Nitriding is a classification of heat treatment process for forgings. Nitriding is a chemical heat treatment process that causes nitrogen atoms to penetrate into the surface layer of a workpiece in a certain medium at a certain temperature. Common liquid nitriding, gas nitriding, ion nitriding. The traditional gas nitriding is to put the workpiece into a sealed container, pass the flowing ammonia gas and heat it. After a long time of heat preservation, the ammonia gas will be thermally decomposed to generate reactive nitrogen atoms, which will continuously adsorb to the surface of the workpiece and diffuse into the surface layer of the workpiece. By changing the chemical composition and texture of the surface layer, excellent surface properties are obtained. If carbon is infiltrated during nitriding to promote the diffusion of nitrogen, it is called nitrocarburizing. Gas nitriding and ion nitriding are commonly used.
Nitriding is often the last step in the entire manufacturing process of nitriding parts, and at most it can be ground or ground. The process flow of nitrided parts is generally: forging → normalizing (annealing) → roughing → tempering → finishing → stress removal → fine grinding → nitriding → fine grinding → assembly.
Preheating before nitriding includes normalizing (annealing), tempering, and stress relieving.
a. Normalizing (annealing), whose purpose is to refine grains, reduce hardness, eliminate forging stress.
b. The quenching and tempering treatment can improve the processing performance of the steel and obtain a uniform tempered sorbite structure, so as to ensure sufficient strength and toughness of the core of the parts, and at the same time, the nitriding layer can be combined with the base firmly.
c. To stress treatment, for the complex shape of the precision parts, should be carried out 1 to 2 times before the nitriding to stress, in order to reduce the deformation during the nitriding process.