What Heat Treatment Should be Carried Out Before Nitriding Steel Parts?

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Nitriding is the last step in the manufacturing process of parts. After the workpiece is nitrided, only fine grinding or grinding is performed.

In order to ensure that the core has good comprehensive mechanical properties, eliminate processing stress, reduce nitriding deformation, and prepare for nitriding, the workpiece generally needs to undergo preliminary heat treatment before nitriding.

The commonly used pre-heat treatment before nitriding of structural steel is quenching and tempering treatment to obtain tempered sorbite structure.

The quenching and tempering process of nitriding parts has a great influence on the quality of nitriding.For example, 38CrMoAlA nitrided steel, due to the high stability of aluminum-containing ferrite, is not easy to dissolve into the austenite when heated. If the quenching and holding time is not enough or the temperature is too low, the ferrite will not be completely dissolved into the austenite In the body, free ferrite exists after quenching and tempering.

Due to the high diffusion rate of nitrogen in ferrite, there will be a higher nitrogen concentration after nitriding, and it is easy to form needle-like nitrides, which makes the nitrided layer more brittle and easy to peel off. Therefore, after quenching and tempering, free ferrite is not allowed to appear on the surface layer, and the amount of free ferrite in the heart is not allowed to exceed 5%. On the contrary, if the quenching temperature is too high, the crystal grains will become coarser after quenching, and the nitrides will preferentially stretch along the grain boundaries, and the corrugated or network structure will appear after nitriding, which will also increase the brittleness of the nitrided layer.

The tempering temperature is too high, the dispersion of carbides in the matrix is reduced, and the core strength and hardness of the nitrided part are insufficient, and it cannot play the role of supporting the hard and brittle nitride layer. Under the action of external force, especially under the action of large contact stress, it will often cause crushing or peeling, causing premature failure of parts.

The tempering temperature is too low, the core strength and hardness are too high, and it is difficult to cut the parts after preliminary heat treatment. In addition, the nitriding speed will be reduced. Therefore, the tempering temperature is appropriately selected to adjust the dispersion of the carbide, so that it can not only make the infiltration layer and the core have better performance, but also obtain a certain infiltration rate.

Because 38CrMoAlA steel contains molybdenum and higher aluminum, it is severely decarburized during heating, and the decarburized layer will increase the brittleness of the nitride layer and reduce the hardness. Therefore, the quenching and quenching of the nitrided parts of 38CrMoAlA steel is best performed in a protective atmosphere. When heating in the furnace, if necessary, the workpiece should have a large machining allowance to ensure that the decarburized layer can be completely processed during mechanical processing.

For parts with complex shapes, strict dimensional stability and deformation requirements, stabilization treatment should be carried out as appropriate after machining and rough grinding, so as to better eliminate the internal stress generated by machining to ensure that the deformation of nitriding treatment is minimized. The organization is stable, and the stabilization temperature should be lower than the tempering and tempering temperature to avoid reducing the strength of the matrix; the holding time is generally 4 to 6 hours.

Stainless steel and heat-resistant steel must undergo special heat treatment before nitriding. Because the raw material structure of these steels often have band segregation and twin crystals, and generally the grains are larger, which affects the quality after nitriding.

Editing by Baccata

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