Vacuum Gas Nitriding Principle

Hangzhou Ruiqin Mechanicial Equipment Co., Ltd. has developed a vacuum gas nitriding process in the past twenty years. Vacuum gas nitriding generally uses anhydrous ammonia (or ammonia + hydrogen, or ammonia + nitrogen) as the nitrogen supply medium.The whole nitriding process can be divided into three stages.

(1)Ammonia decomposition
Ammonia is a very unstable gas and is easy to decompose under certain conditions.Its decomposition rate increases with the increase of temperature. In the temperature range of 400~600℃, its natural decomposition rate tends to be completely decomposed.

The decomposition reactions are as follows:

2NH3=====2[N]+6[H]
The active nitrogen atom decomposed in ammonia gas is a new ecological nitrogen atom, which has great chemical activity. Some of it is absorbed by the surface of the workpiece and then diffuses from the surface to the interior. The remaining [N] is quickly combined into molecular N2 and H2, etc. and discharged from the exhaust gas together.

2NH3======2[N]+6H=====3H2+N2
In order for nitriding to continue uninterrupted, it is necessary to continuously feed ammonia gas and continuously produce reactive nitrogen atoms.

(2) The surface of steel parts absorbs nitrogen atoms
After the active nitrogen atoms are absorbed by the surface of the steel, they go deep into the ferrite to form ferrite with high nitrogen content, and then form nitrides after supersaturation.

(3) Diffusion
After nitrogen atoms are absorbed on the surface of steel parts, there is a nitrogen concentration gradient on the surface and the inner layer, which promotes the diffusion of nitrogen atoms from the surface to the inner layer to form a certain thickness of nitride layer.

At the nitriding temperature, the active nitrogen atoms in the adsorption layer move to the interior of the metal lattice, and the void left is quickly filled by the nitrogen atoms in the adsorption layer, so that the active nitrogen atoms continuously infiltrate into the metal surface all the time.So the diffusion process is as follows.

(1) Continuous input of nitrogen-containing gas into the furnace;
Ammonia molecules migrate to the metal surface;
Ammonia molecules are adsorbed on metal surfaces;
Ammonia decomposes continuously at the interface to form nitrogen and hydrogen atoms.
Absorb the remaining active atoms compound into molecules, constantly discharged from the furnace;
The nitrogen atoms adsorbed on the surface were dissolved in γ-Fe and α-Fe.

(2)Nitrogen atom diffuses from the metal surface to the center, and produces a certain concentration gradient.

(3)When the solubility of nitrogen in α-Fe is exceeded, nitrides begin to form in the surface layer.

(4)Nitrides grow along the vertical and parallel directions of the metal surface.

(5)The surface forms γ phase and ε phase in turn.

(6)The nitride layer is thickened continuously.

(7) Nitrogen diffuses from the nitride layer to the metal interior.

There are many factors that affect the above basic conditions, such as temperature, time, pressure, medium composition (or nitrogen potential), steel composition and structure of parts, etc.Gas nitriding process is to control these influencing factors reasonably and obtain satisfactory nitriding layer.

Editing by Francis Lee
Copyright © RQ Furnace

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