Pre-heat Treatment of Ion Nitriding Parts

---

The main purpose of the pre-heat treatment before nitriding of the parts is to make the core have the necessary mechanical properties, eliminate the internal stress during the processing, reduce the nitriding deformation, and prepare the organization for obtaining good nitriding layer performance.

After forging, the parts can be normalized to refine grains, uniform structure, eliminate stress and improve machining performance. The tool steel is annealed after forging. Generally, the pre-heat treatment of structural steel before nitriding should be quenched and tempered. It is not only to improve the machining performance of the parts, but also to ensure that it has a certain strength and obtains a uniform and finely distributed sorbite structure. After quenching and tempering, large ferrite and coarse sorbite structure are not allowed.

The heat treatment before nitriding of tool and die steel is generally quenching + tempering. The heat treatment before nitriding of stainless steel acid-resistant steel is to eliminate the processing stress and improve the structure, and quenching + tempering is generally used. If the hardness is not high, it can be annealed. In order to eliminate the band structure, quenching can be used twice. Austenitic stainless steel is usually solid solution treated.

The stress removal process before nitriding+ plays a very important role in reducing nitriding deformation. Because it can eliminate most of the stress caused by the previous forging, quenching and tempering (or normalizing) and machining. Generally, high temperature aging treatment slightly lower than the tempering temperature is used. Slender parts, thin-walled parts, complex parts and precision parts must undergo one or more stress relief treatments. After the parts are stress-relieved, the smallest nitriding deformation can be obtained.

Nitriding deformation and deformation correction. Small deformation is one of the advantages of ion nitriding compared with traditional gas nitriding. However, there are many factors that affect the deformation of nitriding, which must be solved one by one to get the minimum deformation effect. Nitriding deformation refers to the following two situations, one is size change (expansion), but shape change (distortion, deformation). The size change is due to the absorption of nitrogen atoms on the surface of the workpiece during nitriding, the lattice constant of the original structure increases, or various nitrides are formed. From a macro point of view, it appears as a slight increase in the volume of the surface layer. Therefore, the size of the workpiece is also slightly increased, that is, the phenomenon of “nitriding expansion”. The amount of swelling during nitriding depends on the amount of nitrogen absorbed on the surface of the part. Therefore, the content of alloying elements in the material is high, the nitriding temperature is high, the time is long, and the nitrogen potential of the nitriding atmosphere is high.

In the entire manufacturing process of nitriding parts, nitriding is often used as the last process. The parts after nitriding are generally not processed, or only finely ground when necessary. Therefore, it is important to understand the processes before and after ion nitriding.

Learn more:

Vacuum Heat Treatment Furnace

Vacuum Carburizing Furnace