Why is quenching and tempering necessary for steel parts? What is the effect?

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Quenching and tempering: The heat treatment method that involves high-temperature tempering after quenching is called quenching and tempering. High-temperature tempering refers to tempering at temperatures between 500-650°C. Quenching and tempering allows for significant adjustments to the properties and quality of steel, resulting in good strength, plasticity, and toughness, as well as excellent overall mechanical properties. Tempered sorbite is obtained after quenching and tempering. Tempered sorbite is formed when martensite is tempered; it can only be identified under an optical light microscope at a magnification of 500–600 times or higher. It is a composite structure in which carbon compounds (including cementite) appear as granules distributed within a ferritic matrix. It is also a tempered structure of martensite, consisting of a mixture of ferrite and granular carbides. At this point, the ferrite no longer has any significant carbon supersaturation, and the carbides are also stable-type carbides. At room temperature, it is an equilibrium tissue. Quenched and tempered steel is divided into two main categories: carbon quenched and tempered steel and alloy quenched and tempered steel. Whether it is carbon steel or alloy steel, the carbon content in it is strictly controlled. If the carbon content is too high, the strength of the workpiece after quenching and tempering is high, but its toughness is insufficient; if the carbon content is too low, toughness improves while strength is inadequate. To achieve good overall properties in quenched and tempered parts, the carbon content is generally controlled between 0.30% and 0.50%. During quenching and tempering, it is required that the entire cross-section of the workpiece be fully quenched, so that the workpiece obtains a microstructure dominated by fine needle-like quenched martensite. Through high-temperature tempering, a microstructure dominated by uniformly tempered sorbite is obtained. Small factories cannot conduct metallographic analysis for each heat; they generally only perform hardness tests. This means that the hardness after quenching must reach the desired level for that material, while the hardness after tempering is checked in accordance with the specifications provided in the diagrams. Case study: Quenching and tempering of 45 steel. 45 steel is a medium-carbon structural steel with good properties for both cold and hot processing, excellent mechanical strength, low cost, and wide availability, which contributes to its widespread use. Its greatest weakness is its low hardenability; it is not suitable for workpieces with large cross-sectional dimensions and high requirements. The quenching temperature for 45 steel is A3 + (30~50)°C; in practical applications, the upper limit of this range is usually adopted. A relatively high quenching temperature can accelerate the heating rate of the workpiece, reduce surface oxidation, and improve work efficiency. To homogenize the austenite in the workpiece, sufficient holding time is required. If the actual loading volume into the furnace is large, the insulation time needs to be appropriately extended. Otherwise, insufficient hardness may occur due to uneven heating. However, if the holding time is too long, it can also lead to problems such as coarse grains and severe oxidation and decarburization, which affect the quality of quenching. We believe that if the charging amount exceeds the specifications in the process documents, the heating and holding time should be increased by 1/5. Since 45 steel has low hardenability, a 10% saline solution with a high cooling rate should be used. After the workpiece is placed in water, it should be hardened thoroughly, but not cooled to extreme temperatures. If the workpiece is cooled to extremely low temperatures in saltwater, it may crack, as austenite rapidly transforms into martensite at around 180°C, resulting in excessive structural stress. Therefore, when the quenched workpiece is rapidly cooled to this temperature range, a slow cooling method should be employed. Since it is difficult to control the outlet water temperature, operation must be based on experience; once the movement of the workpiece in the water stops, the water can be discharged for air cooling (oil cooling would be better if possible). Additionally, when the workpiece enters the water, it should move rather than remain stationary, and it ought to move in a regular manner according to its geometric shape. A stationary cooling medium combined with a stationary workpiece leads to uneven hardness and stress, resulting in significant deformation of the workpiece or even cracking. The hardness of 45 steel parts after quenching in the tempering process should reach HRC 56–59. In cases where the cross-sectional area is large, this value may be slightly lower, but it must not fall below HRC 48; otherwise, it indicates that the part has not been fully quenched, and structures such as sorbite or even ferrite may form within the material. Such structures remain in the matrix even after tempering, failing to achieve the objectives of tempering. For the high-temperature tempering of 45 steel after quenching, the heating temperature is usually between 560 and 600°C, with a required hardness of HRC22 to 34. Since the purpose of tempering is to achieve comprehensive mechanical properties, the hardness range is relatively wide. However, if the drawings specify requirements for hardness, the tempering temperature must be adjusted accordingly to ensure the desired hardness. If some shaft components require high strength, then a high hardness level is also necessary ； For some gears and shaft parts with keyways, since milling and interpolation machining are required after quenching and tempering, the hardness requirement is lower. Regarding the holding time during tempering, it depends on the required hardness level and the size of the workpiece. We believe that the hardness after tempering is determined by the tempering temperature and not so much by the tempering time; however, it is necessary to ensure complete tempering, and generally, the holding time for tempering a workpiece is always more than one hour.