How To Distinguish Between Annealing, Normalizing, Quenching, And Tempering...
Aug 13, 2025
Annealing, normalizing, quenching, and tempering... Do you know the difference between these heat treatments? Heat treatment improves the mechanical properties of materials, eliminates residual stresses, and improves the machinability of metals. Based on their different purposes, heat treatment processes can be divided into two categories: preliminary heat treatment and final heat treatment.
01 Preliminary Heat Treatment
The purpose of preliminary heat treatment is to improve machinability, eliminate internal stresses, and prepare a good metallographic structure for final heat treatment. Heat treatment processes include annealing, normalizing, aging, and tempering.
1) Annealing and Normalizing
Annealing and normalizing are performed on hot-worked blanks. Carbon steels and alloy steels with a carbon content greater than 0.5% are often annealed to reduce their hardness for easier cutting. Carbon steels and alloy steels with a carbon content less than 0.5% are normalized to prevent the hardness from sticking during cutting. Annealing and normalizing also refine the grain size and homogenize the microstructure, preparing for subsequent heat treatment. Annealing and normalizing are often performed after blank manufacturing and before rough machining.
2) Aging Treatment
Aging treatment is primarily used to eliminate internal stresses generated during blank manufacturing and machining.
To avoid excessive transportation, for parts requiring average precision, a single aging treatment before finishing is sufficient. However, for parts requiring higher precision (such as the housing of a coordinate boring machine), two or more aging treatments should be performed. Simple parts generally do not require aging.
In addition to castings, for some precision parts with less rigidity (such as precision lead screws), multiple aging treatments are often performed between rough machining and semi-finishing to eliminate internal stresses generated during machining and stabilize part accuracy. Some shaft parts also require aging after straightening.
3) Quenching and Tempering
Quenching and tempering involves performing a high-temperature tempering treatment after quenching. This treatment produces a uniform and fine tempered bainite structure, preparing for subsequent surface hardening and nitriding treatments to minimize distortion. Therefore, quenching and tempering can also serve as a preparatory heat treatment. Because the overall mechanical properties of parts after quenching and tempering are excellent, it can also be used as a final heat treatment for parts with lower requirements for hardness and wear resistance.
02 Final Heat Treatment
The purpose of final heat treatment is to improve mechanical properties such as hardness, wear resistance, and strength.
1) Quenching
Quenching can be divided into surface quenching and through quenching. Surface quenching is widely used because it minimizes deformation, oxidation, and decarburization. It also offers the advantages of high external strength and good wear resistance while maintaining good internal toughness and impact resistance. To improve the mechanical properties of surface-quenched parts, a preliminary heat treatment such as quenching or normalizing is often required. The general process route is: blanking → forging → normalizing (annealing) → rough machining → quenching → semi-finishing → surface quenching → finishing.
2) Carburizing and Quenching
Carburizing and quenching are suitable for low-carbon and low-alloy steels. It first increases the carbon content in the surface layer of the part. After quenching, the surface layer achieves high hardness while the core retains a certain degree of strength, high toughness, and high ductility. Carburizing can be performed in both general and partial carburizing. During partial carburizing, non-carburized areas require anti-seepage measures (copper plating or coating with an anti-seepage material). Because carburizing and quenching cause significant deformation, and the carburizing depth typically ranges from 0.5 to 2 mm, the carburizing process is typically performed between semi-finishing and finishing.
The general process route is: blanking → forging → normalizing → rough and semi-finishing → carburizing and quenching → finishing. When partially carburizing a part, the non-carburized areas are stocked and then the excess carburized layer is removed. This removal should be performed after carburizing and before quenching.
3) Nitriding Treatment
Nitriding is a treatment process in which nitrogen atoms penetrate the metal surface to create a layer of nitrogen-containing compounds. This nitrided layer improves the surface hardness, wear resistance, fatigue strength, and corrosion resistance of the part. Since 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.
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