Fasteners are a type of mechanical parts that are used for fastening connections and are widely used. Fasteners are widely used in all walks of life, and various types of fasteners can be seen in various machinery, equipment, vehicles, railways, etc., and are one of the most widely used mechanical basic parts. It is characterized by a wide variety of specifications, different performance uses, and a very high degree of standardization, serialization and generalization. Failure of fasteners can have serious consequences. Therefore, it is necessary to strengthen the analysis of the reasons for the failure of fasteners and find corresponding improvement measures.
1. Surface quenching cracks
Surface quenching cracks refer to cracks generated during the quenching process or in the process of placing at room temperature after quenching, and the latter is also called aging cracks. During the quenching process, when the stress generated by quenching is greater than the strength of the material itself and exceeds the plastic deformation limit, cracks will occur. Quenching cracks often occur soon after the martensitic transformation begins, and the distribution of cracks has no certain rules, but it is generally easy to form at the sharp corners of the workpiece and the sudden change of the cross-section. Quenching cracks caused by too fast cooling in the martensitic transformation zone are often distributed in transgranular, and the cracks are relatively straight and small cracks without branches around them.
Quenching cracks caused by too high quenching heating temperature are distributed along grains, with sharp ends of cracks and overheating characteristics. Coarse acicular martensite can be observed in structural steels, and eutectic can be observed in tool steels. or angular carbide. The surface decarburized high carbon steel workpiece is more likely to form network cracks after quenching. This is because the volume expansion of the surface decarburized layer during quenching and cooling is smaller than that of the non-decarburized core, and the surface material is affected by the expansion of the core. It was torn apart into a mesh. The presence of quenching cracks on the surface can cause sudden fracture of the bolt, and the source of such fractures is located on the surface.
2. Torque overrun
Torque alarms commonly occur during bolt assembly where torque is controlled by the angle method.
The failure modes and causes of fastener torque overrun are as follows:
(1) After the assembly is completed, the final torque of the part is higher than the upper control limit or lower than the lower control limit. The reason is that the assembly torque control range of the parts is unreasonable, which is manifested as the set control range is too small, and the control range is shifted up or down.
(2) There is no preload to the preset angle, and the torque reaches the upper limit and alarms. The reason is that the friction coefficient of the parts itself exceeds the upper limit, the friction coefficient of the fitting of the parts exceeds the upper limit, and the interference between the parts causes the assembly torque to rise sharply.
(3) Normal installation, torque lower limit alarm. The reason is that the friction coefficient of the part itself exceeds the lower limit or the friction coefficient of the fitting of the part exceeds the lower limit.
3. Hydrogen embrittlement
Fasteners are prone to hydrogen embrittlement, and hydrogen embrittlement is the main reason for the fracture of fasteners. Hydrogen embrittlement is the phenomenon when hydrogen atoms enter and diffuse throughout the material matrix. When hydrogen atoms enter the material matrix, lattice distortion occurs in the material matrix, which destroys the original equilibrium state, and is easily cracked by external force. When the external load is applied to the screw, the hydrogen atoms migrate to the highly stress-concentrated area, causing great stress between the crystal boundary edges and leading to the fracture between the crystal grains of the fastener. When the fastener contains hydrogen in a critical state before installation, it will break within 24 hours. It is impossible to predict when the fracture will occur when hydrogen enters the fastener.
4. Improvement measures
4.1. Measures to prevent surface quenching cracks:
(1) Reasonably adjust the gap between the induction hardener and the workpiece, and select the appropriate intermediate frequency power supply parameters and quenching process parameters in strict accordance with the process requirements to ensure that the circumference of the product heats up evenly, and prevent the local temperature from being too high and exceeding the normal quenching temperature.
(2) Improve the structure of the quenching inductor, change the circular section structure of the upper end and the tail end of the inductor to a rectangular section structure, reduce the heating speed of the end and tail inductors, and prevent the end and the tail from heating up quickly, exceeding the process control temperature, Overburning occurs, resulting in cracks.
(3) Reduce the number of magnetic conductors of the quenching inductor in the transition area at the end of quenching, and appropriately reduce the heat there.
(4) The preheating-heating-cooling and quenching method is adopted to make the heating temperature of the product uniform.
(5) Properly extend the cooling time delay after intermediate frequency heating.
(6) Implement self-tempering. In strict accordance with the technical parameters of the process, the pressure, flow rate, temperature and cooling time of the quenching coolant are reasonably controlled. After the liquid spray is stopped, the temperature of the hardened layer is recovered by the residual heat of the workpiece, so as to carry out self-tempering, so as to maintain high surface hardness and good quality. The wear resistance can stabilize the quenched structure in time and reduce the peak tensile stress.
4.2, torque system
The torque control method is to first tighten the bolt to a small torque, generally 40% to 60% of the tightening torque (formulated after the process verification), and then start from this point to tighten a specified angle of control. This method is based on a certain rotation angle, the bolt produces a certain axial extension and the connector is compressed. The purpose of this is to screw the bolts to the close contact surface, and to overcome some uneven factors of surface unevenness, and the axial clamping force required later is generated by the corner. After calculating the rotation angle, the influence of frictional resistance on the axial clamping force no longer exists, so its accuracy is higher than that of the simple torque control method. The point of the torque control method is to measure the starting point of the rotation angle. High tightening accuracy.
4.3 Preventive measures for hydrogen embrittlement
(1) Normal electroplating and strict removal of hydrogen. Using the reversibility of hydrogen in metal to remove hydrogen from electroplated bolts is an important method to slow down or eliminate hydrogen embrittlement. For processing, place the plated steel bolts in an oven to heat. The baking temperature is about 200°C, and the baking time varies according to the strength of the steel. The higher the strength, the longer the baking time. The hydrogen in the bolt material forms hydrogen overflow at high temperature to achieve the purpose of removing hydrogen.
(2) Low hydrogen embrittlement electroplating. Low hydrogen embrittlement electroplating is a process developed for hydrogen embrittlement of aircraft parts in the 1960s and 1970s, including low hydrogen embrittlement cadmium plating, low hydrogen embrittlement cadmium titanium plating, and low hydrogen embrittlement zinc plating. Low hydrogen embrittlement electroplating requires stress relief and tempering before plating. Pickling with strong acid is not allowed, but sandblasting is used to remove scale and surface dirt, or vacuum heat treatment is used to prevent scale. In the electroplating process, on the one hand, the formula of the bath is adjusted, and on the other hand, the current density is strictly controlled by reducing the voltage to reduce the adsorption amount of hydrogen particles. The subsequent process also requires strict baking to remove hydrogen, and the hydrogen removal time should be at least 18h.
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