Aging Period Of Shoulder Screws

What is the aging period of shoulder screws? The aging period of shoulder screws mainly refers to the cycle for eliminating internal stress in the shoulder (the limiting shaft between the head and thread) through aging treatment. During the production of shoulder screws, internal stress is generated in the raw material rolling and cold heading forming stages. This internal stress is not solely caused by uneven carbon distribution, but more importantly by lattice distortion and uneven structure resulting from metal plastic deformation during cold working. After cold heading into finished products, if used directly, internal stress may gradually release during subsequent storage or operation, leading to screw deformation or even fracture. Therefore, after the production of shoulder screws, they need to be first cooled naturally and placed in a well-ventilated environment for aging treatment to reduce internal stress through natural or artificial means. The period from the generation of internal stress to the stabilization of the screw after treatment is called the aging period of shoulder screws.

If internal stress is still not effectively eliminated after the aging period, the shoulder screws are prone to fracture during use. It is worth noting that if the raw material itself has excessive internal stress due to composition segregation (not simply uneven carbon distribution), rolling defects, etc., it will not only cause uneven surfaces or bending deformation of the material, but also lead to quality issues such as substandard surface finish and dimensional tolerance out of range in the shoulder screws produced from it. Such raw materials are inherently unqualified.

To save time costs, some manufacturers directly use heat treatment to mitigate the impact of internal stress when they know that shoulder screws have internal stress. This method may have a certain effect on low-strength ordinary screws, but for high-strength shoulder screws of grade 10.9 and 12.9, simple conventional heat treatment cannot effectively solve the internal stress problem-it is not that "it doesn't work", but that precise low-temperature stress relief annealing processes are required to avoid high-temperature treatment affecting their established high-strength performance. The higher the strength grade of the bolt, the greater the potential damage risk of internal stress. Therefore, control must start from the raw material stage: raw materials need to undergo aging treatment first, and the finished products must be further processed to eliminate internal stress after forming.

Generally, for shoulder screws with a strength grade ≥ 8.8, after completing heat treatment (quenching and tempering) and surface treatment, it is recommended to first perform artificial stress relief treatment: place them in a constant temperature furnace at around 300℃ for 2 hours of heat preservation (this is low-temperature stress relief annealing, and the temperature must be strictly controlled below its tempering temperature to avoid reducing strength), then take them out for natural cooling, followed by natural aging.

The natural aging period of shoulder screws is usually 15 to 20 days. There is no unified standard for the specific time, which needs to be adjusted according to factors such as raw material batches, cold working deformation, and the effect of artificial stress relief. Raw materials from different manufacturers may require slightly different aging times due to differences in rolling processes. If internal stress is still detected after 15 days (e.g., deformation is found through precision measurement, or abnormalities occur during sampling for torque testing), the aging period needs to be extended to 20 days. If internal stress still causes problems such as deformation or fracture after 20 days, it is necessary to re-evaluate the raw material quality and stress relief process, and if necessary, directly scrap the products-they must not be used forcibly.