The "swelling" phenomenon of parts after ion nitriding and its prevention countermeasures
I. The essence of "swelling"
the "swelling" of parts after ion nitriding is actually a manifestation of the size change of parts. The size change is caused by the absorption of a large number of nitrogen atoms on the workpiece surface during nitriding, the formation of various nitrides or the increase of lattice constant of the original structure on the surface of the workpiece. On the macro level, it shows a slight increase in the surface volume
the "swelling" of parts after nitriding is a common phenomenon. Parts treated by various nitriding methods (gas nitriding, liquid nitriding and ion nitriding) will always have some "swelling" more or less. However, it should be noted that the "swelling" of parts after ion nitriding is smaller than that of other nitriding methods. This is because the "cathode sputtering" in ion nitriding can reduce the size, thus offsetting part of the nitriding "swelling"
II. Factors affecting "swelling"
the expansion of dimension after nitriding depends largely on the nitrogen absorption of the surface layer of the part. Therefore, the factors that affect the amount of nitrogen absorption are the factors that affect the "swelling"
the main factors affecting "swelling" are: the content of alloy elements in the material, nitriding temperature, nitriding time, nitrogen potential in nitriding atmosphere, etc
the higher the content of alloy elements in the material, the greater the "swelling" of parts after nitriding
the higher the nitriding temperature and the longer the nitriding time, the greater the "swelling" of the parts after nitriding
the higher the nitrogen potential of nitriding atmosphere, the greater the "swelling" of parts after nitriding
III. Prevention and treatment of "swelling"
as mentioned above, "swelling" is an inevitable phenomenon of extrusion pressure fluctuation in the nitriding process, so it is unrealistic to completely eliminate "swelling". The "prevention" of the main electrostatic phenomenon here will bring a lot of harm to our experimental machine. There are two meanings: one is to reduce the "swelling" as much as possible; The second is to master automatic diagnosis when "swelling" is inevitable: the system has the automatic diagnosis function "swelling" law, eliminating the reprocessing after nitriding
1. Methods to reduce "swelling"
① select materials correctly according to the service conditions of the workpiece. Avoid blindly using "good" materials (high alloy steel) due to the pursuit of workpiece performance
② according to the service conditions of the workpiece, put forward reasonable nitriding requirements to avoid one-sided pursuit of nitriding layer depth and hardness
③ properly carry out the pre heat treatment and "stabilization" treatment before nitriding. The pre heat treatment process parameters must be correctly formulated and the operation must be reasonable. For parts with complex shapes, one or several "stabilization" treatments must be carried out before final finishing
④ properly reduce nitriding temperature and nitriding time on the premise of process permission
⑤ adjust the nitriding atmosphere on the premise of ensuring the performance of nitriding layer
⑥ charge the furnace reasonably to ensure the uniformity of workpiece temperature in the same furnace
2, "swelling" law, eliminating the reprocessing after nitriding
generally speaking, under the premise of correct material selection and process formulation, if the furnace can be reasonably charged and operated correctly, the "swelling" of the workpiece has a certain law. After mastering the "swelling" law, the workpiece size can be in a negative deviation according to the "swelling" amount in the last processing procedure before nitriding treatment. The size of the workpiece after nitriding treatment can be just within the required dimensional tolerance range, so the re processing after nitriding can be omitted, and then this layer of metal film can play a role. (end)