ã€China Aluminum Network】 Metal heat treatment is a process in which a metal workpiece is heated to a suitable temperature in a certain medium and maintained at this temperature for a certain period of time and cooled at different speeds. Metal heat treatment is one of the more important processes in material production. Compared with other processing technologies, heat treatment generally does not change the shape and overall chemical composition of the workpiece, but by changing the internal microstructure of the workpiece or changing the surface of the workpiece. The chemical composition imparts or improves the performance of the workpiece. Its characteristic is to improve the intrinsic quality of the workpiece, which is generally not visible to the naked eye. "Four fires" in metal heat treatment refers to annealing, normalizing, quenching (solid solution) and tempering (aging).
Annealing refers to heating the workpiece to a suitable temperature, according to the material and the size of the workpiece using different insulation time, and then slowly cooling, its purpose is to reduce the hardness of the material, improve the plasticity, in order to facilitate the subsequent processing, reduce residual stress, improve Homogenization of organization and composition. Annealing is divided into recrystallization annealing, stress-relief annealing, spheroidizing annealing, and full annealing depending on purposes. Normalizing heats the workpiece to a suitable temperature and then cools it in air. The effect of normalizing is similar to that of annealing, but the resulting structure is finer and is often used to improve the cutting performance of materials. Sometimes it is also used for parts that do not require high quality. The final heat treatment. Quenching is the rapid cooling of water, oil or other inorganic salts, and organic aqueous quenching media after heating and holding the workpiece. After quenching, the material is an unbalanced structure, which is usually very hard and brittle. It requires a long-term heat preservation at a temperature higher than room temperature, and then it is cooled. This process is called tempering (aging).
From the above definition, it can be seen that whether the annealing, normalizing, quenching or tempering, the workpiece must be heated, kept warm and cooled in the heat treatment process. Therefore, in the metal heat treatment, the heating rate, holding time and cooling rate become more important process parameters in the heat treatment process.
In the "four fires", the relationship between quenching and tempering (ageing) is relatively close, and they are often used together. Both are indispensable. However, in actual production, in order to save costs and improve production efficiency, for products with low performance requirements, quenching is often replaced by quenching on-line quenching, and tempering is replaced by natural aging. For example, in the extrusion of 6060, 6063 and other low-alloyed alloy profiles, due to the low quenching sensitivity of these alloys, silicon, magnesium solid solution quickly above the solution temperature. Therefore, in order to ensure that the outlet temperature of the extrusion material is higher than the solution temperature, a supersaturated solid solution having a large solid solution degree can also be obtained by air-cooling and quenching.
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