Quenching and tempering treatment: The heat treatment method of high temperature tempering after quenching is called quenching and tempering treatment. High temperature quenching refers to quenching between 500-650℃. Quenching and tempering can significantly adjust the properties and materials of steel. Its strength, plasticity and toughness are all better, and it has good comprehensive mechanical properties.
After quenching and tempering treatment, a tempered sorbit is obtained. The quenched sorbit is formed by the martensite during tempering. It can only be distinguished under a metallographic optical microscope at 500 to 600 times magnification. It is a ferrite matrix with a composite structure of carbides (including cementite). It is also a martensite-tempered structure, which is a mixture of ferrite and granular carbides. At present, ferrite has almost no carbon supersaturation and the carbides are also stable carbides. It is a balanced fabric at room temperature. There are two categories of quenched and tempered steel: carbon quenched and tempered steel and alloy quenched and tempered steel. Whether it is carbon steel or alloy steel, the control of carbon content is relatively strict. If the carbon content is too high, the strength of the workpiece after quenching and tempering will be high, but the toughness is not enough. If the carbon content is too low, the toughness will be increased but the strength will be insufficient. In order to obtain good overall performance of quenched parts, the carbon content is generally controlled between 0.30 and 0.50%. During tempering and quenching, the entire cross-section of the part must be quenched, so that the part obtains a microstructure dominated by fine needle-shaped quenched martensite. Thanks to high-temperature tempering, a microstructure dominated by a uniformly tempered sorbital is obtained. It is impossible for small factories to perform metallographic analysis for each furnace and generally only perform hardness tests. This means that the hardness after quenching should reach the quenching hardness of the material, and the hardness after tempering should be checked according to the document requirements. the drawing.
Quenched and tempered 45 steel is a medium carbon structural steel with good hot and cold processing properties, 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 parts with large cross sections and high requirements. The quenching temperature of 45 steel is A3+(30~50)℃ In actual operation, the upper limit is generally taken. Higher quenching temperature can accelerate the heating of the workpiece, reduce surface oxidation and improve work efficiency. In order to homogenize the austenite of the part, sufficient holding time is necessary. If the actual furnace load is large, the holding time should be extended appropriately. Otherwise, insufficient hardness may occur due to uneven heating. However, if the holding time is too long, coarse grains and significant oxidative decarburization will also occur, which will affect the quenching quality. We believe that if the loading capacity of the furnace is higher than the stipulations of the process documents, the heating and holding time should be extended by 1/5. Since 45 steel has low hardenability, a 10% brine solution with a high cooling rate should be used. Once the piece enters the water, it should be completely soaked, but not completely cooled. If the part is completely cooled in salt water, it may crack, because when the part is cooled to around 180°C, the austenite quickly turns into. martensite. Caused by excessive tissue tension. Therefore, when the quenched workpiece is rapidly cooled within this temperature range, a slow cooling method should be adopted. Since the outlet water temperature is difficult to control, it should be used according to experience. When the part in the water stops shaking, the water can be taken out to be cooled by air (it would be better if it could be cooled by oil). . Additionally, when the part enters the water, it should be moving rather than static. It must move regularly according to the geometric shape of the room. The static coolant and the static part cause uneven hardness and stress, resulting in significant deformation of the part or even cracking. The hardness of 45 steel parts quenched and tempered after quenching should reach HRC56~59. The possibility of large cross sections is lower, but it cannot be lower than HRC48. Otherwise, it means the part has not been completely soaked and sorbed. or even ferrite may appear in the structure. The structure remains in the matrix after income and cannot achieve the income target.
For high temperature tempering of 45 steel after quenching, the heating temperature is generally 560~600℃, and the hardness requirement is HRC22~34. The purpose of tempering is to obtain complete mechanical properties, the hardness range is relatively wide. However, if the drawing has hardness requirements, the tempering temperature must be adjusted according to the drawing requirements to ensure hardness. For example, some shaft parts require high strength and hardness requirements; while some gears and shaft parts with keyways require milling and inserting after quenching, so the hardness requirements are lower. Regarding the tempering and holding time, it depends on the hardness requirements and the size of the part. We believe that hardness after tempering depends on tempering temperature and has little to do with tempering time, but it should generally be tempered. the quenching and holding time of the part is always more than one hour.
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