According to the different purposes of heat treatment, heat treatment processes can be divided into two categories: preliminary heat treatment and final heat treatment.
1. Preliminary heat treatment
The purpose of preliminary heat treatment is to improve processing performance, eliminate internal stress, and prepare a good metallographic structure for final heat treatment. Its heat treatment processes include annealing, normalizing, aging, and tempering.
(1) Annealing and normalizing: Annealing and normalizing are used for blanks that have been heat-treated. Carbon steel and alloy steel with a carbon content greater than 0.5% are often annealed to reduce their hardness and facilitate cutting; carbon steel and alloy steel with a carbon content less than 0.5% are normalized to avoid sticking to the knife when cutting due to their low hardness. Annealing and normalizing can also refine the grains and make the structure uniform, preparing for subsequent heat treatment. Annealing and normalizing are often arranged after blank manufacturing and before rough processing.
(2) Aging treatment: Aging treatment is mainly used to eliminate internal stress generated during blank manufacturing and machining. To avoid excessive transportation workload, for parts with general precision, one aging treatment is sufficient before finishing. However, for parts with higher precision requirements (such as the housing of a coordinate boring machine, etc.), two or more aging treatments should be arranged. Simple parts generally do not require aging treatment.
In addition to castings, for some precision parts with poor rigidity (such as precision lead screws), multiple aging treatments are often arranged between roughing and semi-finishing to eliminate the internal stress generated during processing and stabilize the processing accuracy of the parts. For some shaft parts, aging treatment should also be arranged after the straightening process.
(3) Quenching and tempering: Quenching and tempering is a high-temperature tempering treatment after quenching. It can obtain a uniform and fine-tempered troostite structure, preparing for the reduction of deformation during subsequent surface quenching and nitriding treatment. Therefore, quenching and tempering can also be used as a preliminary heat treatment.
Since the comprehensive mechanical properties of the parts after quenching and tempering are better, it can also be used as the final heat treatment process for some parts with low requirements for hardness and wear resistance.
2. Final heat treatment
The purpose of final heat treatment is to improve mechanical properties such as hardness, wear resistance, and strength.
(1) Quenching: Quenching includes surface quenching and overall quenching. Surface quenching is widely used because of its small deformation, oxidation, and decarburization. Surface quenching also has the advantages of high external strength and good wear resistance, while maintaining good toughness and strong impact resistance inside. To improve the mechanical properties of surface quenched parts, heat treatment such as tempering or normalizing is often required as a preliminary heat treatment. The general process route is blanking-forging-normalizing (annealing)-rough machining-tempering-semi-finishing-surface quenching-finishing.
(2) Carburizing quenching: Carburizing quenching is suitable for low-carbon steel and low-alloy steel. It first increases the carbon content of the surface layer of the parts, and after quenching, the surface layer obtains high hardness, while the core still maintains a certain strength and high toughness and plasticity. Carburizing is divided into overall carburizing and local carburizing. During local carburizing, anti-seepage measures (copper plating or anti-seepage material plating) should be taken for the non-carburized parts. Since carburizing and quenching cause large deformation and the carburizing depth is generally between 0.5 and 2 mm, the carburizing process is generally arranged between semi-finishing and finishing. The process route is generally: cutting—forging—normalizing—rough and semi-finishing—carburizing and quenching-finishing. When the non-carburized part of the local carburized part adopts the process plan of increasing the allowance and cutting off the excess carburized layer, the process of cutting off the excess carburized layer should be arranged after carburizing and before quenching.
(3) Nitriding treatment: Nitriding is a treatment method that allows nitrogen atoms to penetrate the metal surface to obtain a layer of nitrogen-containing compounds. The nitrided layer can improve the hardness, wear resistance, fatigue strength, and corrosion resistance of the part surface. 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. To reduce the deformation during nitriding, high-temperature tempering is generally required to eliminate stress after cutting.
Post time: Oct-15-2024