Why Do Steel Parts Need To Be Quenched And Tempered? What Is The Purpose?

Quenching and tempering:
The heat treatment method of high-temperature tempering after quenching is called quenching and tempering. High temperature tempering refers to tempering between 500 and 650 ℃. Quenching and tempering can greatly adjust the properties and materials of steel, with good strength, plasticity, and toughness, and excellent comprehensive mechanical properties.

Tempered martensite is obtained after quenching and tempering treatment. Tempered sorbite is a composite structure formed during tempering of martensite, which can only be distinguished by magnification of 500-600 times or more under an optical metallographic microscope. It is composed of carbide (including cementite) spheres distributed within the ferrite matrix. It is also a tempering structure of martensite, which is a mixture of ferrite and granular carbides. At this point, the ferrite has almost no carbon supersaturation, and the carbides are also stable carbides. At room temperature, it is an equilibrium tissue.

Quenched and tempered steel can be divided into two categories: carbon quenched and tempered steel and alloy quenched and tempered steel. Whether it is carbon steel or alloy steel, its carbon content is strictly controlled. If the carbon content is too high, although the strength of the quenched and tempered workpiece is high, the toughness is not enough. If the carbon content is too low, the toughness will increase and the strength will be insufficient. To achieve good comprehensive performance of the quenched and tempered parts, the carbon content is generally controlled between 0.30% and 0.50%.

When quenching and tempering, it is required that the entire cross-section of the workpiece be quenched to obtain a microstructure mainly composed of fine needle shaped quenched martensite. By high-temperature tempering, a microstructure dominated by uniformly tempered martensite is obtained. Small factories cannot conduct metallographic analysis on each furnace, and generally only conduct hardness tests. This means that the hardness after quenching must reach the quenching hardness of the material, and the hardness after tempering should be checked according to the requirements of the diagram.

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 sources, so it is widely used. Its biggest weakness is its low hardenability, and it is not suitable for workpieces with large cross-sectional dimensions and high requirements.

The quenching temperature of 45 steel is A3+(30-50) ℃, and in practical operation, the upper limit is generally taken. A higher quenching temperature can accelerate the heating speed of the workpiece, reduce surface oxidation, and improve work efficiency. To homogenize the austenite of the workpiece, sufficient insulation time is required. If the actual furnace loading is large, it is necessary to extend the insulation time appropriately. Otherwise, there may be insufficient hardness due to uneven heating. However, if the insulation time is too long, it can also lead to the drawbacks of coarse grains and severe oxidation decarburization, which affects the quality of quenching. We believe that if the furnace loading exceeds the requirements of the process documents, the heating and insulation time needs to be extended by 1/5.

Due to the low hardenability of 45 steel, a 10% saline solution with a high cooling rate should be used. After the workpiece is immersed in water, it should be quenched through, but not cooled through. If the workpiece is cooled through in salt water, it may crack. This is because when the workpiece cools to around 180 ℃, austenite rapidly transforms into martensite, causing excessive structural stress. Therefore, when the quenched workpiece rapidly cools to this temperature range, a slow cooling method should be adopted. Due to the difficulty in controlling the outlet temperature, it is necessary to operate based on experience. When the workpiece in the water stops shaking, it can be cooled by air (oil cooling is better if possible).

In addition
the workpiece should be moved rather than static when entering the water, and regular movements should be made according to the geometric shape of the workpiece. The combination of a stationary cooling medium and a stationary workpiece results in uneven hardness and uneven stress, leading to significant deformation and even cracking of the workpiece. The hardness of quenched and tempered 45 steel parts should reach HRC56~59, and the possibility of a large cross-section is lower, but it cannot be lower than HRC48. Otherwise, it indicates that the workpiece has not been fully quenched, and there may be martensite or even ferrite structure in the structure. This structure is still retained in the matrix through tempering, which cannot achieve the purpose of quenching and tempering.

The high-temperature tempering of 45 steel after quenching usually requires a heating temperature of 560-600 ℃ and a hardness requirement of HRC22-34. Because the purpose of tempering is to obtain comprehensive mechanical properties, the hardness range is relatively wide. But if the drawing has hardness requirements, the tempering temperature should be adjusted according to the drawing requirements to ensure hardness. Some shaft parts require high strength and high hardness; Some gears and shaft parts with keyways require lower hardness requirements due to milling and insertion processing after quenching and tempering. Regarding the tempering insulation 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 penetrated back. Generally, the tempering insulation time of the workpiece is always more than one hour.