Fishplate Heat Treatment Process Optimization and Rail Joint Fatigue Strength Enhancement Scheme
What are the fatigue failure mechanism and main influencing factors of the fishplate joint?
The fatigue failure mechanism of the fishplate joint is crack initiation and propagation under alternating stress. During train operation, there is a certain stiffness mutation at the rail joint. The wheel-rail load is transmitted to the fishplate through the rail, causing the fishplate to bear repeated tensile and compressive alternating stresses around the bolt holes and at the ends. Microcracks first initiate in stress concentration areas, and the microcracks continue to propagate under alternating stress. When the crack length reaches the critical value, the fishplate fractures. The main influencing factors include three aspects: first, material performance factors. If the 45# steel commonly used for fishplates is not subjected to reasonable heat treatment, its tensile strength and toughness are insufficient, and cracks are prone to occur in stress concentration areas; second, structural factors. The periphery of the bolt holes and the ends of the fishplate are stress concentration areas. If the periphery of the holes is not chamfered, the stress concentration factor will be greatly increased, accelerating crack initiation; third, process factors. Unreasonable heat treatment process parameters, such as too low quenching temperature and insufficient tempering time, will lead to uneven metallographic structure of the fishplate, residual stress, and reduced fatigue resistance. In addition, the maintenance condition of the line will also affect fatigue failure. Excessive height difference at the rail joint will increase the additional stress of the fishplate and shorten the service life.
What are the core process parameters and optimization methods of quenching and tempering heat treatment for fishplates?
The core process of quenching and tempering heat treatment for fishplates is quenching + high-temperature tempering. The core process parameters include five aspects: quenching temperature, holding time, cooling rate, tempering temperature, and tempering time. For 45# steel fishplates, the optimized range of quenching temperature is 830-850℃. Too low temperature will lead to insufficient austenitization, and there will be undissolved ferrite in the metallographic structure, reducing the strength of the fishplate; too high temperature will lead to coarse grains and reduced toughness. The holding time is adjusted according to the thickness of the fishplate. For fishplates with a thickness of 20mm, the holding time is optimized to 60-90 minutes to ensure complete austenitization inside the material. The cooling rate adopts the method of water quenching and oil cooling. The quenched fishplate is first cooled to 300℃ in water, then transferred to oil for cooling. This method can reduce quenching stress and avoid cracking of the fishplate. The tempering temperature is optimized to 550-580℃, and the tempering time is optimized to 120-150 minutes. High-temperature tempering can transform the metallographic structure into tempered sorbite, which has both high strength and high toughness, and can significantly improve fatigue resistance. The optimization method adopts the orthogonal test method, with tensile strength and impact toughness as assessment indicators, and optimizes the combination of five process parameters to finally determine the optimal parameter combination, so that the tensile strength of the fishplate ≥ 650MPa and the impact toughness ≥ 30J/cm².
What are the surface strengthening treatment process and function of the fishplate bolt hole periphery?
The surface strengthening treatment of the fishplate bolt hole periphery adopts a composite process of rolling strengthening + carburizing treatment. Rolling strengthening is the first process. A special rolling tool is used to apply pressure to the surface around the bolt hole, causing plastic deformation on the surface around the hole, forming a work-hardened layer with a thickness of 0.2-0.3mm. The hardness of the work-hardened layer can reach HV350 or more, which is 30% higher than the matrix hardness. Rolling strengthening can also reduce the surface roughness around the hole to below Ra0.8 and reduce stress concentration sources. Carburizing treatment is the second process. The rolled fishplate is placed in a carburizing furnace and held at a temperature of 920-940℃ for 4-6 hours to allow carbon atoms to penetrate into the surface around the hole. The thickness of the carburized layer is controlled at 0.8-1.0mm, and the carbon content of the carburized layer is 0.8%-1.0%. After carburizing treatment, low-temperature tempering is required at a temperature of 180-200℃ for 60 minutes to eliminate residual stress generated during the carburizing process. The composite strengthening process has two main functions: first, it improves the hardness and wear resistance of the surface around the hole, preventing friction and wear between the bolt and the hole periphery; second, it forms residual compressive stress on the surface around the hole, which can offset part of the alternating tensile stress, reduce the probability of crack initiation, and increase the fatigue strength of the fishplate by more than 40%.
What are the differentiated design points of heat treatment processes between national standard and foreign standard fishplates?
The differentiated design of heat treatment processes between national standard and foreign standard fishplates stems from the differences in their materials and service requirements. National standard fishplates mainly use 45# steel, adapted to national standard 60kg/m and 75kg/m rails, while foreign standard fishplates mostly use European standard C45E steel, adapted to UIC60 and UIC54 rails. Quenching process differentiation: The quenching temperature of national standard 45# steel fishplates is 830-850℃, using water quenching and oil cooling; the quenching temperature of foreign standard C45E steel fishplates is 820-840℃, using air cooling. Because C45E steel has better hardenability, uniform martensite structure can be obtained by air cooling, avoiding the cracking risk caused by water cooling. Tempering process differentiation: The tempering temperature of national standard fishplates is 550-580℃, and the tempering time is 120 minutes, aiming to obtain tempered sorbite structure; the tempering temperature of foreign standard fishplates is 520-550℃, and the tempering time is 150 minutes, aiming to obtain tempered troostite structure, which has higher strength and is suitable for the needs of European heavy-haul lines. Surface strengthening differentiation: National standard fishplates adopt the composite strengthening process of rolling + carburizing; foreign standard fishplates adopt shot peening strengthening process, which can form residual compressive stress on the overall surface of the fishplate, more suitable for the maintenance mode of European lines.
What are the testing items and acceptance standards for the heat treatment quality of fishplates?
The testing items for the heat treatment quality of fishplates include four core items: metallographic structure testing, mechanical property testing, surface hardness testing, and residual stress testing. Metallographic structure testing uses a metallographic microscope to observe the microstructure of the fishplate. The qualified structure of national standard fishplates is tempered sorbite, the grain size grade ≥ 8, and no network ferrite or coarse grains are allowed; the qualified structure of foreign standard fishplates is tempered troostite, the grain size grade ≥ 7. Mechanical property testing includes tensile strength, yield strength, elongation and impact toughness. For national standard fishplates, tensile strength ≥ 650MPa, yield strength ≥ 355MPa, elongation ≥ 16%, impact toughness ≥ 30J/cm²; for foreign standard fishplates, tensile strength ≥ 700MPa, yield strength ≥ 400MPa, elongation ≥ 14%, impact toughness ≥ 25J/cm². Surface hardness testing uses a Rockwell hardness tester to detect the hardness around the bolt holes. The surface hardness of national standard fishplates is HRC28-32, and that of foreign standard fishplates is HRC30-35. Residual stress testing uses an X-ray stress measuring instrument to detect the residual stress around the bolt holes. The residual compressive stress value should be ≥ 100MPa, and no residual tensile stress is allowed. The acceptance standard is that all four testing items are qualified. The sampling ratio of each batch of fishplates is not less than 5%. If there are unqualified items, double sampling is required. If they are still unqualified, the entire batch will be scrapped.