According to different purposes of heat treatment, heat treatment process can be divided into two categories: preliminary heat treatment and final heat treatment.
1. Preparing for heat treatment
The purpose of preliminary heat treatment is to improve the processing performance, eliminate internal stress and prepare a good metallurgical structure for the final heat treatment.
The heat treatment processes include annealing, normalizing, aging, and tempering.
a. Annealing and normalizing
Annealing and normalizing are used for thermally processed blanks. Carbon steel and alloy steel with a carbon content of more than 0.5% are often annealed in order to reduce their hardness. Carbon steel and alloy steel with a carbon content of less than 0.5% are used to avoid sticking the knife when the hardness is too low. Instead, it is normalized. Annealing and normalizing can still refine grains and uniform structure, and prepare for the subsequent heat treatment. Annealing and normalizing are usually arranged after rough manufacturing and before roughing.
b. Aging treatment
Aging treatment is mainly used to eliminate the internal stress generated in the blank manufacturing and mechanical processing.
In order to avoid excessive transportation workload, aging treatment can be arranged before finishing for parts of general precision. However, for parts with high accuracy requirements (such as the box body of the coordinate boring machine, etc.), two or several aging treatment procedures should be arranged. Simple parts are generally not subject to aging treatment.
In addition to castings, for some precision parts with poor rigidity (such as precision screws), in order to eliminate the internal stress generated during processing and stabilize the processing accuracy of parts, many aging treatments are often arranged between roughing and semi-finishing. For some shaft parts processing, aging treatment should also be arranged after the straightening process.
Tempering is to carry out high temperature tempering treatment after quenching, which can obtain a uniform and fine tempered sorbite structure, which is prepared for reducing deformation during subsequent surface quenching and nitriding treatment, so tempering can also be used as a preliminary heat treatment.
Due to the better overall mechanical properties of the parts after tempering, some parts that do not require high hardness and wear resistance can also be used as the final heat treatment process.
2. Final heat treatment
The purpose of the final heat treatment is to improve the mechanical properties such as hardness, wear resistance and strength.
Quenching has surface quenching and overall quenching. Among them, surface quenching is widely used because of less deformation, oxidation and decarburization, and surface quenching also has the advantages of high external strength and good wear resistance, while maintaining good toughness and strong impact resistance inside. In order to improve the mechanical properties of surface quenched parts, heat treatment such as quenching and tempering or normalizing is often required as a preliminary heat treatment. The general process route is: blanking-forging-normalizing (annealing)-roughing-tempering-semi-finishing-surface quenching-finishing.
b. Carburizing and quenching
Carburizing and quenching is suitable for low-carbon steel and low-alloy steel. It first increases the carbon content of the surface layer of the parts. After quenching, the surface layer obtains high hardness, while the core still maintains a certain strength and high toughness and plasticity. Carburization is divided into overall carburization and local carburization. In case of partial carburization, anti-seepage measures shall be taken for non-carburized parts (copper plating or anti-seepage material). Since the carburizing and quenching deformation is large, 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: blanking-forging-normalizing-rough, semi-finishing-carburizing and quenching-finishing.
When the non-carburized part of the local carburized parts adopts the process of removing the excess carburized layer after increasing the margin, the process of cutting off the extra carburized layer should be arranged after carburizing and before quenching.
c. 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. Because 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 far as possible. To reduce the deformation during nitriding, it is generally necessary to Perform high-temperature tempering to relieve stress.