Under the current "carbon peak" and "carbon neutralization" policies, fasteners for national major equipment manufacturing such as automobile, high-speed railway, aviation, aerospace and national defense have an urgent demand for long-life and ultra-high strength. Ultra high strength fasteners can reduce weight and increase installation space by reducing their own size under the same clamping force. Therefore, the function and volume of connected parts can be optimized, so as to make the equipment achieve the purpose of overall weight reduction and performance optimization. Public information shows that the highest strength grade of fasteners of foreign advantageous enterprises has reached grade 17.8. According to the international development situation, China's 973 project has developed 13.9-15.9 fastener materials and fasteners.
On November 28, 2021, the high-performance steel materials team led by Professor Dong Han of the school of materials of Shanghai University, Hebei Longfengshan Casting Co., Ltd., Qifeng Seiko technology Co., Ltd., Zhoushan 7412 factory, Jiangsu metallurgical technology research institute, Shanghai University (Zhejiang) high-end equipment basic parts and Materials Research Institute and other six units, after more than a year of joint research, passed the With the cooperation of the whole industry chain of "material production fastener manufacturing service evaluation", based on the high-performance theory of iron and steel materials, b17.8 and b19.8 steels for ultra-high strength fasteners have been successfully developed by using the high-purity iron raw materials produced by Longfengshan casting industry, forming grade 16.8 and 19.8 fastener manufacturing technologies. The physical fasteners are shown in Figure 1.
Physical fasteners of grade 16.8 and 19.8 (specification: mj8) × one × 50）
Through the optimization of materials and heat treatment process, the room temperature tensile strength of grade 16.8 and 19.8 bolts meets 1600 ~ 1770mpa and 1900 ~ 2070mpa respectively. The results of shear test (according to GJB 3376-1998) show that the shear stress of grade 16.8 and 19.8 bolts reaches 97kn and 115KN respectively; the room temperature tensile fatigue strength test (according to GJB 3376-1998 and gjb715.30a-2002, load ratio = 0.1) The results show that the developed fasteners meet the fatigue life requirements (the average fatigue life is not less than 65000 times, and the single fatigue life is not less than 45000 times); the delayed fracture resistance test (according to GJB 715.12-1990, the constant tensile stress is maintained for 96 hours in atmospheric environment) The results show that there is no fracture failure of the bolt within the test time, and no crack is found on the bolt surface and thread after unloading. The comprehensive properties of the newly developed ultra-high strength bolt meet the service requirements.
It is reported that foreign ultra-high strength fasteners have formed corresponding design specifications (such as VDA 235-2005) In contrast, the technology and standards of China's relevant products are seriously lagging behind, so it is urgent to formulate relevant specifications. The project has successfully developed grade 16.8 and 19.8 fasteners, which is a breakthrough in climbing the peak of strength of fasteners and materials in the world. It will effectively promote the lightweight manufacturing of China's major equipment (such as realizing the goal of miniaturization of engine and improving power density) This breakthrough in the development of ultra-high strength bolts indicates that the process of China's basic parts and materials from following to leading is accelerating.
