Do you know the difference between these heat treatments?
1. Preparatory heat treatment
The purpose of preparatory heat treatment is to improve processing performance, eliminate internal stress and prepare excellent metallographic structure for final heat treatment. Its heat treatment processes include annealing, normalizing, aging, and tempering.
(1) Annealing and normalizing
Annealing and normalizing are used for blanks that have undergone heat treatment. Carbon steel and alloy steel with a carbon content greater than 0.5% are often annealed to reduce their hardness for easy cutting; carbon steel and alloy steel with a carbon content less than 0.5% are normalized to avoid sticking to the knife when cutting due to their low hardness. Annealing and normalizing can also refine the grains and make the structure uniform, preparing for subsequent heat treatment. Annealing and normalizing are often carried out after blank production and before rough processing.
(2) Aging treatment
Aging treatment is mainly used to eliminate internal stress generated during blank production and machining.
In order to avoid excessive transportation workload, for parts with general precision, one aging treatment is sufficient before finishing. However, for parts with higher precision requirements (such as the box of the coordinate boring machine, etc.), two or more aging treatment processes should be carried out. Simple parts generally do not need aging treatment.
In addition to castings, for some precision parts with poor rigidity (such as precision screws), in order to eliminate the internal stress generated during processing and stabilize the processing accuracy of parts, multiple aging treatments are often carried out between rough processing and semi-finishing. For some shaft parts processing, aging treatment is also required after the straightening process.
Quenching and tempering is a high-temperature tempering treatment after quenching. It can obtain a uniform and fine tempered troostite structure, preparing for the reduction of deformation during subsequent surface quenching and nitriding treatment. Therefore, quenching and tempering can also be used as a preparatory heat treatment.
Because the comprehensive mechanical properties of the parts after quenching are better, it can also be used as a final heat treatment process for some parts with low requirements for hardness and wear resistance.
2. Final heat treatment
The purpose of final heat treatment is to improve mechanical properties such as hardness, wear resistance and strength.
(1) Quenching
Quenching includes surface quenching and overall quenching. Surface quenching is widely used because of its small deformation, oxidation and decarburization. Surface quenching also has the advantages of high external strength and good wear resistance, while maintaining excellent toughness and strong impact resistance inside. In order to improve the mechanical properties of surface quenched parts, heat treatment such as tempering or normalizing is often required as a preliminary heat treatment. The general process route is: cutting-casting-normalizing (annealing)-rough machining-tempering-semi-finishing-surface quenching-finishing.
Carburizing quenching is suitable for low carbon steel and low alloy steel. It first increases the carbon content of the surface of the parts, and after quenching, the surface obtains a high hardness, while the core still maintains a certain strength and high toughness and plasticity. Carburizing is divided into overall carburizing and partial carburizing. When partially carburizing, anti-seepage measures (copper plating or anti-seepage material plating) should be taken for the non-carburizing parts. Because carburizing and quenching have large deformation and the carburizing depth is generally between 0.5~2mm, the carburizing process is generally arranged between semi-finishing and finishing.
Its process route is generally: cutting-casting-normalizing-rough and semi-finishing-carburizing and quenching-finishing. When the non-carburizing part of the partially carburized part adopts the process plan of increasing the allowance and cutting off the remaining carburized layer, the process of cutting off the remaining carburized layer should be arranged after carburizing and before quenching.
Nitriding is a treatment method that allows nitrogen atoms to enter the metal surface to obtain a layer of nitrogen-containing compounds. The nitriding layer can improve the hardness, wear resistance, fatigue strength and corrosion resistance of the part surface. Because the nitriding treatment temperature is low, the deformation is small, and the nitriding layer is thin (generally not more than 0.6~0.7mm), the nitriding process should be arranged as late as possible. In order to reduce the deformation during nitriding, high-temperature tempering is generally required to eliminate stress after cutting.
