The heat treatment of 45 steel, also called 45 steel Tempering .
45 steel is a medium carbon structural steel with good cold and hot working properties, good mechanical properties, low price and wide sources, so it is widely used in Fixture Jigs making or machine parts manufacturing. Its biggest weakness is the low hardenability, large cross-section size and high requirements of the workpiece is not suitable for use.
Steel heat treatment temperature:
The quenching temperature of 45 steel is A3+ (30-50). In practice, the upper limit is usually taken. Higher quenching temperature can accelerate the heating speed of the workpiece, reduce surface oxidation and improve work efficiency. In order to homogenize the austenite of the workpiece, sufficient holding time is needed. If the actual load is large, the holding time should be prolonged appropriately. Otherwise, the phenomenon of insufficient hardness due to uneven heating may occur. However, if the holding time is too long, there will also be serious defects such as coarse grains and oxidative decarbonization, which will affect the quenching quality. In our opinion, if the quantity of furnace is larger than that stipulated in the process document, the heating and holding time should be extended by 1/5.
Steel cooling control:
Because 45 steel has low hardenability, 10% brine solution with high cooling rate should be adopted. The workpiece should be hardened after entering water, but not cooled. If the workpiece is cooled in brine, it may cause cracking of the workpiece. This is because when the workpiece is cooled to about 180 C, the rapid transformation of austenite into martensite causes excessive structural stress. Therefore, when the quenched workpiece is rapidly cooled to the temperature region, a slow cooling method should be adopted. Because the temperature of effluent is difficult to control, it must be operated by experience. When the shaking of the workpiece in the water stops, the effluent air-cooled (if oil-cooled is better). In addition, the workpiece should not be static when entering water. It should move regularly according to the geometric shape of the workpiece. Static cooling medium plus stationary workpiece results in uneven hardness and uneven stress, resulting in large deformation and even cracking of workpiece.
Steel hardness after heat treatment:
Hardness of quenched and tempered parts of 45 steel should reach HRC56-59, the possibility of large cross section is lower, but not lower than HRC48. Otherwise, it means that the workpiece has not been completely quenched, and there may be sorbite or even ferrite in the structure. This kind of structure is still retained in the matrix through tempering, which can not achieve the purpose of quenching and tempering.
After quenching, 45 steel is tempered at high temperature, the heating temperature is usually 560 ~ 600 C, 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. But if the drawing has hardness requirement, the tempering temperature should be adjusted according to the drawing requirement to ensure the hardness. For example, some axle parts require high strength and high hardness, while some gears and axle parts with keyway need milling and interpolation after quenching and tempering, so the hardness requirement is lower. As for tempering holding time, it depends on hardness requirement and workpiece size. We believe that the hardness after tempering depends on tempering temperature and has little relation with tempering time, but it must be reversed. Generally, the tempering holding time of workpiece is more than one hour.
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