First, the main content of the mechanical properties of the tight parts
Mechanical properties of fasteners. Can reflect the ability to use fasteners, including: mechanical (or mechanical) properties, physical properties and performance:
1. Physical properties
Physical properties are primarily intrinsic properties of the materials used to make fasteners. In the finished product - generally does not change, such as melting point, expansion coefficient, magnetic permeability and the like. The other part, in combination with the actual situation of the fastener product, also includes some physical changes in the fastener manufacturing process, such as "surface defects", and even "surface decarburization" and "surface hardness (carbon increase)" .
2 . Mechanical behavior
Mechanical properties (or mechanical properties) refer to the mechanical properties of a fastener product, reflecting the ability to react when it is subjected to a certain load. Such as: tensile strength, yield strength, elongation, reduction in area and hardness. In the production process, the fasteners are finished into the finished fasteners through the processing of raw materials (cold drawing), cold heading, cold extrusion, shrinking or hot pressing, rolling (rolling) and other heat treatment. At this time, its mechanical performance indicators, compared with the raw materials, has changed a lot. And achieved the mechanical properties of a certain fastener product.
3 . Work performance
Work performance is a property obtained through the machining process according to functional design requirements. Therefore, the fastener can be made to meet certain specific requirements in use, such as torque, locking performance, elasticity, and the like. The performance of the work can also be expressed by the properties of nylon materials such as "non-metallic insert lock nuts".
Second, why the mechanical properties of fasteners are specified
1 . Only the age of the material grade
In the 1950s, China's first national standards such as GB 38-58 "Bolt Technical Conditions" and GB 61-58 "Nut Technical Conditions" were issued, and only the material grades of fasteners were specified. At the same time, in foreign countries (mainly the former Soviet Union, East Germany and other "socialist countries") "assisted construction projects", the standards for fastener products at all levels only specify the material grade of the product, and the "standard parts" of the enterprise The workshop/station is responsible for the production of the required fastener products.
2. Specify the mechanical properties of the fastener
The professional development of fasteners promotes the design and selection of fasteners, as well as the improvement of standardization level. The national standard “fasteners†are implemented nationwide, especially by the former Second Industrial Bureau of the Ministry of Machinery Industry. And the “Ten Standard Parts Factory†that has been built one after another has played a professional and concentrated production of fastener products with many advantages, fast, good and provincial advantages.
The mainframe production plant that produces electromechanical products gradually terminates self-made, but instead sells fastener products. However, due to the specialization of centralized production, the process is different. Therefore, the contradiction between “specified material grade†and “specialized centralized production process†is prominent and directly affects the development of specialized production. The so-called advanced technology requires new equipment and tooling molds, especially the use of special materials suitable for cold heading process, otherwise the expected results will not be achieved.
3 . Mechanical properties of fasteners depend on materials and manufacturing processes
If you need the same fastener type, such as bolts, the same specifications, such as M16 × 80, when using the same material grade. Such as 35 steel. When using different production processes, the mechanical properties of the bolt products obtained are different, if expressed in terms of approximate mechanical properties:
1) Cutting process: maintain the original performance index of the material, equivalent to 5.8;
2) Hot forging process: basically maintain the original performance index of the material, which is basically equivalent to 5.8 or 5.6;
3) Cold heading process: the tensile strength is improved by cold work hardening, which is equivalent to 6.8 grade;
4) Cold heading forming a quenching and tempering (tempering) treatment: equivalent to 8.8.
It can be seen that for general-purpose (or commodity) fasteners, even if a material grade is specified, the mechanical properties that can be obtained are different due to different processes. therefore. For general purpose (or commodity) fasteners, specifying material grades is neither scientific nor reasonable. This design concept has long since fallen behind and was abandoned. Relatively speaking, replacing the specified material grade with the mechanical performance grade as much as possible is an internationally recognized, advanced and reasonable fastener design and selection concept as early as the 20th century.
Third, the fastener products that specify mechanical properties
1 . Standard fastener products include: bolts, studs, nuts, screws, wood screws, self-tapping screws, washers, rivets, pins, retaining rings, fasteners and joints, and other (such as welding nails, profiles) Nail and other 12 categories. At present, in the international scope, ISO 2nd Technical Committee "fasteners" (ISO/TC2) only have bolts, nuts, screws, self-tapping screws (including self-drilling self-tapping screws and self-squeezing) for metric fasteners. 9 types of products such as screws, flat washers, blind rivets, pins, fasteners, and welding nails have been developed and released with international standards, including bolts, nuts, screws, self-squeezing screws, self-tapping screws, Self-drilling self-tapping screws and flat washers specify mechanical properties (grades) without specifying material grades.
2. China's fastener standards are the same as those specified in ISO/TC2. Two more are added:
First, the provisions of the "core rivet mechanical performance grade marking code" (such as 06, 08, 10, 20, 50). As early as 1990 GB/T 12619--1990 (the technical conditions of the blind rivet) Chapter 3 specifies the performance grade of the blind rivet, the material grade of the nail body and the nail core. The current standard is: GB/T 3098 .19—2004 “Mechanical Performance of Fastener Rivetsâ€;
Second, with the rapid growth of national economic construction, China's steel industry has also developed rapidly. In 1996, steel output surpassed 100 million tons and ranked first in the world. In 2005, the output has reached 300 million tons. The demand for steel for fasteners has reached 5.2 million tons in 2007. In this development, in the face of the current situation of the steel industry, the fastener industry with steel as the main raw material must take into account the unbearable pressures that resources, energy, and environmental protection will face. How to use the "circular economy" as a strategic idea and thoroughly implement the scientific development concept. The improvement, development and development of fastener steel is a major issue for the scientific and technological workers in the steel industry and fastener industry for decades.
Non-quenched and tempered steel is developed on the basis of the basic theory of microalloying and the achievements of modern industrial technology. Its performance and price ratio is superior to traditional quenched and tempered steel, which can replace part of carbon structural steel and alloy structural steel. It is a kind of high-tech steel with high technology content, strong driving force and large influence surface. Cold-worked non-tempered steel is an important branch in the field of non-quenched and tempered steel. It micro-alloys, controlled rolling and controlled cooling, which causes the precipitation of carbonitrides of microalloying elements to produce precipitation strengthening and fine grain strengthening. On the basis of this, work hardening is generated by cold deformation, so that the strength of the steel is further improved, and the required performance index can be achieved without undergoing quenching and tempering treatment.
Maanshan Iron and Steel Group Co., Ltd. has formed a complete set of key technologies for the production of non-tempered cold-rolled steel wire in technology, equipment and technology. It has realized the mass production and application of non-tempered cold-rolled steel for the first time in China, and the physical quality has reached the international advanced level. Some indicators exceed the international advanced level. At the same time, together with the China National Machinery Productivity Promotion Center of the General Institute of Mechanical Science and the fastener manufacturing enterprise, the national standard GB/T 3098.22-2009 "Mechanical properties of fasteners - fine-grained non-tempered steel bolts, screws and studs 》. By 2012, only Maanshan Iron & Steel Co., Ltd. has mass-produced and sold 40,000 tons of fine-grained non-tempered cold-rolled steel wire rods, all of which are used to produce 8.8-grade and 10.9-grade fastener products with stable and reliable quality. Fasteners eliminate the need for annealing and quenching and tempering processes when manufacturing high-strength fasteners (especially for fasteners that are easily deformed after quenching such as U-bolts and long bolts), with significant energy savings The effect is significant in economic and social benefits.
Fine-grained non-tempered steel wire is used to manufacture products equivalent to 8.8~10.9 hex bolts, hex head screws, screws and U-screws. At present, in addition to supplying domestic products, some of these products have already been exported.
The approval of GB/T 3098.22-2009 will accelerate the promotion and application of this green material.
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