The Four Major Processes Of Heat Treatment
The four major processes of heat treatment are annealing, normalizing, quenching, and tempering. Each of them has different characteristics and functions. The following is a detailed explanation of these four processes:
Annealing
Annealing is a heat treatment process that heats metal to a certain temperature, keeps it warm for some time, and then slowly cools it. The purpose is to make the internal structure of the metal reach or approach a balanced state, eliminate internal stress, reduce hardness, improve plasticity, refine grains, and improve Organize and prepare the tissue for subsequent processing or handling.
Annealing is mainly divided into complete annealing, spheroidizing annealing, and stress relief annealing.
Normalizing
Normalizing is a heat treatment process that heats the workpiece to 30~50℃ above the critical temperature, holds it for a suitable period, and then cools it in still air. Normalizing can eliminate the network cementite of hypereutectoid steel. For hypoeutectoid steel, normalizing can refine the crystal lattice and improve the comprehensive mechanical properties. It is more economical to use normalizing instead of annealing for parts with low requirements.
Quenching
Quenching is a metal heat treatment process in which the metal is heated to a certain appropriate temperature and maintained for some time, and then immersed in a quenching medium for rapid cooling. Quenching can improve the hardness and wear resistance of the metal, enable the workpiece to obtain the required martensite structure and control the morphology and distribution of martensite by adjusting the cooling rate to improve material properties.
Tempering
Tempering is a metal heat treatment process in which the quenched workpiece is reheated to an appropriate temperature below the lower critical temperature, kept warm for some time, and then cooled in air, water, oil, and other media. The main purpose of tempering is to eliminate quenching stress, stabilize the structure of the steel, and transform it into tempered martensite, troostite, or troostite to improve the toughness, plasticity, and strength of the steel and reduce its brittleness.
According to the different tempering temperatures, tempering can be divided into three types: low-temperature tempering, medium-temperature tempering, and high-temperature tempering.
In general, these four heat treatment processes play an important role in metal processing. By controlling heating temperature, holding time, and cooling method, precise control of metal material properties can be achieved to meet the needs of different engineering applications.
Choice of annealing and normalizing
Annealing and normalizing are both important processes for metal heat treatment. The main difference between them is indeed the difference in cooling rate, which directly affects the properties of the treated metal material.
Annealing is a relatively slow process, usually accomplished by natural cooling in the furnace. This slower cooling rate helps refine the grains, even out the chemical composition, and eliminate or reduce internal stresses in the metal. Annealed metal materials usually have lower strength and hardness, but relatively good plasticity and toughness. Therefore, the annealing process is particularly useful where it is necessary to improve the processing properties of the material, increase toughness, and eliminate residual stress.
In contrast, normalizing has a faster cooling rate and is usually performed in air. This rapid cooling can increase the strength and hardness of the metal, but correspondingly, its plasticity and toughness may be affected to some extent. Normalizing is mainly used to improve the mechanical properties of metal materials, especially for applications that need to withstand high loads or require good cutting performance.
Used together with quenching and tempering
Quenching and tempering are two processes commonly used in metal heat treatment. They each have unique effects, but when used together they can produce superior performance. Quenching is the rapid cooling of steel to achieve high hardness and strength, but this is often accompanied by an increase in brittleness. Tempering is performed after quenching. The purpose is to reduce the internal stress generated by quenching, reduce brittleness, and improve the toughness and plasticity of the steel by reheating it to a certain temperature maintaining it for a certain time, and then slowly cooling.
After quenching, although the hardness and strength of the steel are significantly improved, the plasticity and toughness become worse, and brittle fracture may occur if it is used directly. Therefore, tempering is usually required after quenching to adjust the properties of the material. Tempering can reduce the internal stress generated by quenching, allowing the steel to maintain a certain strength while improving toughness and plasticity to prevent brittle fracture.
The combined use of quenching and tempering can comprehensively adjust the properties of steel so that it has sufficient strength and hardness as well as good toughness and plasticity. This kind of treatment process is particularly suitable for manufacturing parts such as bearings that need to withstand high loads and friction and can meet their performance requirements during use.
In addition, quenching and tempering can improve other properties of steel, such as wear and corrosion resistance. By adjusting the parameters of the heat treatment process, the performance of the steel can be further optimized to meet different usage needs.