First, the characteristics of wind power fasteners
Wind power and firmware have a series of technical features: high strength, high precision level, and harsh service conditions. It will withstand the test of severe heat and extreme temperature with the host group, and withstand high temperature and low temperature erosion: high power, up to 6MW unit , large speed difference, vibration, corrosion, heavy load, etc.; in addition to the axial preloading tensile load, it will receive additional tensile alternating load, transverse shear alternating load or The effect of the combined bending load is accompanied by an impact load, and the additional lateral alternating load causes the loose axial axial alternating load of the snail to cause fatigue fracture of the tamping bolt. Under the action of the environmental medium, the axial tensile load causes delayed fracture of the bolt and creep of the bolt under high temperature conditions.
Due to the randomness of the power source, the harshness of the operating environment, the particularity of manufacturing and installation, and the cost of maintenance costs, wind turbines impose extremely high requirements on bolting, and need to proceed from their inherent characteristics. Design, manufacturing processes, shop floor production, and field assembly must take the necessary steps to ensure the reliability of the bolted connections.
Most of the high-strength bolts for wind power use 10.9, and a small amount uses 8.8 and 12.9. The high-strength tightness of wind power is greatly affected by the performance of raw materials. The appearance quality, low-fold structure, decarburization depth weaving (grain size) and upsetting experiments have a significant impact on the quality of high-strength fasteners.
At present, the use of fasteners in wind turbines in China is roughly divided into the following categories:
(1) Tower bolts: bolts used on towers of wind turbines, mainly used for hexagonal steel bolts such as GB/T1228~1231, DIN6914~6916 and DAST;
(2) The whole machine bolt, that is, the bolt used on the wind turbine generator, mainly uses hexagonal head bolts, nuts and washers such as GB/T5782, GB/T5783, GB/T70.1, GB/T6170, GB/T97;
(3) Blade screw: the bolt used to connect the wind turbine blade to the hub, mainly used to customize the map.
Non-standard double studs.
Second, the material requirements
Most of the wind power equipment technology is introduced from Europe. According to the high strength and the same standard, the high-strength tight-fitting parts of wind power are more complicated, and the medium-carbon steel and medium-carbon alloy steel with a carbon-bearing enthalpy of 0 Z5~0.55 are widely used. . List of fasteners used by wind power at home and abroad, see Table 1:
Table 1 List of domestic and foreign brands of high-strength bolt materials for wind power
Under normal circumstances, the wind power nut is 45, 35 steel, some products are designated 35CrMoA steel; the gasket material is 45 steel.
The elements of the materials selected for bolts, screws, studs, nuts and washers are directly related to the mechanical properties of the fastener and should not be less than the mechanical properties of the recommended material. Other inspection items and standards are shown in Table 2:
Table 2 Test items as standards
Third, performance requirements
1. General requirements
GB/T3098.1-2010 "Mechanical performance bolts, screws and studs of fasteners" have specific data for each grade of fasteners. Most wind power bolts use 10.9 grade strength, hardness grade is 32 ~ 39HRC, tensile strength Strength ≥1040Mpa, elongation after breakage ≥9%, shrinkage after breakage ≥48%, low-temperature impact absorption energy Akv (-40～45°C)≥27J, fastener manufacturers need to make bolts, screws and stud materials Manufactured into specimens, in accordance with the experimental items of FFl and FF2 "Standards for bolts, screws or studs with full load capacity" specified in GB/T3098.1-2010 "Mechanical performance bolts, screws and studs for fasteners" Mechanical and physical performance tests, all meet the requirements specified in GB/T3098.1-2010.
In order to meet the requirements of GB/T3101.1-2002B grade products, the straightness error of the wind power bolt is: ≤0.0025XL+0.05 (where L is the nominal length of the bolt), which is generally straightened after heat treatment to reach the standard.
The mechanical properties of the nut shall comply with all the standards specified in GB/T3098.2-2000.
2, bolt mechanical properties
The high-strength bolts for wind power must guarantee the torque coefficient. The average torque coefficient of the same batch of fasteners is 0.11~0.15, and the standard deviation of the torque coefficient should be ≤0.01. The torque coefficient experiment was performed with the preload being guaranteed to be 75% of the yield strength. High-strength bolts for wind power, because the surface is coated with Dacromet, the torque factor is guaranteed by applying Mos2 during installation. If MoS2 is applied to both the thread surface and the gasket, the torque coefficient is generally in the range of 0.08 to 0.12, and the standard deviation of the torque coefficient should be ≤0.01. If the M0S2 is only applied to the surface of the thread, the torque coefficient value will increase slightly. The larger the diameter of the bolt, the more obvious the increase. The test method is carried out in accordance with GB/T50205-2001 "Steel Structure Engineering Construction Quality Inspection and Acceptance Specification". Each bolt connection pair consists of 1 bolt, 1 nut and 2 washers and should be manufactured in the same batch.
The bolts used for the through-hole connections are supplied by the supplier directly to the torque factor after the Dacromet (zinc-chromium coating); the torque factor is supplied by the supplier with the bolts attached.
The torque coefficient of the high-strength bolt connection pair is directly related to the tightening force of the high-strength bolt during the installation of the wind turbine. The mean value of the torque coefficient and the inaccuracy of the standard deviation will directly lead to the over-tightening or under-tightening of the bolting auxiliary force. , has an impact on the quality of the installation.
In the GB/T1231-2006 standard, the experimental method and acceptance of the high-strength large hexagon bolt connection torque coefficient for steel structures are strictly regulated. The GB/T50205-2001 "Steel Structure Engineering Construction Quality Inspection and Acceptance Specification" standard also explains and stipulates the acceptance of high-strength hex bolt connection pairs for steel structures. However, with the expansion of the application range of the high-strength large hex head bolt connection, especially with the increase of the capacity of the wind turbine assembly machine, the importance of the bolt coupling torque coefficient is gradually increased.
Fourth, size and tolerance requirements
The dimensional tolerances and geometric tolerances of fasteners shall be strictly in accordance with the requirements of the corresponding dimensions and geometric tolerances of the grades; straightness and full runout shall be carried out in accordance with GB/T3103.1-2002B, and the remaining unfilled tolerances shall be in accordance with GB/T3103.1- 2002, GB/T3103.3-2000Cc level implementation. The basic dimensions of the bolt and nut thread are in accordance with the provisions of GB/T196-2003 coarse-tooth common thread. The bolt thread tolerance band is 6g before plating according to GB/T197-2003; the 6h level after plating is carried out according to GB/T5267.2-2002. The thread tolerance of the nut is 6G before plating and is carried out according to GB/T197-2003; the 6H after plating is carried out according to GB/T5267.2-2002. The threaded end of the bolt is specified in GB/T5779.1 and GB/T5779.2.
The maximum parameter value Ra of the surface roughness of the thread side should be not less than 3.2 um. Threads must be rolled after heat treatment, and machining is not allowed. The length of the thread must be processed according to the requirements of the purchaser.
V. Quality requirements
The bolted joints shall be surface treated for corrosion protection. The tightness of Dacromet's anti-corrosion is in accordance with GB/T5267.2-2002 or GB/T18684-2002 zinc-chromium coating technical conditions; at least 720 hours of salt spray test. Anti-corrosion treatment must ensure that the mechanical and physical properties of the fastener are not compromised.
Metallographic microstructure examination was carried out according to GB/T13298-1991, quenching martensite about 90%, tempering sorbite 90% tissue detection; according to GB/T3098.1-2010 decarbonization test, low-fold tissue according to GB/T1979 -2001 Loose, segregation defects ≤ 1.5 ~ 2 for testing, random sampling according to the batch number of each batch of 3 pieces.
The surface crack test shall be carried out in accordance with 9.1.b of GB/T4730.4-2005 “The fasteners and shaft parts are not allowed to show any lateral defectsâ€; the ultrasonic inspection test shall be performed in all inspection and acceptance standards in JB/T4730.3-2005. Class I requirements for Ultrasonic Testing and Quality Grading of Bolt Blanks.
The product shall have a completed quality certificate and certificate of conformity. For each specification of M27 and above, each batch must have a high-strength bolt mechanical performance test report issued by the third-party testing organization. The test items shall be in accordance with GB/T3098.1. -2010 implementation.
Sixth, wind power fastener manufacturing process
In addition to the cold heading process, the wind power high-strength fastener manufacturing process includes warm forging, cold extrusion, and cutting. The production process of warm forging bolts is: cold drawing material, warm forging forming, hexagonal shaping, quenching and tempering, processing thread, and surface treatment. High-strength bolts for wind power need to be spheroidized by two heat treatments, fire and quenching, to a strength level of 10.9.
For high strength bolts of class 10.9 and above, the uniformity of the quenched structure is particularly important. In order to ensure the austenitization of the high-strength bolts during quenching, the quenching structure is uniform, and there is no undissolved ferrite and non-martensitic structure. The metallographic analysis of the quenched structure should be fully considered. Foreign high-strength bolt and bolt heat treatment attaches great importance to sufficient austenitization to ensure the uniformity of its structure to obtain the best combination of toughness and to ensure the safety of bolts in service. Domestic high-strength bolt manufacturers have not paid enough attention to this, and the common problem is the unevenness of bolt quenching structure. This unevenness cannot be eliminated in the subsequent tempering process; although the strength and hardness of the bolt can reach the performance of 10.9 grade, due to the poor uniformity of the structure, the bolt contains a region with a large amount of ferrite. Easy to cause early effects. Therefore, the control of the production process should be strengthened in the early heat treatment and quenching and tempering process.
In recent years, the conversion film technology in surface treatment has developed rapidly. On high-strength fasteners, the bolts use more surface treatments such as phosphate (phosphating) or oxidation (blackening), nuts, washers. Phosphorus saponification process is generally used. High-strength fasteners for wind power guarantee a 10-year service life to reduce the risk of hydrogen embrittlement during pickling and plating. The shot peening + SARS contact coating is used to protect outdoor fasteners. The function has the functions of mechanical shielding, self-passivation and surface anti-corrosion of sacrificial anode electrochemical protection. The coating layer should be larger than 8-12 microns, and the salt spray resistance test can reach more than 1000h.
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