1. Main contents of mechanical properties of fasteners
Mechanical properties of fasteners. It can reflect the use ability of fasteners, mainly including: mechanical (or mechanical) properties, physical properties and working performance:
1. Physical properties
Physical properties are mainly the inherent characteristics of the materials used to make fasteners. They generally do not change in the finished product, such as melting point, expansion coefficient, magnetic permeability, etc. The other part, combined with the actual situation of fastener products, includes some physical changes produced during the manufacturing process of fasteners, such as "surface defects", and even "surface decarburization" and "surface hardness (carbon increase)".
2. Mechanical properties
Mechanical properties (or mechanical properties) refer to the mechanical properties of fastener products, reflecting their ability to respond when they are subjected to a certain load. Such as tensile strength, yield strength, elongation, cross-sectional shrinkage and hardness. In the production process, fasteners undergo raw material reforming (cold drawing), cold heading, cold extrusion, rod shrinkage or hot pressing, thread rolling (rolling) and other processing processes, as well as heat treatment, and finally become fastener products. At this time, its various mechanical performance indicators have changed significantly compared with the unprocessed raw materials, and have reached the mechanical properties of a certain fastener product.
3. Working performance
Working performance is a characteristic obtained through the processing process according to the functional design requirements. Therefore, the fastener can meet certain specific requirements in use, such as torque, locking performance, elasticity, etc. Working performance can also be expressed by materials, such as the nylon material performance of "non-metallic insert locking nuts".
II. Why the mechanical properties of fasteners are specified
1. The era of only specifying material grades
In the 1950s, my country's first promulgation of GB 38-58 "Technical Conditions for Bolts" and GB 61-58 "Technical Conditions for Nuts" and other national standards only specified the material grades of fasteners. At the same time, in foreign "aid projects" (mainly "socialist countries" such as the former Soviet Union and East Germany), the standards at all levels for fastener products only specified the material grades of the products, and the "standard parts workshop/section" of the enterprise was responsible for producing the required fastener products.
2. Specified fastener mechanical properties
The specialized development of fasteners promotes the design and selection of fasteners, as well as the improvement of standardization level. With the implementation of the national standard "fasteners" throughout the country, especially the "Ten Standard Parts Factory" built by the former Second Industrial Bureau of the First Ministry of Machinery Industry, the advantages of specialized centralized production of fasteners, such as more, faster, better and more economical, have gradually emerged. Many
The host manufacturers of electromechanical products have gradually stopped self-production and switched to outsourcing fastener products. However, due to the different processes used in specialized centralized production, the contradiction between "specified material grades" and "specialized centralized production processes" is prominent and directly affects the development of specialized production. As the saying goes, advanced technology requires new equipment and tooling, especially the use of special materials suitable for cold heading processes, otherwise the expected effect cannot be achieved.
3. The mechanical properties of fasteners depend on materials and manufacturing processes
If the same fastener variety, such as bolts, and the same specifications, such as M16×80, are required, the same material grade should be used. For example, No. 35 steel. When different production processes are used, the mechanical properties of the bolt products obtained are different. If expressed in approximate mechanical property grades:
1) Cutting process: Maintain the original material performance indicators, equivalent to grade 5.8;
2) Hot forging process: Basically maintain the original material performance indicators, basically equivalent to grade 5.8 or grade 5.6;
3) Cold heading process: Due to cold work hardening, the tensile strength is increased, equivalent to grade 6.8;
4) Cold heading forming-quenching and tempering (quenching and tempering): equivalent to grade 8.8.
It can be seen that for general (or commercial) fasteners, even if a material grade is specified, the mechanical properties that can be obtained are different due to the use of different processes. Therefore. For general (or commercial) fasteners, specifying material grades is neither scientific nor reasonable. This design concept has long been outdated and abandoned. Relatively speaking, replacing the specified material grade with the specified mechanical property grade as much as possible is an advanced and reasonable fastener design and selection concept that was internationally recognized as early as the 20th century.
