Stress Relaxation Suppression Technology and Long-Term Fastening Performance Guarantee of Elastic Clips
What is the generation mechanism of stress relaxation of elastic bars?
The generation mechanism of stress relaxation of elastic bars is that under the action of long-term constant strain, the microstructure inside the elastic bars undergoes slow changes, leading to the gradual attenuation of elastic stress. After installation, the elastic bar is in a state of continuous elastic deformation, with high residual stress inside. Under the dual effects of train vibration load and ambient temperature changes, the microscopic grains will undergo slow slip and dislocation movement. This microscopic movement gradually transforms the elastic deformation of the elastic bar into plastic deformation, and the elastic stress decreases accordingly, which is manifested as the continuous attenuation of clamping force. The rate of stress relaxation is closely related to temperature: for every 10℃ increase in temperature, the relaxation rate increases by 1-2 times, so the problem of elastic bar relaxation is more prominent in high-temperature areas. In addition, impurity elements (such as sulfur and phosphorus) in the elastic bar material will accelerate grain slip and further increase the relaxation rate, which is also an important reason for the high relaxation rate of ordinary spring steel elastic bars. When stress relaxation develops to a certain extent, the clamping force of the elastic bar will be lower than the design value, which cannot effectively restrain the rail and cause potential line safety hazards.
What are the core material formula optimization measures for stress relaxation inhibition of elastic bars?
The core material formula optimization measures for stress relaxation inhibition of elastic bars are using low-alloy spring steel and precisely controlling the content of alloy elements to improve the anti-relaxation performance of the material. The base material selects 60Si2CrVA low-alloy spring steel. Compared with ordinary 60Si2Mn steel, this material adds chromium (Cr) and vanadium (V) alloy elements. Chromium can refine grains, improve the yield strength and toughness of the material, and reduce the possibility of grain slip; vanadium can form stable carbides, pin grain boundaries, hinder dislocation movement, and significantly reduce the stress relaxation rate. The content of alloy elements must be strictly controlled: chromium content is 0.9%-1.2%, vanadium content is 0.15%-0.25%, silicon content is 1.4%-1.6%. Excessively high alloy content will increase the brittleness of the material, while excessively low content cannot achieve the ideal anti-relaxation effect. At the same time, the content of impurity elements such as sulfur and phosphorus must be strictly controlled: sulfur content ≤0.02%, phosphorus content ≤0.025%, to avoid impurity elements damaging the stability of the grain structure. The elastic bar made of the optimized material can control the 1000-hour stress relaxation rate below 3%, which is much lower than 10% of ordinary materials.
What are the key heat treatment process points for stress relaxation inhibition of elastic bars?
The key heat treatment process points for stress relaxation inhibition of elastic bars are adopting the three-stage process of quenching + medium-temperature tempering + stabilization treatment to precisely control the microstructure. The quenching process adopts oil quenching: heat the elastic bar to 860-880℃, keep it warm for 30-40 minutes, fully austenitize the material and then cool it quickly to obtain a uniform martensite structure. The hardness after quenching should reach HRC58-62, laying the foundation for anti-relaxation performance. The medium-temperature tempering process heats the elastic bar to 420-440℃, keeps it warm for 2-3 hours, and transforms the martensite structure into tempered troostite, which has both high strength and high toughness and can effectively resist stress relaxation. Excessively high tempering temperature will reduce hardness, while excessively low temperature will result in insufficient toughness and brittle fracture. Stabilization treatment is the key step to inhibit stress relaxation: heat the elastic bar to 150-180℃, keep it warm for 10-12 hours, simulate the stress state of long-term service, promote the early release of internal residual stress, and reduce the stress relaxation during service. The elastic bar treated by the three-stage heat treatment can improve the anti-relaxation performance by more than 50% and maintain stable clamping force for a long time.
What are the differentiated requirements of anti-relaxation performance of elastic bars for different line types?
The differentiated requirements of anti-relaxation performance of elastic bars for different line types are mainly determined according to three core indicators: annual total passing weight, operating speed and ambient temperature. High-speed railways have fast train operating speed and high vibration frequency, so they have the highest requirements for the anti-relaxation performance of elastic bars: the 10-year stress relaxation rate ≤5%, and the clamping force attenuation ≤10%. Elastic bars made of 60Si2CrVA material and treated with stabilization must be used to adapt to the service environment of high-frequency vibration. Heavy-haul railways have large train axle load and high clamping force demand: the 15-year stress relaxation rate ≤8%, and the clamping force attenuation ≤15%. The initial clamping force of elastic bars should be ≥12kN, and the material can be 60Si2CrVA or 55SiCr to ensure that it can withstand heavy-haul impact loads. Ordinary-speed railways have low annual total passing weight, so the requirements for the anti-relaxation performance of elastic bars are relatively loose: the 20-year stress relaxation rate ≤10%, and the clamping force attenuation ≤20%. Elastic bars made of 60Si2Mn material can be selected to balance performance and economy. The underground lines of urban rail transit have stable temperature but frequent starts and stops and many vibration times: the 15-year stress relaxation rate ≤7%. Elastic bars treated with stabilization must be used to avoid accelerated relaxation caused by frequent vibration.
What are the detection methods and acceptance standards for the stress relaxation performance of elastic bars?
The detection method for the stress relaxation performance of elastic bars mainly uses a stress relaxation testing machine in accordance with GB/T 10120-2013, and the acceptance standards must comply with TB/T 3013-2015 special standards for railway elastic bars. During detection, install the elastic bar on a special fixture, apply a constant strain equivalent to the initial clamping force, set the test temperature to 120℃ (accelerated aging temperature), test for 1000 hours, record the stress value at different time points, and calculate the stress relaxation rate. The acceptance standard stipulates that the 1000-hour stress relaxation rate of elastic bars for high-speed railways ≤3%, for heavy-haul railways ≤5%, and for ordinary-speed railways ≤8%. At the same time, a normal-temperature clamping force test should be carried out: the initial clamping force of the elastic bar should reach 100%-110% of the design value, and after 1000-hour relaxation test, the clamping force retention rate should be ≥90%. The sampling ratio for detection is 10 elastic bars per batch. If one is unqualified, double sampling shall be conducted; if it is still unqualified, the batch of elastic bars shall be judged as unqualified. In addition, a fatigue performance test should be carried out: after 2×10⁷ vibrations, the elastic bar shall have no fracture or deformation, and the clamping force retention rate shall be ≥85%.