In order to make the metal workpiece have the mechanical properties, physical properties, and chemical properties required, in addition to a reasonable selection of materials and various forming processes, heat treatment technology is often essential.
Heat treatment generally does not change the shape of the workpiece and the overall chemical composition, but by changing the internal microstructure of the workpiece, or changing the chemical composition of the workpiece surface, to give or improve the performance of the workpiece. Its characteristic is to improve the intrinsic quality of the workpiece.
The function 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: preparatory heat treatment and final heat treatment.
1. Prepare for heat treatment
The purpose of pre-heat treatment is to improve machining performance, eliminate internal stress, and prepare a good metallographic structure for final heat treatment. The heat treatment process includes annealing, normalizing, aging, tempering, and so on.
(1) Annealing and normalizing
Annealing and normalizing are used for hot machined blanks. Carbon steel and alloy steel with more than 0.5% carbon content are usually annealed to reduce their hardness and make them easy to be cut. Carbon steels and alloy steels with carbon content less than 0.5% are normalized to prevent them from hardening through low cutting and sticking to the tool. Annealing and normalizing can also refine grain and uniform structure, and prepare for future heat treatment. Annealing and normalizing are usually arranged after fabrication and before roughing.
(2) Aging treatment
Aging treatment is mainly used to eliminate the internal stress in the blank manufacturing and machining.
In order to avoid excessive transport workload, for the general accuracy of the parts, before finishing the arrangement of aging treatment can be. But for the parts with high precision (such as the box body of the coordinate boring machine, etc.), two or several aging processes should be arranged. Simple parts are generally not aging treatment.
In addition to castings, for some precision parts with poor rigidity (such as a precision screw), in order to eliminate the internal stress generated in the processing, to stabilize the processing accuracy of the parts, often in rough machining, the semi-finishing arrangement of aging treatment. Some shaft parts processing, in the straight after the process, should also be arranged aging treatment.
(3) Tempering
Quenching and tempering are to conduct high-temperature tempering treatment after quenching, it can obtain uniform and meticulous tempering soxhlet structure, for the subsequent surface quenching and nitriding treatment to reduce deformation preparation, so quenching and tempering can also be used as preparatory heat treatment.
Due to the better comprehensive mechanical properties of tempered parts, some parts with low hardness and wear resistance requirements 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.
(1) Quenching
There are surface quenching and integral quenching. The surface quenching is widely used because of the small deformation, oxidation, and decarburization, and the surface quenching also has the advantages of high external strength, good wear resistance, and good internal toughness, strong impact resistance. In order to improve the mechanical properties of surface quenched parts, it is often necessary to conduct tempering or normalizing heat treatment as preparatory heat treatment. Its general process line is: blanking -- forging -- normalizing (annealing) -- rough machining -- tempering -- semi-finishing -- surface quenching -- finishing.
(2) Carburizing quenching
Carburizing and quenching are suitable for low carbon steel and low alloy steel. Firstly, the carbon content of the surface layer of the parts is increased. After quenching, the surface layer can obtain high hardness, while the core still maintains certain strength and higher toughness and plasticity. Carburizing is divided into overall carburizing and local carburizing. Local carburizing to the non-carburizing part to take impervious measures (copper plating or plating impervious material).As a result of carburizing quenching deformation, and the depth of carburizing is generally between 0.5~2mm, so carburizing process is generally arranged in the semi-finishing and finishing.
Its process line is generally: blanking - forging - normalizing - rough, semi-finishing - carburizing - quenching - finishing.
When the local carburizing parts of the non-carburizing part of the increase of the margin, the removal of the excess carburizing layer of the process plan, the removal of an excess carburizing layer of the process should be arranged after carburizing, quenching before.
(3) Nitriding treatment
Nitriding is a treatment in which nitrogen atoms infiltrate into the metal surface to obtain a layer of nitrogenous compounds. Nitriding layer can improve the surface hardness, wear resistance, fatigue strength, and corrosion resistance. As a result of nitriding treatment temperature is low, deformation is small, and nitriding layer is thin (generally not more than 0.6~0.7mm), nitriding process should be arranged as far back as possible, in order to reduce the deformation of nitriding, generally need to remove the stress after the high-temperature tempering.