Factors and Prevention Methods of Ion Nitriding “Swelling”

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The “swelling” of the parts after ion nitriding is actually a manifestation of the dimensional changes of the parts. The dimensional change is caused by the absorption of a large number of nitrogen atoms on the surface of the workpiece during nitriding, and the formation of various nitrides or the increase of the lattice constant of the original structure of the workpiece surface. Macroscopically, the surface volume is slightly increased.

“Swelling” of parts after ion nitriding is a common phenomenon. The parts treated by various nitriding methods (gas nitriding, liquid nitriding and ion nitriding) will more or less always have a certain “swelling”. But it should be noted that the “swelling amount” of the parts after ion nitriding is smaller than other nitriding methods. This is because the “cathode sputtering” in ion nitriding has the effect of reducing the size, thus offsetting part of the “swelling” of nitriding.

1. Factors affecting the “swelling” of ion nitriding

The size expansion after ion nitriding depends on the amount of nitrogen absorbed on the surface of the part. Therefore, the factors that affect the amount of nitrogen uptake are all factors that affect “swelling”.

The main factors affecting “swelling” are: the content of alloying elements in the material, the nitriding temperature, the nitriding time, and the nitrogen potential in the nitriding atmosphere.

The higher the content of alloying elements in the material, the greater the “swelling” of the part after nitriding.

The higher the nitriding temperature and the longer the nitriding time, the greater the “swelling” of the part after nitriding.

The higher the nitrogen potential of the nitriding atmosphere, the greater the “swelling” of the part after nitriding.

2. Prevention and treatment of “swelling”

As mentioned earlier, “swelling” is an inevitable phenomenon in the process of ion nitriding, so it is unrealistic to completely eliminate “swelling”. The “prevention” we are talking about here mainly has two meanings: one is to reduce the amount of “swelling” as much as possible; the other is to master the law of “swelling” when “swelling” is unavoidable, and eliminate the need for repetition after nitriding. Processing.

2.1 Ways to reduce “swelling”

According to the service conditions of the workpiece, select the material correctly. Avoid blindly using “good” materials (high-alloy steel) due to the pursuit of workpiece performance.

According to the service conditions of the workpiece, reasonable nitriding requirements are put forward to avoid the phenomenon of one-sided pursuit of the depth and hardness of the nitriding layer.

The pre-heat treatment and “stabilization” treatment before nitriding must be done correctly, the preparation of the pre-heat treatment process parameters must be correct, and the operation must be reasonable. For parts with complex shapes, one or several “stabilization” treatments must be carried out before final finishing.

If the process permits, appropriately lower the nitriding temperature and shorten the nitriding time.

Under the premise of ensuring the performance of the nitriding layer, adjust the nitriding atmosphere.

Install the furnace reasonably to ensure the uniformity of the temperature of the workpieces in the same furnace.

2.2 The law of “swelling” eliminates the need for reprocessing after nitriding

Generally speaking, under the premise of correct material selection and process formulation, if the furnace can be installed reasonably and operated correctly, the “swelling” of the workpiece will have a certain law. After mastering the law of “swelling”, in the last processing procedure before nitriding treatment, the size of the workpiece can be in a negative deviation according to the amount of “swelling”. After the nitriding treatment, the size of the workpiece can be within the required dimensional tolerance range. Therefore, reprocessing after ion nitriding can be omitted.

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