How to address the stress relaxation issue of fasteners during long-term use
To address the stress relaxation issue in the long-term use of fasteners, the core approach lies in material optimization, structural design improvement, standardization of installation processes, and enhancement of operation and maintenance management. By doing so, we can reduce the relaxation rate from the source and maintain stable pre-tightening force.
1. Material selection: Improving anti-relaxation performance
Choosing materials with stronger inherent anti-relaxation capabilities is the foundation for solving the problem.
Choose high-strength anti-relaxation material
Priority is given to alloy materials that have undergone special processing, such as high-temperature alloys commonly used in high-temperature environments (such as the Inconel series) and high-strength alloy structural steels for medium and low-temperature environments (such as 42CrMo and 35CrMo).
Avoid using ordinary carbon steel, as its stress relaxation rate is much higher than that of alloy materials under long-term stress or temperature fluctuations.
Optimization of material heat treatment process
Perform quenching and tempering (quenching + high-temperature tempering) on the fasteners to refine the grain size, enhance the yield strength and elastic limit of the material, and delay relaxation.
For fasteners operating under high-temperature conditions, aging strengthening treatment can be applied to further enhance the material's resistance to relaxation stability at high temperatures.
II. Structural Design: Optimizing Stress Distribution and Anti-Loosening
By improving the design of fasteners and connection structures, we can reduce stress concentration and enhance anti-loosening effects.
Optimize the structure of fasteners
Using fine thread, compared to coarse thread, its thread angle is smaller and the thread contact area is larger, which can effectively distribute stress and reduce the risk of loosening.
Elastic components such as spherical washers and Belleville spring washers are selected to compensate for the loss of pre-tightening force caused by relaxation of fasteners, thereby maintaining the tightness of the connection.
Improve the design of connection structure
To avoid stress concentration at connection points, measures such as chamfering bolt holes and optimizing the structure of flanges or connectors can be taken to ensure even distribution of stress.
For critical connections (such as pressure vessels and steam turbines), structures like double nut locking and bolt tensioner pre-tightening are employed to enhance the stability of the pre-tightening force.
III. Installation process: Ensure reasonable and stable pre-tightening force
Standard installation techniques can prevent insufficient or uneven initial pre-tightening force, reducing slack in the later stages.
Accurately control the pre-tightening force
Professional tools such as torque wrenches and hydraulic tensioners are used to replace manual tightening, ensuring that the pre-tightening force of each fastener meets the design requirements, and that the pre-tightening force of fasteners at the same connection point is uniform.
Avoid overtightening, as it can lead to plastic deformation of the fasteners, which in turn accelerates stress relaxation; meanwhile, avoid insufficient preload, as it may fail to meet the connection strength requirements.
Control the installation environment and sequence
When installing in high or low temperature environments, it is necessary to consider the impact of temperature on materials and adjust the pre-tightening force appropriately (for example, in high temperature environments, the pre-tightening force can be appropriately increased to compensate for later high temperature relaxation).
For multi-bolt connections (such as flanges), strictly follow the symmetrical and step-by-step tightening sequence to avoid excessive local stress, which can lead to connection deformation and stress relaxation.
IV. Operation and maintenance management: Regular monitoring and maintenance
During long-term use, timely compensation for preload loss can be achieved through regular inspection and maintenance.
Regular inspection and retightening
Based on the operating conditions of the equipment (such as temperature, vibration, and corrosivity of the medium), a regular inspection plan should be formulated. Torque wrenches or ultrasonic preload detectors should be used to monitor changes in the preload of fasteners.
When the pre-tightening force is found to be insufficient, timely re-tightening should be carried out to avoid problems such as connection loosening and leakage caused by slackness.
Ensure proper corrosion prevention and protection
The surface of fasteners is treated with galvanizing, chromium plating, Dacromet coating, or applied with anti-rust grease to prevent corrosion, which can cause thread jamming or material performance degradation, indirectly reducing stress relaxation.
For fasteners exposed to harsh environments (such as moisture and corrosive media), regularly clean the surface dirt and corrosion products to extend their service life.