Why should steel parts be tempered

Quenching and tempering treatment: the heat treatment method of high temperature tempering after quenching is called quenching and tempering treatment. High temperature tempering refers to tempering between 500-650 ℃. Quenching and tempering can adjust the properties and materials of steel to a great extent. Its strength, plasticity and toughness are good, and it has good comprehensive mechanical properties.

Tempered sorbite is obtained after quenching and tempering. Tempered sorbite is formed by tempering of martensite. It can be distinguished by magnification of 500-600 times under optical microscope. It is a composite structure of carbide (including cementite) spherulites distributed in ferrite matrix. It is also a tempering structure of martensite, a mixture of ferrite and granular carbide. At this time, the ferrite has almost no supersaturation of carbon, and the carbide is also a stable carbide. It is a kind of equilibrium structure at room temperature.

There are two kinds of quenched and tempered steel: carbon steel and alloy 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. For example, if the carbon content is too low, the toughness is increased, but the strength is insufficient. In order to obtain good comprehensive properties of the quenched and tempered parts, the carbon content is generally controlled at 0.30-0.50%.

During quenching and tempering, the whole section of the workpiece is required to be quenched thoroughly, so that the microstructure of the workpiece is mainly fine needle like quenched martensite. Through high temperature tempering, the microstructure of sorbite was obtained. It is impossible for a small factory to conduct metallographic analysis for each furnace. Generally, it only conducts hardness test, that is to say, the hardness after quenching must reach the quenching hardness of the material, and the hardness after tempering shall be checked according to the requirements of the drawing.

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 that the workpieces with low hardenability, large section size and high requirements should not be used.

The quenching temperature of 45 steel is A3 + (30 ~ 50) ℃. In practice, the upper limit is generally taken. The higher quenching temperature can accelerate the heating speed of the workpiece, reduce the surface oxidation and improve the work efficiency. In order to homogenize the austenite of the workpiece, enough holding time is needed. If the actual charging capacity is large, it is necessary to extend the holding time properly. Otherwise, the hardness may be insufficient due to uneven heating. However, if the holding time is too long, the defects of coarse grains and serious oxidation and decarburization will also occur, which will affect the quenching quality. In our opinion, if the charging capacity is greater than the requirements of the process documents, the heating and holding time needs to be extended by 1 / 5.

Because the hardenability of 45 steel is low, 10% brine solution with high cooling rate should be used. After the workpiece is cooled into water, it should be hardened, but not cooled through. If the workpiece is cooled through in brine, it may cause the workpiece to crack. This is because when the workpiece is cooled to about 180 ℃, the austenite rapidly transforms into martensite, resulting in excessive structural stress. Therefore, when the quenched workpiece is rapidly cooled to this temperature area, the method of slow cooling should be adopted. Because the outlet water temperature is difficult to master, it must be operated by experience. When the workpiece shaking in the water stops, the outlet water can be air-cooled (if oil cooling is better). In addition, the work piece should not be static, but should move regularly according to the geometry of the work piece. Static cooling medium and static workpiece lead to uneven hardness, uneven stress and large deformation, even cracking of workpiece.

The hardness of quenched and tempered 45 steel should reach hrc56 ~ 59, and the possibility of cross section is lower, but it can not be lower than hrc48. Otherwise, it means that the workpiece is not completely quenched, and there may be sorbite or even ferrite structure in the structure. This kind of structure still remains in the base body through tempering, so the purpose of quenching and tempering can not be achieved.

45 steel is tempered at high temperature after quenching, the heating temperature is usually 560 ~ 600 ℃, and the hardness is required to be hrc22 ~ 34. Because the purpose of tempering is to obtain comprehensive mechanical properties, so the hardness range is relatively wide. However, if the drawing has hardness requirements, the tempering temperature shall be adjusted according to the drawing requirements to ensure the hardness. For example, some shaft parts require high strength and high hardness, while some gears and shaft parts with keyway need milling and slotting after tempering, so the hardness requirements are lower. As for the tempering holding time, it depends on the hardness requirements and the size of the workpiece. We think that the hardness after tempering depends on the tempering temperature, which has little to do with the tempering time, but it must be penetrated back. Generally, the tempering holding time of the workpiece is more than one hour.

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