Tempering treatment: The heat treatment method of high temperature tempering after quenching is called tempering treatment. High temperature tempering refers to tempering between 500-650℃. Tempering can adjust the performance and material of steel to a great extent. Its strength, plasticity and toughness are good, and it has good comprehensive mechanical properties.
After tempering treatment, tempered sorbite is obtained. Tempered sorbite is formed when martensite is tempered. It can only be distinguished by magnification of 500~600 times or more under an optical metallographic microscope. It is a composite structure with carbide (including cementite) spherules distributed in the ferrite matrix. It is also a tempered structure of martensite, which is a mixture of ferrite and granular carbide. At this time, the ferrite has basically no carbon supersaturation, and the carbide is also a stable carbide. It is a balanced structure at room temperature.
There are two major types of tempered steel: carbon tempered steel and alloy tempered steel. Whether it is carbon steel or alloy steel, its carbon content is strictly controlled. If the carbon content is too high, the strength of the workpiece after quenching and tempering is high, but the toughness is not enough. If the carbon content is too low, the toughness is improved but the strength is insufficient. In order to obtain good comprehensive performance of the quenched and tempered parts, the carbon content is generally controlled at 0.30~0.50%.
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During quenching and tempering, the entire cross-section of the workpiece is required to be hardened through, so that the workpiece can obtain a microstructure dominated by fine needle-shaped quenched martensite. Through high-temperature tempering, a microstructure dominated by uniform tempered troostite is obtained. It is impossible for a small factory to perform metallographic analysis on each furnace. Generally, only hardness testing is performed. That is to say, the hardness after quenching must reach the quenching hardness of the material, and the hardness after tempering is checked according to the requirements of the drawing.
Quenching and tempering of 45 steel
45 steel is a medium-carbon structural steel with good cold and hot processing performance, good mechanical properties, low price and wide source, so it is widely used. Its biggest weakness is low hardenability, and it is not suitable for workpieces with large cross-sectional sizes and relatively high requirements.
The quenching temperature of 45 steel is A3+(30~50)℃. In actual operation, the upper limit is generally taken. A relatively high quenching temperature can speed up the heating speed of the workpiece, reduce surface oxidation, and improve work efficiency. In order to make the austenite of the workpiece uniform, sufficient insulation time is required. If the actual furnace loading is large, the insulation time needs to be appropriately extended. Otherwise, the phenomenon of insufficient hardness due to uneven heating may occur. However, if the insulation time is too long, there will also be coarse grains and serious oxidation and decarburization, which will affect the quenching quality. We believe that if the furnace loading is greater than the provisions of the process documents, the heating and insulation time needs to be extended by 1/5.
Because the hardenability of 45 steel is low, a 10% brine solution with a high cooling rate should be used. After the workpiece enters the water, it should be hardened, but not cooled. If the workpiece is cooled in the brine, it may cause the workpiece to crack. This is because when the workpiece is cooled to about 180℃, austenite quickly transforms into martensite, causing excessive organizational stress. Therefore, when the quenched workpiece is quickly cooled to this temperature range, a slow cooling method should be adopted. Since the outlet water temperature is difficult to grasp, it must be operated based on experience. When the workpiece in the water stops shaking, the water can be discharged for air cooling (oil cooling is better). In addition, the workpiece should be moving rather than static when entering the water, and should move regularly according to the geometric shape of the workpiece. The static cooling medium plus the static workpiece will lead to uneven hardness and uneven stress, which will cause large deformation of the workpiece and even cracking. The hardness of 45 steel quenched and tempered parts after quenching should reach HRC56~59. The possibility of a large cross-section is lower, but it cannot be lower than HRC48. Otherwise, it means that the workpiece has not been completely quenched, and sorbite or even ferrite may appear in the organization. This organization is still retained in the matrix through tempering, and the purpose of quenching and tempering cannot be achieved.
The high-temperature tempering of 45 steel after quenching is usually 560~600℃, and the hardness requirement is HRC22~34. Because the purpose of quenching and tempering is to obtain comprehensive mechanical properties, the hardness range is relatively wide. However, if the drawings have hardness requirements, the tempering temperature must be adjusted according to the requirements of the drawings to ensure hardness. For example, some shaft parts require high strength and high hardness requirements; while some gears and shaft parts with keyways require lower hardness because they need to be milled and inserted after quenching and tempering. Regarding the tempering and holding time, it depends on the hardness requirements and the size of the workpiece. We believe that the hardness after tempering depends on the tempering temperature and has little to do with the tempering time, but it must be tempered thoroughly. Generally, the tempering and holding time of the workpiece is always more than one hour.
Post time: Jan-06-2025
