For a long time, China's high-strength bolts have three major problems: short life, poor reliability and heavy structure. The three major problems seriously restrict the development of high-end mechanical equipment and safe service. High-strength bolts determine the main connection function of high-end mechanical equipment, reflecting the life, reliability and economic affordability of mechanical equipment. Heat treatment is the core of high-strength bolts.
For the more frequent discussion of "Industry 4.0" and "The Third Industrial Revolution", we must consider the future of fasteners and consider the role of fasteners in the third industrial revolution. The next 10 years will be a crucial period for the transformation and upgrading of China's fastener industry, from “manufacturing big country†to “manufacturing power countryâ€. The era of dividends in China's manufacturing industry is coming to an end. Many developing countries have accepted the manufacturing of fasteners, which has created new challenges for China's fastener industry. It is imperative for the fastener industry to accelerate the transformation of economic development.
The main failure mode of high-strength bolts is fatigue
Fatigue is a form of failure in which one or more bolts break or break under the repeated action of cyclic stress and strain. Fatigue is a process of cumulative mechanical damage and delayed failure. Fatigue, including crack initiation and crack propagation, occurs at a yield strength below the material and abruptly fails without significant deformation. Therefore, fatigue is one of the most dangerous forms of failure compared to other forms of failure.
Usually, the connecting bolts on the vehicle are pulled-pull fatigue failure. For example, wind turbines have complex and harsh service environments. They are subjected to dynamic load shocks and corrosive environments. The fatigue fracture of connecting bolts is as high as 85%. The main failure mode of high-strength bolts is fatigue.
The "three major problems" of high-strength bolts originate from stress concentration sensitivity
High-strength bolts are generally made of high-strength low-alloy steel, ultra-high-strength steel and high-temperature alloy. With the development of high-end mechanical equipment, the strength of alloys for high-strength bolts will continue to increase. High-strength materials have high fatigue strength, and are light in weight, small in size, long in life, and highly reliable. However, the outstanding weakness of high-strength materials is that fatigue strength is sensitive to stress concentration.
A large number of high-strength bolt fatigue failure analysis results show that more than 80% of the fatigue failures are caused by surface damage, decarburization at the head bar intersection, obvious fine cracks in the thread processing, or discontinuities in the cutting of the machining marks and surface corrosion, quenching. The tissue is uneven because of the high concentration of stress there.
Opportunities and challenges in manufacturing high-strength bolts
High-end mechanical equipment brings unprecedented opportunities for high-strength bolt manufacturing. It will be difficult to achieve the expected goals without solving the "three major problems", and it is even harder to say that it has reached the international advanced level. Solving the "three major problems" needs to solve three levels of problems: first, independent intellectual property rights; second, economic affordability; third, long-life, high-reliability, and structural weight-reduction technologies.
First of all, to change the concept, we must recognize the time and space, China must develop high-end machinery manufacturing, master key technologies and enhance core competitiveness, and abandon those flickering words that change the rhetoric of the rhetoric, and become the second-hander of technology in developed countries, market-changing technology and other errors. The concept, from the past "introduction, imitation - backward, re-introduction, imitation...", liberation from paste, copy, and cottage products, speeding up the development of fastener heat treatment is an urgent task.
Secondly, it is a revolution to fully understand and solve the "three major problems" and realize the long life, high reliability and structural weight loss of high-strength bolts: 1 from the "formed" manufacturing of high-strength bolts to the technological revolution of anti-fatigue manufacturing; Solving the "with or without" transformation into a key high-strength bolt to support the sustainable development of high-end machinery manufacturing; 3 from the final inspection of indicators to the management revolution of process control; 4 from low-tech, passive to high-tech, active practitioners Quality revolution; 5 from cheap labor, abuse of the environment and resources to high value-added, humanized, low-emission, environmentally friendly environmental revolution.
Heat treatment imparts ultimate properties to fastener materials
Accelerating the development of heat treatment should become China's national strategy. In the context of major changes in the international environment, starting from the current situation and existing problems of heat treatment in China, the necessity, urgency and importance of accelerating the development of fastener heat treatment have been highlighted in front of us.
"Heat treatment" is a technique that uses heating and cooling methods to control phase transitions, microstructures, residual stress fields, and to impart extreme properties to materials and high strength bolts for serviceability. It is both a theoretically strong science and a very practical type of technology. The practicality is that the advanced heat treatment process is ever-changing with materials and high-strength bolts, and it is constantly innovating, continuously developing and continuously improving. It can be seen that no material can leave the heat treatment, and no high strength bolts do not rely on heat treatment. It is important to note that heat treatment is one of only two techniques that today gives the ultimate performance of materials and gives them the ultimate serviceability of high-strength bolts.
Heat treatment differs from blanking technology and cutting technology. Cold-formed blanks, forged blanks, cast blanks and welded blanks can produce blanks of a certain shape and size, which are conspicuous. The cutting process can produce a variety of precise and beautiful high-precision bolts, which are extremely eye-opening. The heat treatment technology is the "internal medicine" of fastener manufacturing, which gives high-strength bolts performance, neither mountain nor water. For this reason, although heat treatment is many times higher than other technical values ​​in the economically sustainable chain of high-strength bolts, it is not taken seriously. Although heat treatment imparts extreme properties to the material and imparts extreme service performance to high-strength bolts, it is marginalized and subordinated. Under the trampling of formalism and quick success, the core technology of heat-treating this material, high-strength bolts and high-end mechanical equipment is excluded from the key technologies of national science and technology development planning, and it is not supported by key research; The loss and shortage of professionals, the backwardness of technology and backwardness, and the gap with the advanced level of foreign countries have widened and widened. It is no exaggeration to say that the heat treatment is marginalized and subsidiary, which is an important reason for the "three major problems" of high-end mechanical equipment and high-strength bolts in China. It is also an important reason why China's fasteners are in the middle and low-end manufacturing and competitive weakness.
Heat treatment technology promotes the development of high-strength bolts
Advanced manufacturing technology is the technical support for the development of advanced high-strength bolts. Heat treatment technology is an important part of advanced manufacturing technology and plays an important role in the development of high-strength bolts and high-end mechanical equipment. It is well known that no "component" does not become a material, and no "heat treatment" does not become a material. However, the traditional habits often recognize the importance of “ingredients†but do not recognize the important role and status of heat treatment.
Heat treatment is directly related to the material power, and there is no material power without heat treatment innovation. The backwardness of heat treatment technology in China has restricted the development of material power, and the material heat treatment technology is original. It is not known that the lack of attention to heat treatment, lack of innovation and control of the innovative development of advanced materials, without heat treatment innovation, there is no advanced materials.
Facing "Industry 4.0" and "Third Industrial Revolution", intelligent, green, lightweight and heat treatment technologies play an important role in the development of high-strength bolt manufacturing. Development should choose the process that plays a fundamental and breakthrough role in the development of the fastener industry as the key to accelerate development. Emphasis on integrated innovation is the key to improving the core competence of fastener enterprises, and the development of heat treatment will be more personalized.
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