According to statistics, there are about 240 commonly used standards for automotive standard parts products that have been released and implemented, covering lumen sealing elements, pipe line connection fasteners, washers, screws, nuts, bolts, etc., of which 115 standards are related to metal fasteners, accounting for about 48%. With the continuous development of the automobile industry, in order to curb the increase in production and management costs caused by the increasing number of metal fasteners (hereinafter referred to as automobile fasteners), various automobile manufacturers have compared and optimized fasteners from four aspects: structural elements, materials, heat treatment and surface treatment. This paper discusses this.
1.1 bolts
For the hexagon bolt + spring washer assembly, when the bolt preload is low, the anti loosening effect is better. However, since the spring washer is not used for important parts, the bolts with spring washers are mainly tightened with pneumatic wrenches with an accuracy of about ± 40% in production. The assembly torque and axial force dispersion are large. The spring washer is often in a flattened state, and even has the risk of expanding the ring. The actual anti loosening performance of the bolt + spring washer assembly is uncontrollable, Can not meet the product design requirements. For hexagon flange bolts, there are the following advantages.
① Under the same thread specification, the support area of hexagon flange bolt is larger than that of hexagon head bolt, which can better disperse the pressure on the support surface and avoid deformation of the connected parts;
② Under the same friction coefficient, the anti loosing effect of flange bolt is obviously better than that of hexagon bolt;
③ In order to avoid the friction between the sleeve end and the connected part damaging the surface of the connected part, the flange bolt is more economical than the hexagon bolt assembly with flat washer.
Hexagon flange bolts are preferred, and hexagon head bolts, hexagon head bolts + spring washers, hexagon head bolts + spring washers + flat washers, and hexagon head bolts + flat washers are restricted.
1.2 screws
The screwing form of the screw is internal screwing. The driving forms include internal hexagon, cross groove and internal hexagon. The head types include round head, cylindrical head flange, flat round head, flat round head flange, pan head, pan head flange, countersunk head and semi countersunk head.
As the requirements for assembly efficiency are also constantly improving, hexagon Torx screws are preferred, standard structures are optimized, and the use of hexagon socket screws and cross recessed screws is gradually limited.
1.3 nuts
The use effect of hexagon flange nut is the same as that of hexagon flange bolt. If the structure allows, hexagon flange nut is preferred. For parts with special anti loosening requirements, effective torque lock nuts, such as all metal lock nuts and non-metal insert lock nuts, shall be considered. Because the all metal lock nut is locked by deformation of the thread, it is not suitable for the parts that are often disassembled; The non-metallic insert lock nut has good reusability, but according to the standard requirements of fasteners, except for the engine, its application temperature is ≤ 120 ℃. In particular, it should be noted that the effective torque type lock nut needs to overcome the additional torque caused by nut deformation or non-metallic inserts during installation, so the torque needs to be confirmed. The clamping force may not be enough when assembled according to the torque value of the ordinary nut, and there is a risk of use.
1.4 thread
Because the bearing capacity and anti loosening capacity of fine thread are higher than that of coarse thread, fine thread should be selected as much as possible when selecting larger threaded fasteners, and the variety of threaded fasteners can also be reduced. It can be seen from table 1 that generally, there are only coarse teeth below M12 and fine teeth above M12. In the threaded fasteners of commercial vehicles, coarse and fine teeth coexist above M12, and there is still room for optimization.
1.5 washer
In order to improve assembly efficiency and reduce the risk of missing and wrong assembly, gaskets are not allowed to exist alone in principle. Suggestions for the use of various gaskets are as follows.
① The flat washer is mainly used to improve the contact state, increase the bearing area and maintain the stability of the friction coefficient of the support surface; ② The spring washer uses elasticity to generate axial preload, which can alleviate the attenuation of axial force. However, since the anti loosening performance is difficult to be effectively controlled, the bolt is easy to bear eccentric load and has the risk of damage; ③ The tooth shaped elastic washer has twisted teeth and has high hardness after heat treatment. During assembly, the teeth will be elastically deformed and partially embedded into the support surface to form a locking effect. The tooth shaped washer should be used with caution at the connection part.
surface treatment
Automotive fasteners include bolts, nuts and washers, most of which must undergo surface treatment to protect them from corrosion, improve their appearance or achieve certain special functions, such as screw and mosquito fasteners that control locking torque. For example, see Table 2 for the service environment and corrosion resistance requirements of a domestic automobile fastener.
2.1 electro galvanizing
The best anticorrosion performance is zinc yellow passivation, followed by zinc green passivation, zinc black passivation and zinc blue white passivation. The corrosion resistance of the general coating is 8 μ m. Yellow passivation white rust time 72h, red rust time 144H; Black and white passivation white rust time 6h, red rust time 72h.
The following three aspects need to be paid attention to in practical application. With the gradual tightening of environmental protection, the use of trivalent chromium passivation, zinc aluminum coating and other more environmentally friendly methods for automotive fasteners is the trend in the future; Automotive fasteners with maximum tensile strength greater than 1000MPa (equivalent to hardness value of 33.5 HRC and 332 HV) shall be subject to hydrogen drive treatment after plating before passivation to reduce the risk of delayed fracture; If chromate passivation film is exposed to the environment above 70 ℃ for a long time, its corrosion resistance will be damaged. Therefore, for areas with high ambient temperature, zinc passivation should be used with caution.
2.2 zinc aluminum coating
The zinc aluminum coating has no hydrogen embrittlement and meets the requirements of environmental protection. The red rust time of neutral salt spray test can reach 720h. The coating colors are black and gray. Adding lubricant to the coating liquid can change the friction coefficient. Bolts of grade 10.9 and above are preferred. In addition, the following aspects should also be considered when using. The adhesion strength between the zinc aluminum coating and the substrate is not as strong as that of zinc plating, and there is powder falling during use. Therefore, it cannot be used inside the transmission parts, and it is not recommended to use the bolts that need to be repeatedly disassembled. In addition, for large-sized bolts and nuts, barrel plating is easy to produce scratches and bumps, reducing the corrosion resistance, which should be considered when selecting; For fasteners with conductivity requirements and fasteners with external thread nominal diameter less than M6 and internal thread less than M10, zinc aluminum coating should not be used to ensure screwing and normal assembly.
2.3 zinc nickel alloy
Compared with zinc plating, the corrosion resistance of zinc nickel alloy has been greatly improved, and the same plating 8 μ After passivation and sealing treatment, the surface can be free of white rust for 240h and red rust for 1000h; In addition, it also meets the requirements of high temperature resistance. Since zinc nickel alloy still has a slight tendency of hydrogen embrittlement, in order to reduce the quality risk of automobile fasteners with tensile strength greater than 1000MPa, necessary verification must be carried out before use.
2.4 copper plating
The melting point of copper is about 1083 ℃. In high-temperature environment, in order to avoid sintering of threaded parts, copper plating is selected for surface treatment, especially for automobile fasteners around the engine exhaust manifold.
Materials and heat treatment
Automobile high-strength bolts generally refer to products of grade 8.8 or above, which are not only required to have high tensile strength and yield ratio, but also have high low-temperature impact performance. One of the difficulties in manufacturing is also the quenching and tempering treatment of high-strength bolts. Swrch35k, 10B21, 10b33, 35CrMo, 42CrMo or 20MnTiB steel are selected as the materials, see Table 3 for details. As we all know, the mechanical performance test results of high-strength fasteners are not only the key product quality indicators, but also important indicators related to safety. The main problem of swrch35k and 10B21 steel is poor hardenability. Effective control of the quenching and tempering process of high-strength bolts plays a vital role in mechanical properties.
In order to improve the heat treatment quality of automobile high-strength bolts, the steel should be controlled from the following three aspects. ① Controlling the carbon content in the middle and upper limits can not only improve the strength and toughness of steel, but also reduce the tendency of segregation. ② Controlling the alloying element to the upper limit can increase the hardenability and improve the strength and toughness of the steel. ③ Minimize the content of harmful residual elements P and s to ensure the purity of steel. Automobile fastener grade and material.
It is one of the difficulties in quality control that the changes of internal structure and properties of bolts during quenching and tempering cannot be monitored in real time. Before loading, carefully check the mark on the bolt head to ensure that the information of the bolts to be processed is accurate, not lost and identifiable after heat treatment. The quenching and heating process shall be strictly controlled, the carbon potential shall be accurate, and the quenching time of each batch of bolts shall be recorded. After the quenching medium is discharged, the surface hardness of the workpiece shall be tested. 10B21 and 20MnTiB steel shall be greater than 43hrc; Swrch35k, 45 and 10b33 steel shall be more than 48hrc. The microstructure after quenching is fine needle martensite, which is evaluated according to JB / t9211-2008 martensite grade of medium carbon steel and medium carbon alloy structural steel. The quenching martensite is grade 3-5, meeting the technical requirements; The uniformity of quenching hardness of surface and core shall not be greater than 3HRC.