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Annealing, normalizing, quenching, tempering... Can you tell these heat treatments apart?
Annealing, normalizing, quenching, tempering... Can you tell these heat treatments apart?

09

2022

-

11

Annealing, normalizing, quenching, tempering... Can you tell these heat treatments apart?

Author:

The purpose of heat treatment is to enhance the mechanical properties of materials, relieve residual stresses, and improve the machinability of metals. According to the different objectives of heat treatment, the process can be divided into two major categories: preliminary heat treatment and final heat treatment. 1. Preliminary Heat Treatment The purpose of preliminary heat treatment is to improve machinability, eliminate internal stresses, and prepare a favorable microstructure for subsequent final heat treatment. The common heat treatment processes included in this category are annealing, normalizing, aging, and tempering. (1) Annealing and Normalizing Annealing and normalizing are typically applied to workpieces that have undergone hot working. For carbon steels and alloy steels with a carbon content exceeding 0.5%, annealing is often employed to reduce their hardness and make them easier to machine.
　　The purpose of heat treatment is to enhance the mechanical properties of materials, eliminate residual stresses, and improve the machinability of metals. According to the different objectives of heat treatment, heat treatment processes can be categorized into two main types: preliminary heat treatment and final heat treatment.
　　1. Preheating treatment
　　The purpose of preliminary heat treatment is to improve machinability, relieve internal stresses, and prepare a favorable microstructure for the final heat treatment. The common heat treatment processes include annealing, normalizing, aging, and tempering.
　　(1) Annealing and Normalizing
　　Annealing and normalizing are both used for workpieces that have undergone hot working. For carbon steels and alloy steels with a carbon content greater than 0.5%, annealing is often employed to reduce their hardness and make them easier to machine. For carbon steels and alloy steels with a carbon content below 0.5%, normalizing is typically used to prevent the material from becoming too soft, which could cause it to stick to the cutting tool during machining. Both annealing and normalizing can also refine grain size and homogenize the microstructure, thereby preparing the material for subsequent heat treatments. Annealing and normalizing are usually carried out after the blank has been manufactured but before rough machining begins.
　　(2) Aging Treatment
　　Aging treatment is primarily used to eliminate internal stresses generated during the manufacturing of blanks and mechanical machining.
　　To avoid excessive handling workload, for parts with general precision requirements, a single aging treatment can be scheduled before finishing. However, for parts with higher precision requirements—such as the housings of coordinate boring machines—two or more aging treatment processes should be scheduled. Simple parts generally do not require aging treatment at all.
　　In addition to castings, for some precision parts with poor rigidity—such as precision lead screws—to eliminate internal stresses generated during machining and stabilize the machining accuracy of the parts, multiple aging treatments are often scheduled between rough machining and semi-finishing. For certain shaft-type parts, an aging treatment is also scheduled after the straightening process.
　　(3) Tempering
　　Tempering involves performing high-temperature tempering after quenching. This process yields a uniform and fine tempered sorbite microstructure, which helps minimize deformation during subsequent surface hardening and nitriding treatments. Therefore, tempering can also serve as a preparatory heat treatment.
　　Since the parts exhibit good overall mechanical properties after tempering, they can also serve as the final heat treatment process for certain parts that do not have high requirements for hardness and wear resistance.
　　2. Final heat treatment
　　The purpose of final heat treatment is to enhance mechanical properties such as hardness, wear resistance, and strength.
　　(1) Quenching
　　Quenching can be categorized into surface quenching and through-hardening. Among these, surface quenching is more widely used due to its advantages of minimal deformation, oxidation, and decarburization. Moreover, surface-quenched parts exhibit high surface strength and excellent wear resistance, while maintaining good toughness and strong impact resistance in their core. To enhance the mechanical properties of surface-quenched parts, it is often necessary to perform preliminary heat treatments such as tempering or normalizing. The typical process flow is as follows: blanking—forging—normalizing (or annealing)—rough machining—tempering—semi-finishing—surface quenching—finishing.
　　(2) Carburizing and quenching
　　Carburizing and quenching is suitable for low-carbon steels and low-alloy steels. This process first increases the carbon content in the surface layer of the part; after quenching, the surface layer achieves high hardness, while the core retains sufficient strength as well as high toughness and ductility. Carburizing can be either full-carburizing or local carburizing. During local carburizing, measures must be taken to prevent carburization of non-carburized areas (e.g., by copper plating or coating with anti-carburizing materials). Because carburizing and quenching cause significant deformation, and because the typical carburizing depth ranges from 0.5 to 2 mm, the carburizing process is usually scheduled between semi-finishing and finishing operations.
　　The typical process route is as follows: blanking—forging—normalizing—rough and semi-finish machining—carburizing and quenching—finish machining. When, for locally carburized parts, the non-carburized sections are treated by increasing the machining allowance and then removing the excess carburized layer, the operation of removing the excess carburized layer should be scheduled after carburizing but before quenching.
　　(3) Nitriding Treatment
　　Nitriding is a treatment process in which nitrogen atoms are diffused into the surface of a metal to form a layer of nitrogen-containing compounds. The nitrided layer can significantly enhance the surface hardness, wear resistance, fatigue strength, and corrosion resistance of the part. Since nitriding is carried out at relatively low temperatures, causes minimal deformation, and produces a relatively thin nitrided layer (typically no more than 0.6–0.7 mm), the nitriding step should be scheduled as late as possible in the manufacturing process. To minimize deformation during nitriding, it is generally necessary to perform high-temperature stress-relief tempering after machining.

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