Fishplate Material Gradient Strengthening Technology and Joint Fatigue Performance Improvement Solution
What are the main forms and causes of fatigue damage of fishplate joints?
The main forms of fatigue damage of fishplate joints include three types: cracks around bolt holes, contact surface wear, and body fracture. Cracks around bolt holes are the most common form of damage. The cause is that the stress concentration factor at the bolt holes is as high as 3.0, and under the action of wheel-rail alternating loads, fatigue cracks will first initiate around the holes. The cause of contact surface wear is that the rail displacement at the joint causes relative sliding between the fishplate and the rail. Sliding friction will cause metal peeling on the contact surface. When the wear depth exceeds 0.5mm, it will affect the fit degree of the joint. The cause of body fracture is insufficient fatigue resistance of the fishplate material. When the crack propagates to the critical length, the fishplate will undergo sudden fracture. This kind of damage mostly occurs at the joint parts of heavy-haul lines. The fatigue damage of fishplate joints is also closely related to the installation process. Insufficient bolt torque will lead to increased joint gaps and intensified stress concentration; excessive torque will lead to plastic deformation of the fishplate and reduce its fatigue resistance. In addition, environmental factors are also important incentives for damage. Corrosion in coastal lines will accelerate crack propagation, and low temperature in alpine lines will reduce the toughness of fishplates and increase the risk of fracture.
What is the core technical principle of fishplate material gradient strengthening?
The core technical principle of fishplate material gradient strengthening is to realize the coordinated improvement of matrix toughness and surface strength. Through the composite process of "matrix quenching and tempering treatment + surface hardening treatment" on the fishplate, the fishplate forms a gradient performance distribution. The matrix quenching and tempering treatment adopts the "quenching + high-temperature tempering" process. The fishplate is heated to 860-880℃ for quenching, and then tempered at 580-600℃ for high temperature, so that the matrix obtains tempered sorbite structure, which has excellent toughness and impact resistance. The toughness index impact energy is ≥50J (-20℃). The surface hardening treatment adopts induction hardening process, which locally heats the stress concentration parts such as the contact surface and bolt hole periphery of the fishplate. The heating temperature is controlled at 900-920℃, and then rapidly cooled, so that the surface forms a tempered martensite structure with a thickness of 2-3mm, the surface hardness can reach HRC55-60, which greatly improves the wear resistance and fatigue resistance of the surface. The key of gradient strengthening is to control the performance of the transition layer. The thickness of the transition layer is controlled at 1-2mm to realize the smooth performance transition between the matrix and the surface, avoiding new stress concentration caused by sudden performance changes. Through gradient strengthening treatment, the fishplate can meet the dual performance requirements of "matrix impact resistance and surface wear resistance" at the same time, adapting to the complex stress environment of the joint.
What are the process measures for wear resistance strengthening of fishplate contact surfaces?
The process measures for wear resistance strengthening of fishplate contact surfaces mainly include three types: induction hardening, plasma spray welding and surface nitriding. Induction hardening is the most commonly used process. It heats the contact surface through electromagnetic induction, increases the surface hardness to above HRC55, and the wear resistance is more than 3 times higher than that of untreated fishplates, which can effectively resist sliding wear of the contact surface. The plasma spray welding process sprays iron-based alloy powder on the contact surface, the thickness of the spray welding layer is controlled at 3-4mm, the hardness can reach HRC60-65, and the wear resistance is 2 times higher than that of the induction hardening process, which is suitable for the strengthening of fishplates in heavy-haul lines. The surface nitriding process adopts the gas nitriding method. At a temperature of 520-540℃, nitrogen atoms are infiltrated into the surface of the fishplate to form a nitrided layer with a thickness of 0.3-0.5mm, the surface hardness can reach HV900-1000. The nitrided layer has excellent wear resistance and corrosion resistance, which is suitable for fishplates in coastal corrosive environments. Regardless of the process adopted, the contact surface needs to be pretreated. The surface oxide scale and defects are removed by grinding, and the surface roughness is controlled below Ra1.6μm to ensure the effect of the strengthening process. After the strengthening treatment, the accuracy of the contact surface needs to be tested to ensure that the flatness and dimensional accuracy of the contact surface meet the design requirements and avoid affecting the fit degree of the joint.
What is the design and process scheme for fatigue resistance strengthening of fishplate bolt holes?
The design and process scheme for fatigue resistance strengthening of fishplate bolt holes adopts a combined strategy of "hole shape optimization + hole periphery strengthening". The hole shape optimization changes the traditional circular hole to an elliptical hole, and the long axis direction of the ellipse is consistent with the stress direction, which can reduce the stress concentration factor around the hole from 3.0 to below 1.5, greatly reducing the probability of crack initiation. For standard fishplates whose hole shape cannot be changed, the hole periphery rolling strengthening process is adopted. The inner wall of the bolt hole is cold-rolled by a rolling tool to form a residual compressive stress layer with a thickness of 0.2-0.3mm around the hole. The residual compressive stress value can reach -300MPa to -400MPa, which can effectively offset the effect of alternating tensile stress and delay the propagation of cracks around the hole. The hole periphery strengthening can also adopt laser quenching process to locally quench the bolt hole periphery to form a hardened ring with a width of 5-8mm. The hardness of the hardened ring can reach above HRC55, which improves the wear resistance and fatigue resistance of the hole periphery. The design scheme also needs to consider the fit accuracy between the bolt hole and the bolt, adopting transition fit, and the fit gap is controlled at 0.05-0.1mm to avoid stress concentration caused by excessive gap. After the implementation of the process scheme, fatigue tests are required to verify the fatigue resistance of the bolt holes to ensure that there are no cracks around the holes under 1 million alternating loads.
What are the core indicators and evaluation standards for fatigue performance detection of fishplate joints?
The core indicators for fatigue performance detection of fishplate joints include three categories: fatigue life, stress around bolt holes, and contact surface wear. The fatigue life detection uses a joint fatigue test bench to simulate wheel-rail impact loads. The fishplate joints for high-speed railway lines need to pass 5 million load cycles without damage, those for heavy-haul lines need to pass 3 million load cycles without damage, and those for ordinary-speed lines need to pass 2 million load cycles without damage. The stress detection around bolt holes adopts the strain gauge test method. Strain gauges are pasted around the holes to measure the stress value under alternating loads. The stress value must be lower than the fatigue limit of the fishplate material, and the stress concentration factor is ≤1.5. The contact surface wear detection is measured by a profiler. After simulating load cycles, the wear depth of the contact surface ≤0.2mm is qualified to ensure that the fit degree of the joint is not affected. The evaluation standard is that all detection indicators meet the standards, the fatigue life of the fishplate joint meets the design requirements, and the qualification rate of the same batch of fishplates is ≥98%. In addition, it is also necessary to detect indicators such as the dimensional accuracy and hardness distribution of the fishplate to ensure the effect of the gradient strengthening process. Unqualified products need to be reworked or scrapped to ensure the safety of engineering applications.