[China Aluminum Network] The sprocket is a critical component of the bicycle crank assembly. Its manufacturing process includes: material cutting, blanking, punching the center hole, forming, tooth cutting, and finally punching 6 accurately positioned holes (25 in total). After that, the part undergoes solution treatment. The solution treatment specifications require a temperature range of 495–503°C, with a holding time of 15–20 minutes, and cooling in water at 25–35°C. After stamping and processing, the flatness of the sprocket initially meets the product drawing requirements. However, after solution treatment, the flatness exceeds the acceptable limits. To address this issue, the sprocket was fixed onto a special hanger using bolts and pressure plates before the solution treatment. Once the treatment was complete, the bolts and pressure plates were removed, and the sprocket was taken out from the hanger. Despite this, the flatness still did not meet the required standards. Using water as a quenching medium offers advantages such as ease of operation, low cost, and fast cooling, which ensures the mechanical properties of aluminum alloys. However, from the perspective of minimizing workpiece deformation, water is not the ideal choice. Its rapid cooling causes significant temperature differences between the surface and core of the part, especially when there are varying wall thicknesses, leading to high internal stresses. At high temperatures, aluminum alloys are soft and prone to deformation, while at lower temperatures, these stresses can result in cracks. To reduce deformation, industry practices often involve increasing the water temperature, using oil quenching, isothermal quenching, or polymer-based quenching media. Oil quenching requires an additional degreasing step, while isothermal quenching involves more complex procedures. Polymer quenching, on the other hand, allows for adjustable cooling rates by changing the concentration. Low-concentration solutions cool faster than cold water, while higher concentrations slow down the cooling rate, approaching that of oil or hot water. Although it offers flexibility, polymer quenching is more expensive and less straightforward in terms of production management compared to water. Based on the above considerations, it was decided to use 100°C hot water as the quenching medium. Following the updated solution treatment process—temperature 495–503°C, holding time 15–20 minutes—the flatness of the sprocket was reduced to less than 0.26 mm, meeting the required specifications.
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