Why flanges should be heat treated

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Why flanges should be heat treated

Release time：

2023-02-15

　　Heat treatment usually does not change the shape and overall chemical composition of the workpiece, but rather gives or improves the properties of the workpiece by altering its internal microstructure or changing the chemical composition of its surface, characterized by improving the internal quality of the workpiece, which is usually invisible to the naked eye.

　　The role of heat treatment is to improve the mechanical properties of the material, eliminate residual stresses and improve the workability of the metal. Depending on the purpose of heat treatment, heat treatment processes can be divided into two main categories: preparatory heat treatment and final heat treatment.

　　The purpose of the preparatory heat treatment is to improve machinability, eliminate internal stresses and prepare a good metallurgical structure for the final heat treatment. Heat treatment processes include annealing, normalizing, aging, quenching and tempering.

　　Annealing and normalizing are used for hot-finished blanks. For carbon and alloy steels with carbon content greater than 0.5%, annealing is usually used to reduce their hardness and ease of cutting; for carbon and alloy steels with carbon content less than 0.5%, normalizing is used to prevent the cutting edge from sticking during low hardness cutting. Annealing and normalizing is usually arranged after the manufacture of the blank and before rough machining. It refines the grain and uniform structure and prepares for future heat treatment.

　　Aging is mainly used to eliminate internal stresses due to roughing and machining. To avoid excessive transport work, the aging treatment can be scheduled prior to precision machining of parts with general accuracy. However, parts should be scheduled for two or more aging treatments. Simple parts usually do not age.

　　Preparation Heat Treatment

　　In addition to castings, for some rigid precision parts (such as precision screws), some shaft parts are also machined in order to eliminate internal stresses generated during processing and stabilize the machining accuracy of the parts, and aging treatment after straightening is also required.

　　Quenching and tempering is a high-temperature tempering treatment after quenching. A uniform and finely tempered sothitic organization can be obtained, which can prepare for the reduction of deformation during surface hardening and nitriding. Therefore, quenching and tempering can also be used as a preparatory heat treatment. Since the parts have better overall mechanical properties after quenching and tempering, some parts with low requirements for hardness and wear resistance can also be used as the final heat treatment process.

　　2. Final heat treatment

　　The final purpose of heat treatment is to improve the mechanical properties, such as hardness, wear resistance and strength.

　　There are surface quenching and overall quenching. Surface hardening is widely used because of its low distortion, oxidation and decarburization. In addition, surface hardening has the advantages of high external strength, good wear resistance, good internal toughness and strong impact resistance. In order to improve the mechanical properties of surface hardened parts, heat treatments such as quenching and tempering or normalizing are usually required as preparatory heat treatments.

　　Carburizing hardening is suitable for low carbon and low alloy steels. It increases the carbon content of the surface of the part. After quenching, the surface layer acquires a higher hardness while the heart still retains some strength, toughness and plasticity. Carburizing is divided into carburizing and partial carburizing. When partial carburizing is performed, the carburized part should be impermeable. Due to the large carburizing and quenching deformation, the carburizing depth is usually between 0.5 to 2mm and the carburizing process is usually arranged between semi-finishing and finishing.

　　When using a carburized portion of a partially carburized part to increase the margin, the carburized layer removal process should be removed. The process of removing the excess carburized layer should be scheduled after carburizing and before quenching.

　　Nitriding is a method of infiltrating nitrogen atoms into the metal surface to obtain a layer of nitrogen compounds. The nitriding layer improves the hardness, wear resistance, fatigue strength and corrosion resistance of the part. To reduce distortion during nitriding, high-temperature tempering is often required to relieve stresses after cutting. Due to the low nitriding treatment temperature, low distortion and thin nitriding layer, the nitriding process should be arranged as much as possible.