and diluent gas, which is the core of controlled nitriding.

Advantages of controlled nitriding

1. Precise control of compound layer thickness and phase composition (ε / γ′);
2. Diffusion layer nitriding with “no compound layer” can be realized;
3. The process is repeatable and suitable for demanding fields such as aerospace, wind power, and automobiles;
4. Save ammonia and reduce emissions;
5. Meets specifications such as AMS2759/10 that focus on nitriding potential control.