Table 1 High-strength bolt materials for wind power, domestic and foreign brands, performance specifications, standard number, remarks, EU grades, Japanese grades, US grades, S-class Chinese grades (borrowed), Chinese grades, high-strength bolts, high-strength bolts in wind power equipment, mostly medium-carbon alloy steel, not only It is required to have high strength while still having sufficient toughness and plasticity.
Practice has proved that the bolt can have high strength, sufficient toughness and good fatigue resistance after quenching and tempering heat treatment. If the martensite is not fully quenched during quenching to obtain some non-martensitic structure, the strength, toughness and fatigue properties of the bolt will be reduced, especially when the ferrite is distributed along the grain boundary. The degree is even more serious. For this reason, for high-strength bolt quenching and quenching, it is required to obtain martensite over 90% of the entire section.
The main alloying elements contained in the medium carbon alloy steel are C', Ni, M+, M, and the like. Their main role is to increase the hardenability of medium carbon alloy steel, to ensure the martensite is obtained by quenching, and to have sufficient tempering stability when tempering at high temperature, and the tempered sorbite structure is strengthened after tempering. In fact, most of these elements are soluble in ferrite, which strengthens the ferrite. The content of such elements in the medium carbon alloy steel ensures that the ferrite is strengthened without significantly reducing its toughness, and even some can simultaneously improve its impact toughness.
The rest are alloying elements such as M, -, B, etc., and their content in medium carbon alloy steel is generally small, especially the content of B is extremely small, but the hardenability can be remarkably increased.
The depth of the hardened layer is not only related to the steel composition, but also affected by the bolt section size, heating temperature, cooling medium and cooling method. Many influencing factors must be paid attention to when formulating the heat treatment process.
Medium carbon alloy steel for high-strength bolts for wind power (see Table 2 for chemical composition).
In Table 2, the chemical composition (mass fraction) of steel alloy steel, such as steel grade, the critical diameter of oil quenching is not more than 25mm, and the B content obviously improves the hardenability. It is mainly used for high strength bolts of steel structure and high strength of 10.9 grade for small power wind turbines. bolt.
Chromium and molybdenum in 35CrM steel have the effect of improving the hardenability of steel and the strength of steel, and at the same time have the effect of increasing steel tempering stability and eliminating temper brittleness. The critical hardenability diameter of 35CrMo steel is 25mm in oil. When the size is large, the core cannot be completely hardened, and non-martensitic structures such as ferrite and bainite are precipitated on the grain boundary. The overall performance of the department is worse than the surface.
Grade 9 high-strength bolt steel, which has good comprehensive performance, high strength and sufficient toughness in quenched and tempered state, hardenability is not high, and can be hardened by 25mm diameter during oil quenching.
B7 steel is the standard grade of American fasteners. Its composition is similar to 42CrM steel. The carbon content and chromium content of 42CrMo steel are higher than that of 35CrMo steel, and the hardenability and strength are improved. It can be used for large-section high-strength fasteners. The critical diameter of the quenching in oil is 42mm. The chromium and molybdenum in 40CrNiMA steel is a strong forming element of carbide. Nickel can improve the strength of steel and improve its plasticity and toughness. Nickel does not form carbides in steel. It is soluble in austenite and ferrite, plays a role in refining grains, strengthening ferrite, improving toughness, especially improving low-temperature impact energy absorption; at the same time, it can increase the hardenability of steel, which is a mechanical property. High-strength bolts with large cross-sections that require high and uniform wind are particularly useful.
The steel has a critical diameter of 45 mm or more in the oil. In addition, the second type of temper brittleness of the steel is particularly sensitive. When 450~550> tempering, the impact energy of fast cooling and slow cooling is 5-7 times different, so it should be cold when tempered at high temperature.
34CrNi3M steel is an alloy steel for large-section quenching and tempering. The critical diameter of oil quenching can reach 805100mm. The hot-worked g steel has been eliminated in China's alloy steel material standard, but its hardenability is unmatched by other steel grades. The market has demand.
High-strength bolts require more than 90% of the entire section to obtain martensite structure during quenching and quenching. The high-strength bolts of wind power are larger than 30mm, and the quenching medium has the greatest influence. When the oil quenching can not meet the mechanical performance requirements, water quenching must be adopted. The principle is to use an aqueous solution as much as possible without quenching. Because water quenching has many advantages, such as deep hardened layer, good mechanical properties, and low production cost.
Impact toughness and hardenability, when the diameter exceeds 45mm, an aqueous solution must be used to ensure the depth of the hardened layer.
According to the critical point of different steel grades to determine the heating temperature (generally around 8503), then quenching + high temperature tempering, high temperature tempering between 500 ~ 6003 (tempering treatment), so that 10.9 wind power high-strength bolts have a good synthesis Mechanical properties. The critical point temperature and heat treatment process of medium carbon alloy steel are shown in Table 3.
Table 3 Carbon steel steel heat treatment process Steel number critical point temperature / 3 heat treatment process 5. Ultra-fine grain steel and new material high-strength bolts are notched parts, have high notch sensitivity, easy to locate at the notch A delayed fracture occurs at the transition or the root of the thread with the head.
The weakness of wind power high-strength bolts is that they are sensitive to delayed fractures in the natural environment. Delayed fracture is usually accompanied by large plastic deformation at normal temperature, and occurs when the yield strength is much lower, which is often difficult to detect and becomes a serious safety hazard.
One of the specific measures for resisting delayed fracture is to refine the crystal grains. For this reason, ultrafine grain steels have been extensively developed at home and abroad. Such as: ADS series of Sumitomo Metal Co., Ltd., KNDS series of Kobe Steel, ADF steel of China Iron and Steel Research Institute. Under the guidance of the Research Institute of Steel Research Institute, Dalian Steel Plant succeeded in developing the content of S, P, Si and Mn, adding the microalloying element Nb and increasing the M- content on the basis of 42C+M-steel. 1300MPD grade high strength bolt steel), its comprehensive mechanical properties have been greatly improved, on the Cummins engine, for the 12.9 cylinder head bolts, has good fatigue performance, the delay fracture test in the natural environment is good, No break was found. The finished 13.9 bolt replaces the 10.9 bolt made of 42C+M-steel on the suspension bracket of the Iveco engine. It passes the 15,000 km reliability road test twice, breaking the international limit of only 12.9 bolts. .
In recent years, a large number of fasteners for steel structures have been used. The tensile strength exceeds 1200MPd. The foreigner uses KDS2 steel (instead of SCM440 steel) to manufacture 12.9 bolts. The delayed fracture has been improved significantly. China uses 45C+NiMTi in the 1500MPd grade steel structure connection. Used above, its performance is superior to tempered martensite (chromium-molybdenum series steel) high-strength fasteners.
At present, U20Si ultra-high strength steel (carb-free bainite/martensitic duplex steel) and domestically developed annealed cold-rolled steel bar or simplified annealing wire have been developed abroad, which have been used in wind power fasteners. It is used in large quantities. 6. Conclusion It is well known that the damage of bolts is mostly fatigue damage, and the bolts are parts with multiple gaps and easy stress concentration. The strength of the bolts is required to be too high, that is, the resistance to static pull is improved, and the dynamic fatigue strength is not The tensile strength increases and increases. For this reason, the tensile strength +m of the high-strength bolt of the 10.9 wind turbine is controlled at 1050-1130MPd during heat treatment, and the thread is formed by rolling after heat treatment, so that a large residual compressive stress is generated inside the bolt, thereby slowing the formation of cracks. Therefore, the fatigue strength of the bolt can be greatly improved.
High-strength bolts for wind power are used as mechanical basic parts. Nowadays, all countries in the world are paying attention to strengthening technology. For fasteners, steels are required to have higher strength, anti-delay damage resistance, corrosion resistance and processing performance.
In recent years, with the technological advancement of China's iron and steel enterprises, the high-strength, high-heat-resistance, high-formed fastener bar wire has been introduced in large quantities, which can meet the requirements of the mechanical properties of high-strength bolts for wind power. Support the localization of wind turbine generator sets and components. MW
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