1. Heat treatment method: Different heat treatment methods can be selected according to different objects and purposes.
Quenched and tempered steel: high-temperature tempering after quenching (500-650℃)
Spring steel: tempered at medium temperature after quenching (420-520℃)
Carburized steel: quenched after carburization and then tempered at low temperature (150-250℃)
After low carbon and medium carbon (alloy) steels are quenched into martensite, with the increase of tempering temperature, the general rule is that the strength decreases, while the plasticity and toughness increase. However, due to the different carbon content in low and medium carbon steels, the effect of tempering temperature on them is different. Therefore, in order to obtain good comprehensive mechanical properties, the following approaches can be taken:
(1) Select low-carbon (alloy) steel, quenched and tempered at a low temperature below 250 °C to obtain low-carbon martensite. In order to improve the surface wear resistance of this type of steel, the only way to increase the carbon content of each surface layer is to carry out surface carburization, which is generally called carburized structural steel.
(2) Use medium carbon steel with high carbon content, and after quenching, carry out high temperature (500-650 ℃) tempering (the so-called quenching and tempering treatment), so that it can maintain sufficient strength in the case of high plasticity, generally called this. Class steel is quenched and tempered steel. If you want to obtain high strength, but would rather reduce the plasticity and toughness, low-temperature tempering can be adopted for the gold-containing quenching and tempering with low carbon content, and the so-called "ultra-high-strength steel" is obtained.
(3) Steels with a carbon content between medium carbon and high carbon (such as 60, 70 steel) and some high carbon steels (such as 80, 90 steel), if used to make springs, in order to ensure a high elastic limit, yield limit and fatigue limit, the medium temperature tempering after quenching is adopted.
(4) Decarburization: refers to the loss of carbon on the surface of ferrous metal materials (steel). There will be decarburization after heat treatment. Slight decarburization is allowed. The depth of the decarburization layer affects the surface hardness. The deeper the decarburized layer, the smaller the surface hardness value.
The specific detection is based on GB3098.1.
2. Operation process: annealing (pearlite steel)
1. Preheating treatment: normalizing
High-temperature tempering (martensitic steel)
(1) The purpose of normalizing is to refine the grains, reduce the degree of banding in the structure, and adjust the hardness to facilitate machining. After normalizing, the steel has equiaxed fine grains.
2. Quenching: Heat the steel body to about 850 ℃ for quenching. The quenching medium can be selected according to the size of the steel piece and the hardenability of the steel. Generally, water or oil, or even air quenching can be selected. Steel in the quenched state has low plasticity and high internal stress.
3. Tempering:
In order to make the steel have high plasticity, toughness, and appropriate strength, the steel is tempered at a high temperature of about 400-500 °C. The steel with greater sensitivity to temper brittleness must be cooled rapidly after tempering to suppress the occurrence of temper brittleness.
If the part is required to have particularly high strength, it is tempered at about 200 ° C to obtain a medium carbon tempered martensite structure.
(2) Spring steel:
1. Quenching: oil quenching at 830-870℃.
2. Tempering: Tempering is performed at about 420-520 °C to obtain a tempered troostite structure.
(3) Carburized steel:
1. Carburizing: a kind of chemical heat treatment, which refers to the infiltration of C element into the surface of steel parts in an active medium containing a certain chemical element at a certain temperature. Preheating (850℃) Carburizing (890℃) Diffusion (840℃) Process
2. Quenching: For carbon and low alloy carburized steel, direct quenching or one-time quenching is generally used.
3. Tempering: Tempering at low temperature to eliminate internal stress and improve the strength and toughness of the carburized layer.
