The importance of wheels in the safety of subway vehicles
Due to its high efficiency and high capacity, rail transport has been established as one of the most important alternatives for reducing traffic congestion, especially in urban areas. Although there have not been any major or serious safety accidents, the development of industry and technological progress have really increased the importance of safe train operation. The wheel is one of the important parts of the subway vehicle. The characteristics of subway lines are more complex than high-speed trains, and frequent braking and acceleration can easily damage the wheels. Common types of wheel damage include rim wear, tread scrapes, tread wear, tread flaking, and rims falling off. In addition, the relative movement between the wheel and the rail during wheel repair may cause the tire tread to wear. It is very important to study the sliding friction performance of wheel to ensure the safety of train.
Laser cladding technology
Laser cladding technology deposits a surface layer of metal alloys on a substrate, which has many potential applications in repairing high value-added parts and has strong resistance to wear and rolling contact fatigue cracks.
Laser cladding technology strengthens the material and makes the substrate surface obtain superior quality. Most researchers believe that laser cladding technology can use various alloy powders to produce alloy coatings with wear resistance, friction reduction, corrosion resistance, fatigue resistance and oxidation resistance.
Choosing the right alloy powder is very important for laser cladding technology. Iron - and nickel-based powders are often used in track and wheel structures. However, the two powders differ in composition, function and characteristics.
Analysis of dry sliding friction and wear properties of Ni and Fe alloys in the repair of metro wheels
It is understood that the researchers compared the performance of Ni-based and Fe-based alloy coatings in the repair of subway wheels. The ER9 material of railway wheels is coated with Ni - and Fe-based alloys by laser melting technology. The resulting coating has dense microstructure, no defects, cracks or pores.
The researchers used energy dispersion spectroscopy (EDS), scanning electron microscopy (SEM), 3D optical topography and X-ray diffractometer (XRD) to study the microstructure morphology, interface elements and phase types of the coating.
The mechanical properties of the coating were tested using the MFT-EC4000 reciprocating electrochemical wear and friction tester and the Vickers Microhardness tester. Ni-based coatings with γ(Ni, Fe), Cr23C6 and Cr7C3 phases and Fe-based coatings with γ-Fe, (Fe-Cr-Ni) and (Fe, Ni) solid solution phases were selected.
In Ni-based alloys, chromium deposition enhances interatomic bonding and contributes to solid solution strengthening. In addition, the presence of hard carbide phase and the solid solution in the cladding structure increase the compressive strength and tensile strength of the material.
In Ni-based alloys, chromium deposition enhances interatomic bonding and contributes to solid solution strengthening. In addition, the presence of hard carbide phase and the solid solution in the cladding structure increase the compressive strength and tensile strength of the material.
When adhesive wear was the primary wear mechanism, the Ni-based coating experienced more severe oxidation at the end of the wear and friction tests.
The hardness of the Fe-based coating is 715 HV0.7, which is 2.86 times that of the base material. The maximum hardness of the Ni-based coating is 268.4HV0.7. In addition, the friction coefficient of the Ni based coating is lower than that of the Fe based coating. In terms of wear and friction, the durability of the Ni based coating is about four times that of the Fe based coating.
Due to the presence of solid solution phase, Fe-based coatings show solid solution strengthening. Local quenching occurs due to the preheating of the melt coating, which increases the hardness and wear resistance of the tissue.
The function of the Ni-based coating at high temperatures meets expectations, and its hardness and friction quality are also low. However, the economic benefits of Ni-based alloys are poor.
This study shows that laser cladding technology can improve the hardness and wear resistance of wheels. In addition, studies have shown that when the alloy coating is the same as the underlying material, the alloy powder can bring more significant performance improvements.
Due to the optimal wear relationship between the track and the wheel and the high wear resistance and hardness of the coating,Fe-based coatings are not suitable for wheel cladding.
Reference:
Xiao, Q., Zhang, B., & Yang, W. (2022) Sliding tribological properties of laser cladding alloy coatings on subway wheels. https://www.mdpi.com/2079-6412/12/10/1561/htm