Fasteners are a type of mechanical parts widely used for fastening connections. They are extensively applied in various industries, including machinery, equipment, vehicles, railways and other fields. Various types of fasteners can be seen everywhere, making them one of the most widely used basic mechanical parts. They are characterized by a wide variety of specifications, different performance and uses, and a very high degree of standardization, serialization and generalization. Once fasteners fail, they will cause serious impacts. Therefore, it is necessary to strengthen the analysis of the causes of fastener failure and find corresponding improvement measures. Combined with the relevant knowledge of fasteners, the details are shared as follows:
1. Surface Quenching Cracks
Surface quenching cracks refer to cracks generated during the quenching process or during storage at room temperature after quenching; 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 be generated. Quenching cracks usually occur shortly after the start of martensitic transformation. The distribution of cracks has no fixed pattern, but they are generally prone to form at the sharp corners and sudden section changes of the workpiece. Quenching cracks caused by excessive cooling rate in the martensitic transformation zone are mostly transgranularly distributed, with straight cracks and no small branch cracks around them.
Quenching cracks caused by excessively high quenching heating temperature are all intergranularly distributed, with sharp and thin crack ends and overheating characteristics; coarse acicular martensite can be observed in structural steel, and eutectic or angular carbides can be observed in tool steel. High-carbon steel workpieces with surface decarburization are more likely to form reticular 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 pulled and cracked by the expansion of the core to form reticular cracks. The presence of surface quenching cracks will lead to sudden fracture of the bolt, and the fracture source of such fracture is located on the surface.
2. Torque Overlimit
Torque alarm is common in the bolt assembly process using the angle method to control torque.
The failure modes and causes of fastener torque overlimit are as follows:
(1) After assembly, 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 part is unreasonable, which is specifically manifested in the set control range being too small, or the control range shifting upward or downward.
(2) The torque reaches the upper limit and alarms before pre-tightening to the preset angle. The reason is that the friction coefficient of the part itself exceeds the upper limit, the friction coefficient of the part fit exceeds the upper limit, or there is interference between parts, resulting in a sharp rise in assembly torque.
(3) Under normal installation conditions, a torque lower limit alarm occurs. The reason is that the friction coefficient of the part itself exceeds the lower limit or the friction coefficient of the part fit exceeds the lower limit, and the fitting torque when the part is screwed in is greater than the initial torque (that is, excessive consumption of screwing torque), which is common in the tightening process of lock nuts.
3. Hydrogen Embrittlement
Fasteners are prone to hydrogen embrittlement, which is one of the main causes of fastener fracture. Hydrogen embrittlement is a phenomenon where hydrogen atoms enter and diffuse into the entire material matrix. When hydrogen atoms enter the material matrix, they cause lattice distortion of the material matrix, destroy the original equilibrium state, and make the material prone to cracking when subjected to external forces. When an external load is applied to the screw, hydrogen atoms migrate to the region of high stress concentration, generating great stress between the edges of the crystal boundaries, leading to intergranular fracture of the fastener. If the fastener contains hydrogen in a critical state before installation, it will usually fracture within 24 hours; once hydrogen enters the fastener, the fracture time cannot be predicted.
4. Improvement Measures
4.1 Measures to Prevent Surface Quenching Cracks:
(1) Reasonably adjust the gap between the induction quencher and the workpiece, strictly select appropriate intermediate frequency power supply parameters and quenching process parameters in accordance with process requirements, ensure uniform circumferential heating of the product, and prevent local temperature from exceeding the normal quenching temperature.
(2) Improve the structure of the quenching inductor, change the circular cross-sectional structure at the upper end and tail end of the inductor to a rectangular cross-sectional structure, reduce the heating speed of the inductor at the end and tail, and prevent the end and tail from heating too fast, exceeding the process control temperature and causing overheating, thereby generating cracks.
(3) Reduce the number of magnetic conductors of the quenching inductor in the transition zone at the end of quenching, and appropriately reduce the heat input in this area.
Adopt the "preheating-heating-cooling" quenching method to ensure uniform heating temperature of the product.
Appropriately extend the delayed cooling time after intermediate frequency heating.
Implement the self-tempering process. Strictly control the pressure, flow rate, temperature and cooling time of the quenching coolant in accordance with the process technical parameters; after stopping the liquid spraying, use the residual heat of the workpiece to raise the temperature of the hardened layer for self-tempering treatment, so as to maintain high surface hardness and good wear resistance, stabilize the quenched structure in time, and reduce the peak tensile stress.
4.2 Improvement Measures for Torque Control
Adopt the torque-angle control method: first screw the bolt to a small torque (usually 40%~60% of the tightening torque, determined after process verification), then start from this torque point and screw to the specified angle. This method is based on a certain angle, causing the bolt to produce a certain axial elongation and the connecting part to be compressed. Its purpose is to screw the bolt to the tight contact surface, overcome uneven factors such as surface unevenness, and the subsequent required axial clamping force is generated by the angle. After determining the angle, the influence of frictional resistance on the axial clamping force can be ignored, so its accuracy is higher than that of the simple torque control method. The key of the torque-angle control method is to determine the starting point of the angle; once the starting point of the angle is determined, high tightening accuracy can be obtained.
4.3 Measures to Prevent Hydrogen Embrittlement
(1) Standardize the electroplating process and strictly implement dehydrogenation treatment. Using the reversibility of hydrogen in metals to perform dehydrogenation treatment on electroplated bolts is an important method to reduce or eliminate hydrogen embrittlement. During treatment, put the electroplated steel bolts into an oven for heating, the baking temperature is about 200℃, and the baking time is adjusted according to the strength of the steel-the higher the strength, the longer the baking time. Hydrogen in the bolt material forms hydrogen gas and overflows at high temperature, thus achieving the purpose of dehydrogenation.
(2) Adopt the low-hydrogen embrittlement electroplating process. Low-hydrogen embrittlement electroplating is a process developed in the 1960s and 1970s for the study of hydrogen embrittlement of aircraft parts, including low-hydrogen embrittlement cadmium plating, low-hydrogen embrittlement cadmium-titanium plating, low-hydrogen embrittlement zinc plating, etc. Low-hydrogen embrittlement electroplating requires stress relief tempering before plating, and strong acid pickling is not allowed; sandblasting should be used to remove oxide scale and surface contaminants, or vacuum heat treatment should be used to avoid generating oxide scale. During the electroplating process, on the one hand, adjust the plating solution formula, and on the other hand, reduce the adsorption of hydrogen particles by reducing the voltage and strictly controlling the current density. The subsequent process still needs to strictly implement baking dehydrogenation, and the dehydrogenation time is not less than 18 hours.
