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Heat treatment process for quenching and tempering of 45 steel and 40Cr steel. Tempering is a two-step heat treatment process that involves quenching followed by high-temperature tempering, and its purpose is to endow the workpiece with excellent overall mechanical properties. 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 uniform tempered sorbite is obtained. Small factories cannot conduct metallographic analysis for each heat; generally, only hardness testing is carried out. This means that the hardness after quenching must reach the quenching hardness of that material, while the hardness after tempering is checked in accordance with the requirements specified in the drawings. The quenching and tempering treatment of workpieces must be carried out strictly in accordance with the process documents; we merely offer some suggestions on how to implement this process during operation. 1. 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 is why it is widely used. 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 holding 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% saltwater solution with a high cooling rate should be used. After the workpiece is placed in water, it should be fully quenched, but not cooled to extreme temperatures. If the workpiece is cooled to extremely low temperatures in saltwater, it may crack, as this occurs when the workpiece cools to around 180°C, causing austenite to rapidly transform into martensite and 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 a hardness requirement, the tempering temperature must be adjusted accordingly to ensure that the required hardness is achieved. 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 broaching operations are required after quenching and tempering, the required hardness 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. 2. Quenching and tempering treatment of 40Cr steel Cr increases the hardenability of the steel, enhances its strength and temper stability, resulting in excellent mechanical properties. For quenched and tempered workpieces with large cross-sectional dimensions or high importance, Cr steel should be used. However, Cr steel has a second type of temper brittleness. For the quenching and tempering of 40Cr workpieces, various parameter values are specified in the process sheets. From our practical experience, it can be seen that: (1) After quenching, 40Cr workpieces should be cooled in oil. 40Cr steel has good hardenability; cooling it in oil allows it to become hardened, while also reducing the tendency for the workpieces to deform or crack. However, in cases of tight oil supply, small enterprises can quench workpieces with simple shapes in water without encountering cracking; it is only necessary for the operator to strictly control the temperature at the time of immersion and removal from the water based on experience. (II) If the hardness of 40Cr workpieces remains high after quenching and tempering, the temperature for the second tempering cycle must be increased by 20~50°C; otherwise, it is difficult to reduce the hardness. (III) After high-temperature tempering of 40Cr workpieces, those with complex shapes are cooled in oil while those with simpler shapes are cooled in water, in order to avoid the effects of secondary temper brittleness. For workpieces that have been quenched rapidly, stress-relief treatment may be applied if necessary. The operator’s skill is an important factor that affects the quality of quenched and tempered workpieces. In addition, there are various other factors such as equipment, materials, and the preprocessing steps prior to quenching. We believe that: (1) If the workpiece is transferred from the heating furnace to the cooling tank too slowly, its temperature upon entering the water drops below the Ar3 critical point, resulting in partial decomposition; this leads to an incomplete quenching structure of the workpiece, failing to meet the required hardness standards. Therefore, for small parts, the speed of the coolant is important, while for large workpieces, it is necessary to control the pre-cooling time. (II) The amount of workpieces loaded into the furnace should be appropriate, with 1–2 layers being ideal; overlapping workpieces can lead to uneven heating, which in turn results in uneven hardness. (III) The workpieces should be arranged at a certain distance when placed in water; too close proximity will prevent the vapor film near the workpieces from breaking, resulting in lower hardness on the surfaces of those workpieces. (IV) For furnace quenching, it is not possible to complete the quenching in one go; depending on the degree of drop in furnace temperature, the furnace should be shut down midway and reheated so that the hardness of the workpieces after quenching is consistent. (5) Pay attention to the temperature of the coolant; if the temperature of 10% salt water is above 60°C, it cannot be used. The coolant must be free of impurities such as oil and mud; otherwise, insufficient or uneven hardness may occur. (VI) Without heat treatment of the untreated blank, the hardness will not be uniform; to achieve good quality after heat treatment, the blank should be rough-turned and the rod material should be forged. (7) Strictly control the quality; if the hardness after quenching is 1–3 units too low, the tempering temperature can be adjusted to meet the required hardness level. However, the hardness of the workpiece after quenching is too low; in some cases it is only around HRC 25–35. It is necessary to carry out quenching again, and it is absolutely not sufficient to rely solely on medium-temperature or low-temperature tempering to meet the requirements specified in the drawings. Otherwise, the purpose of quenching and tempering is lost, and serious consequences may arise.