Bolt fracture, possible cause analysis

1 Background

Fascination is one of the most widely used basic parts, and its reliability is closely related to the safe and reliable operation of the entire equipment or structure. According to the statistical analysis results of the failure case of fasteners in many years, it can be seen that the form and reasons for the failure of the fastener part of the fastener can be seen that the break accident caused by the inadequate control of the manufacturing process is mainly due to the failure and bolt caused by non -metal mixed materials. Cold R -angle folding, design or manufacturing of the head structure of the head is unreasonable, thermal treatment of carbon increase and decarburization, the failure of the phosphorus -rich layer, the threading teeth failure, delayed quenching, insufficient quality and quenching The failure caused by quenching cracks, large tissue, lack of heat treatment, failure, fatigue failure, etc.

Today, the screwdriver will discuss several reasons for the invalidation accident of wind power and nuclear power sets. According to the analysis of calculation and pre -tightening settings based on the bolt connection strength, the pre -tightening force does not meet the requirements. Poor corrosion and lubrication, installation process and design defects, material defects, etc. can also cause bolt breaks.

 2 The pre -tightening force does not meet the requirements

The pre -tightening force does not meet the requirements is the most important cause of bolt break. During the on -site installation and later technical maintenance of the bolt, it is necessary to operate according to the prescribed torque value to prevent problems such as excessive torque or insufficient torque. Excessive pre -tightening force may cause bolt stretch stress to exceed the limit of the bolt material's yield strength limit, and produces plastic deformation or even break. If the pre -tightening force is too small, the load load amplitude of the bolt fatigue load will be increased, and the friction between the bolt and the connector will be reduced, so that the bolt connection is unlocked to the design requirements. A relative movement is generated, so that the bolt is to withstand complex alternating loads such as additional bending moment, stretching, and shearing, which exacerbates the failure of the bolt. The bolt loosening also increases the fatigue load of the bolt and reduce the life of the bolt.

The main manifestations of the pre -tightening force do not meet the requirements of high -strength bolts are:

1. When installing a high -strength bolt, the "ten" word cross -fastening is not performed as required, causing the existence of a gap between connecting the flange surface and causing the bolt stress value to be too large, and finally the bolt breaks;

    

    

    

   (2)High -strength bolts produce neck shrinking due to excessive pre -tightening force, and cracks produced by the threaded roots of the roots of the thread, causing the bolt to break;

    

    

    

   (3) The insufficient pre -tightening force of the high -strength bolt causes the bolt loosening, resulting in interference with the flanged pore diameter and bolt rod.

    

   For example, there is a broken problem with the connection bolt of a unit, and there are repeated breaks in some units of bolts. The initial bolt suppliers reduce the occurrence of bolt breaks by replacing the form of thickened pads. Although the thickness of the cushion has a certain relief effect on the bolt break, it has not completely solved the problem of bolt fracture. Based on the analysis of the chemical composition of the fracture bolt failure reporting bolt bolt bolt, the results of stretching performance tests, impact tough tests, hardness tests, and gold phase tests meet the requirements of GB/T3098.1-2010.

    

   Therefore, it can be judged that the design and materials of the bolt can meet the on -site operating needs. The cause of the break is caused by stress concentration during operation. In order to effectively solve the problem of bolt breaks, the bolt supplier and the demand party change the design, and use the characteristics of the characteristics to ensure the pre -tightening force of the bolt and enhance the intensity of the bolt to ensure the safe operation of the unit.

    

   3  Poor corrosion and lubrication

   Poor bolts and poor lubrication can cause the bolt connection part to do not meet the requirements of the torque coefficient, resulting in a deviation of the bolt torque coefficient, which affects the insufficient pre -tightening force of the bolt, and eventually causes the bolt to crack or break. During the assembly of high -strength bolts, effective lubrication will fill the gap between contact points, reduce contact between metals and metals, prevent wear or stuck, and play a smooth assembly.

    

   Due to the processing accuracy, it is impossible for 100%contact between the two metal surfaces and the thread surface; especially the thread surface, only about 15%to 20%of contact due to the existence of the threaded corner. When a high -load torque is applied to the bolt, the high contact between the two thread surfaces occurs, and the elastic variable will occur until the pressure that can be transformed by the torque can be carried. If there is no lubricating surface, the surface is more prone to bruises. At a specific load or temperature, the metal may bite, causing killing during the assembly or disassembly process. In order to prevent this phenomenon from failing, the two contact surfaces need to be separated and ensure that lubrication and anticorrosion are effective during the installation process.

    

   When the high -strength bolt rust is rust, the surface layer peels off. When more surface areas are sent to the corrosion, the metal substance, oxygen, water and carbon dioxide consumption will be accelerated, and the material will be completely dissolved. The rust of the bolt will not only affect the material itself, but also affect the friction connection performance and torque coefficient of high -strength bolts. The degeneration of connection performance will further cause the pre -tightening force of the high -strength bolt to reduce the connection.

    

   Research shows:

   As the degree of corrosion increases, the anti -slip coefficient of the high -strength bolt connection test parts is within the range of the corrosion rate ηs <4.5 %, which increases as the rust rate increases, and the test parts sliding the load of the test parts in different corrosion time Also improve accordingly. The anti -corrosion coating of high -intensity bolts can prevent the surface from rusting. Therefore, when installing high -strength bolts, the appropriate anticorrosive coating material should be selected according to the local temperature and humidity characteristics and the unit model and the anti -corrosion coating is seriously applied according to the construction process.

    

   4 Installation process and design defects

   For example, the connection between the unit tower and the wheel hub is stitching on the spot. The installation process is difficult to achieve a unified standard. When the high -strength bolt is installed, it is not carried out according to the operation guidance letter, which will cause the bolt rod and the bearing hole wall to fit the overlapping or bolt loosening. It can also cause high -strength bolts to break. This is due to the abnormal stress and alternative load caused by the rotation of the unit. When the bolt is under the action of the cyclic cross -alternating transformation for a long time, the thread at the thread of the nut fastener surface is prone to cracks, which eventually leads to fatigue breaks.

    

   If there is a problem of repeated breaks in the bolt of a wind turbine, the problem of breaking after the manufacturer has replaced the bolt has not been resolved. After testing the bolts and load testing of the bolt, it was found that its bolt was too large, and further traceable on the spot found that during the installation of the crew during the on -site installation process, there was a dislocation and interference in the bolt and the bearing hole wall. It will cause the bolt stress amplitude to increase fatigue life, and dislocation interference will cause bolts to withstand additional bending stress and cause blade bolt to break.

    

   5 Materials and manufacturing defects

   Materials and manufacturing defects are mainly manifested as: unqualified mechanical properties of bolt, unreasonable thermal treatment system, unreasonable processing process, unreasonable processing process, non -chemical component analysis, and surface decarburization of the heat treatment system. In particular, inappropriate thermal treatment equipment or processes will lead to the irresistible and unstable of the quenching process, affect the quality of the bolts, cause instability among the individual products, and even the uniform organization of different parts of the same product. At this time, even if the performance indicators of the bolt meet the requirements, the product is still the possibility of potential failure.

    

   Bolt breaks caused by material defects require failure analysis. For example, a certain wind power high -strength bolt breaks are broken at the thread. After the analysis of metal scanning electron microscopy, it was found that the sample break source area was broken along the crystal, and the section had slender cracks. There were more tough nests in the expansion area.

    

   Generally, factors such as pickled treatment, electroplating or hydrogen repellent, hydrogen and brimming have become one of the main reasons for the failure of bolts. During the occurrence of hydrogen, hydrogen and gruits are sudden and harmful, and the hydrogen and brimming are irreversible after the occurrence. The mass score of hydrogen in the steel generally generates hydrogen cracks when it reaches 5 × 10−6, while high -strength bolts will also occur if the hydrogen mass score is lower than 5 × 10−6. That is, under the action of stress, the hydrogen atom that is in the dotted gap spreads and concentrated in the stress concentration area generated at the gap, prompting the interaction between the hydrogen atom and the bit wrong, so that the position of the wrong line is nailed, thereby causing the substrate to make the substrate The toughness becomes low, the brittleness intensifies, and the fracture stress of the material decreases. Because the hydrogen atoms that are adsorbed on the steel surface during oil removal, electroplating, or pickling are saturated, hydrogen penetrates into the steel.

    

   Therefore, it is recommended to shorten the pickling time during pickling to avoid excessive acid washing, and the corrosion agent should be added during the picking process. Protecting the surface of the bolt before galvanizing and galvanizing hydrogenation treatment before galvanizing.

    

   Screwdriver experience and summary

   L.S.Engineer Experie and Summary

   With the frequency of bolt fracture, it is necessary to further standardize the fault analysis process and do the operation and maintenance work of the unit. When the bolt accident occurs, the breeding of the bolt breaks is collected, including the broken position, quantity, number of fractures, the shape of the bolt fracture, screw wear marks, etc.; Whether the bolt is loosened at the time of qualification and maintenance; pay attention to whether the anti -loose label of the bolt is misplaced during on -site inspections, whether the unit has abnormal vibration, and pay attention to whether there are abnormal conditions in the flange surface of the fracture bolt. wait.

    

   In short, there may be a risk of failure during the entire life cycle of fasteners. The quality management of fasteners is to learn from the experience of failure cases, and find potential to find potential failure from the design, materials, assembly and maintenance of fasteners. The factors of reducing the incidence of failure are of real significance for improving the level of manufacturing and application of fasteners in my country, and even the reliability of machinery and equipment.