Annealing, normalizing, quenching, tempering… Do you know these heat treatments clearly? The role of heat treatment is to improve the mechanical properties of materials, eliminate residual stress and improve the machinability of metals. According to the different purposes of heat treatment, heat treatment processes can be divided into two categories: preliminary heat treatment and final heat treatment.
01Preparatory 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, tempering, etc.
1) Annealing and normalizing
Annealing and normalizing are used for blanks that have been heat-processed. Carbon steel and alloy steel with a carbon content greater than 0.5% are often annealed to reduce their hardness and make them easier to cut; 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 machining.
2) Aging treatment
Aging treatment is mainly used to eliminate the internal stress generated during blank manufacturing and machining.
To avoid excessive transportation workload, for parts with general precision, one aging treatment can be arranged before finishing. However, for parts with higher precision requirements (such as the box of a coordinate boring machine, etc.), two or more aging treatment processes 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 screws), in order to eliminate the internal stress generated during processing and stabilize the processing accuracy of parts, multiple aging treatments are often arranged between rough machining and semi-finishing. For some shaft parts processing, 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, which is prepared for reducing 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 parts after quenching and tempering are good, some parts with low requirements for hardness and wear resistance can also be used as the final heat treatment process.
02 Final heat treatment
The purpose of the final heat treatment is to improve mechanical properties such as hardness, wear resistance and strength.
1) Quenching
Quenching includes surface quenching and overall quenching. Among them, 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. In order to improve the mechanical properties of surface quenched parts, heat treatments such as quenching and normalizing are often required as preliminary heat treatments. The general process route is: cutting → forging → normalizing (annealing) → rough machining → quenching and tempering → semi-finishing → surface quenching → finishing.
2) Carburizing and quenching
Carburizing and quenching are suitable for low-carbon steel and low-alloy steel. First, the carbon content of the surface of the parts is increased, and after quenching, the surface obtains a high hardness, while the core still maintains a certain strength and high toughness and plasticity. Carburizing is divided into overall carburizing and local carburizing. When carburizing locally, anti-seepage measures (copper plating or anti-seepage material plating) should be taken for the non-carburized parts. Since carburizing and quenching have 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 general process route is: cutting → forging → normalizing → rough and semi-finishing → carburizing and quenching → finishing. When the non-carburized part of the partially carburized part adopts the process plan of increasing the margin 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 into the metal surface to obtain a layer of nitrogen-containing compounds. The nitriding 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. In order to reduce the deformation during nitriding, high-temperature tempering is generally required to eliminate stress after cutting.