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 greatly adjust the properties and materials of steel, with good strength, plasticity and toughness, and good comprehensive mechanical properties.
 
Tempered sorbite was obtained after quenching and tempering. Tempered sorbite is formed by martensite during tempering. It can be distinguished only when it is magnified more than 500 – 600 times under the optical metallographic microscope. It is a composite structure with carbide (including cementite) spheres distributed in the ferrite matrix. It is also a tempering structure of martensite and 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 kind of balanced tissue at room temperature.
Quenched and tempered steel includes 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, 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 and the strength is insufficient. In order to obtain good comprehensive performance of quenched and tempered parts, the carbon content is generally controlled at 0.30 – 0.50%.
During quenching, tempering and quenching, the whole section of the workpiece is required to be quenched thoroughly, so that the workpiece can obtain the microstructure dominated by fine needle quenched martensite. The microstructure dominated by uniformly tempered sorbite was obtained by high temperature tempering. It is impossible for small factories to conduct metallographic analysis for each furnace. Generally, they only conduct hardness test, that is, 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.
Case sharing: quenching and tempering of 45# steel
45# steel is a medium carbon structural steel with good cold and hot workability, good mechanical properties, low price and wide source, so it is widely used. Its biggest weakness is that it is not suitable for workpieces with low hardenability, large section size and high requirements.
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, sufficient holding time is required. If the actual furnace loading is large, the holding time needs to be extended appropriately. Otherwise, insufficient hardness may occur due to uneven heating. However, if the holding time is too long, the disadvantages of coarse grain and serious oxidation decarburization will also appear, which will affect the quenching quality. We believe that if the charging amount is greater than that specified in the process documents, the heating and holding time needs to be extended by 1/5.
Because 45# steel has low hardenability, 10% brine solution with high cooling rate should be used. After the workpiece enters the water, it should be quenched thoroughly, but not cold thoroughly. If the workpiece is cold thoroughly in salt water, it may crack the workpiece, which is caused by the rapid transformation of austenite into martensite when the workpiece is cooled to about 180 ℃, resulting in excessive structural stress. Therefore, when the quenched workpiece is rapidly cooled to this temperature area, the slow cooling method should be adopted. Since the outlet water temperature is difficult to master, it must be operated by experience. When the shaking of the workpiece in the water stops, the outlet water can be air cooled (oil cooling is better). In addition, the workpiece should move rather than static when entering the water, and make regular movement according to the geometry of the workpiece. Static cooling medium and static workpiece lead to uneven hardness and stress, resulting in large deformation and even cracking of workpiece.
The quenched hardness of 45# steel quenched and tempered parts should reach HRC56 – 59, and the possibility of large section is lower, but not lower than hrc48. Otherwise, it means that the workpiece has not been completely quenched, and sorbite or even ferrite structure may appear in the structure. This structure is still retained in the substrate through tempering, which can not achieve the purpose of quenching and tempering.
For high temperature tempering of 45# steel after quenching, the heating temperature 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 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; For some gears and shaft parts with keyway, the hardness requirements are lower because they have to be milled and inserted after quenching and tempering. The tempering holding time 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 re penetrated. Generally, the tempering holding time of the workpiece is more than one hour.