High-speed Railway Rail Welding Technology: Aluminothermic Welding, Pressure Welding, Flash Welding

As the construction of high-speed railway projects continues to deepen, the country has imposed stricter quality requirements on railway tracks. The steel rail is a key component of railway tracks, and choosing the appropriate steel rail welding technology plays a significant role in improving the quality of high-speed railway construction.

Common steel rail welding technologies include aluminothermic welding technology, pressure welding technology, and flash welding technology, among others. Different welding technologies have different applicable scopes. To standardize the application of high-speed railway rail welding technology and enhance the level of high-speed railway construction, this paper elaborates on the common types of high-speed railway rail welding technology, their advantages and disadvantages, analyzes the application of high-speed railway rail welding technology, and proposes measures to improve the application level of high-speed railway rail welding technology, with the aim of providing reference for high-speed railway construction projects.

I. Common High-Speed Railway Rail Welding Technologies

(1) Aluminothermic Welding Technology

Aluminothermic welding technology is a common high-speed railway rail welding technology. This technology uses the heat released by the chemical reaction between aluminum powder and heavy metal oxides to weld the steel rails. Technicians can place the aluminothermic agent in a specially designed crucible and ignite it, generating an aluminothermic reaction. During this process, the high temperature can cause the metal in the crucible to melt, forming molten steel, which can then be directed into a sand mold for rail welding.

Aluminothermic welding technology has the advantages of being convenient to operate and having low costs. Its advantage is that it can be welded at the original position of the steel rail without the need for large-scale movement or dismantling of the steel rail. However, it is limited by the casting structure at the welding site, resulting in poor comprehensive mechanical properties and difficulties in ensuring the quality and performance of the welded rail after welding due to factors such as temperature, weld size, and human operation.

Compared with pressure welding technology and flash welding technology, aluminothermic welding technology has the highest probability of rail fracture during application. Therefore, this technology is suitable for use in scenarios where the welding space for high-speed railway rail welding is limited or for emergency repair of rail fractures. In addition, technicians need to conduct inspections and analyses to standardize the application of aluminothermic welding technology and improve the quality of high-speed railway rail welding.

The following problems may occur when applying this technology.

① Cracking. Cracking refers to the formation of voids or holes in the castings due to volume contraction during rail welding. This problem is directly related to the excessive preheating time of the steel rail, the contraction of molten steel in the high-temperature state, the uneven temperature distribution within the steel rail, improper use of preheating equipment, improper handling of high-temperature molten steel, etc.

② Porosity. Porosity refers to the widespread and evenly distributed fine cavities in high-speed railway rail welding. The occurrence of this problem has a negative impact on the quality and compactness of the weld seam and the safety of the rail operation. It is directly related to insufficient preheating, improper rail gap cleaning, improper sealing of the sand mold, excessive rail gap, insufficient steel water volume, and insufficient steel water quenching time.

③ Gas holes. Gas holes refer to cavities formed inside or on the surface of the rail weld seam, which are directly related to the release of gas during the welding process, the performance of the sand mold, the quality of rail welding, and the cleanliness of the rail welding end.

④ Inclusions. Inclusions are a common problem in aluminothermic welding technology applications in high-speed railway rail welding. This problem is directly related to the presence of slag in the weld seam, impurities in the weld cross-section, improper sealing of the sand mold, excessive rail gap, insufficient steel water volume, and insufficient steel water quenching time, which may cause slag to enter the sand mold or remain in the weld seam, or prevent slag from being promptly discharged from the sand mold.

(2) Pressure Welding Technology

Pressure welding technology mainly uses the high-temperature flame generated by the combustion of oxygen-ethane to heat the pre-pressed ends of the steel rail, converting them into a molten or plastic state and connecting them. The application of pressure welding technology in the welding of high-speed railway rails has the characteristics of low energy consumption, high efficiency, low cost, and excellent welding quality. Moreover, the application of pressure welding technology does not require repeated adjustment of the rail position. It only requires technicians to control the top forging pressure at both ends of the rail based on factors such as the rail welding environment temperature and the resistance of the rail during operation, thereby enhancing the pre-pressurization, welding protection, and heating effects, and improving the welding quality of high-speed railway rails. Therefore, this technology is more suitable for application in the stretching locking welding environment (where there is a certain stretching force during the welding process). However, this may be constrained by the specific environment of high-speed railways and the operational level of welding technicians, resulting in welding joint quality problems.

The pressure welding technology has a high requirement for the cleanliness of the rail end surfaces before heating. Additionally, the temperature generated by the oxygen-acetylene combustion is largely affected by the construction environment and the concentrations of oxygen and acetylene. The pressure welding technology heats the rail weld surface with an external heat source, resulting in a large welding heating area. During the welding process, it is difficult to achieve a balanced temperature between the joint end face, the rail surface, the top and bottom of the rail, and the sides of the rail. Therefore, the application of pressure welding technology may not be able to ensure the shape, smoothness, and flatness of the rail welding joint, and technicians need to calculate the top forging amount based on data such as rail dimensions, material, and pressure welding temperature to ensure the quality of the rail welding joint.

At present, the common operation equipment for pressure welding technology is a large-scale numerical control pressure rail welding vehicle. The large-scale numerical control pressure rail welding vehicle adopts a clamp-type clamping structure and an on-board automatic operation mode, which can effectively overcome the operational deficiencies of traditional small-scale numerical control pressure welding equipment and improve the strength of the rail welding joint, the smoothness of the welding structure, and the welding efficiency of the rails, reducing the workload of the welding technicians. However, the operation of the large-scale numerical control pressure rail welding vehicle still uses oxygen-acetylene combustion temperature as the heat source, and the temperature differences between various parts of the rail during the welding process are difficult to handle effectively.

(III) Flash Welding Technology

Flash welding technology welds rails by the resistance heat generated when a large current flows through the rails. During the welding process, a large amount of power resources are consumed, and the application cost is relatively high. Flash welding technology uses high current to rapidly increase the temperature of the joint end faces of the rails, making them in a molten or plastic state. During the heating process of the rails, the resistance heat forms an appropriate temperature field and uses a liquid bridge to protect the high-temperature area of the rails, achieving the purpose of releasing harmful substances and overheated metals within the rails through top forging operations to connect the rails firmly.

Flash welding technology is suitable for application in rail factories or high-speed railway rail welding construction sites. The actual application of flash welding technology at construction sites can be controlled automatically and precisely through numerical control technology to effectively improve the welding quality, welding efficiency, and construction stability of high-speed railway rails.

At present, the application of flash welding technology in high-speed railway rail welding has gradually replaced aluminothermic welding technology and pressure welding technology. In the practical application of flash welding technology, two sections of rails can be connected and current can be passed through, generating electrical heat energy in the rails to melt the contact position and form a liquid metal beam. During this process, a flash phenomenon occurs between the contact gap of the two rails, further heating the rails and balancing the temperature of each area of the rail cross-section. At the same time, carbon elements in the rail are released during the heating process, which can to some extent protect the stability of the rail welding ends. After that, technicians can apply top forging pressure to the rail end surface to discharge the internal metal liquid and weld the rails.

The application flash welding technology in high-speed railway rail welding should be attention to the following issues.

① Rolling or rolling rod type rail sliding equipment can be selected during the welding process to provide convenience for construction. The rolling or rolling rod type rail sliding equipment contacts the rail through rollers, which can reduce the friction between the equipment and the rail; In the application of the roller-type rail sliding equipment, wooden sleepers are placed at the bottom of the steel rods, and the friction coefficient at the bottom is the same as that between the rail and concrete. To effectively control the friction force between the sliding equipment and the rail during rail welding, technicians can preferentially choose the roller-type sliding equipment.

② Technicians can use the rail bending method to gently bend the ends of the rail, forming a slight curvature to improve the straightness of the rail welding joint and reduce welding stress. At the same time, when using the rail bending method, they need to closely monitor the changes in the straightness of the welding joint and strictly control the distance between the welding joint and the bending area of the rail ends.

③ Flash welding technology may encounter sinking problems in the welding of high-speed railway rails. Technicians can appropriately increase the pre-arching degree of the rail to improve the load-bearing capacity of the rail joint and ensure the stability of rail operation. Generally, the pre-arching degree value can be controlled at around 0.2 mm.

II. Measures to improve the application level of high-speed railway rail welding technology

(1) Strictly control the selection and application process of rail welding technology

When choosing rail welding technology, various factors such as the maturity, application scope and economic benefits of different rail welding technologies should be fully considered. Select the rail welding technology with efficient, automated and stable welding quality as the preferred option. At the same time, conduct small-scale tests before applying the technology to verify the application effect of rail welding technology. During the calculation of test data, evaluate the strength performance of the welding joint and the quality of the weld seam to ensure the application effect of the welding technology. In addition, detail the technical application standards for rail welding, such as welding parameter settings, application of welding equipment, and monitoring of the welding process, and monitor each welding data in real time through a computer system to improve the stability of the welding technology application.

(2) Implement inspection work to improve the comprehensive quality of rail welding technicians

Technicians can use non-destructive testing methods such as ultrasonic testing and magnetic particle testing to inspect the quality of rail welding joints and weld seams, including the flatness of the weld seam surface, etc. Timely discover and handling of problems such as pores and slag in rail welding, and conducting mechanical performance tests such as tensile and bending tests on the rail welding joints to verify whether the strength and tensile force of the welding joint meet the requirements for high-speed railway track construction quality.

At the same time, organize rail welding technicians for technical training regularly to deepen their understanding of the principles of welding technology, the operation standards of welding technology, and the usage methods of welding equipment. They can also be evaluated and required to hold certificates for their positions. In addition, the high-speed railway construction department can organize welding technicians to participate in industry exchanges and seminars to enable them to master the latest welding technology and industry trends, and continuously improve their professional skills.

(3) Test the insulation performance of the insulation joints in the early stage of rail welding, and implement rail welding management

Select high-quality insulation materials to ensure the insulation performance of the rail welding joints in the early stage. This can improve the comprehensive mechanical performance of the rail. Technicians should install insulation joints at the ends of the rail according to standard processes and pay attention to protecting the insulation materials during the installation of the insulation joints from external force. Before welding, test the electrical performance of the insulation joints, combined with the design requirements of the high-speed railway and the rail layout diagram, carefully plan the position and welding process of rail welding, and mark the position, length and welding parameters of the welding joint in the rail layout diagram to guide the application of the welding technology.

In addition, prepare a detailed rail welding construction plan, clearly define the work responsibilities of each technician in rail welding, and implement on-site management to supervise technicians to weld according to the rail layout diagram and construction plan, and guide technicians to operate, helping them solve problems in rail welding. The construction units of high-speed railways also need to meticulously record the construction parameters and operation procedures of rail welding, such as welding time, current, voltage, upsetting force, etc. This will provide a basis for subsequent steel rail quality inspection and analysis of quality issues. They can also hold regular summary meetings to establish an effective feedback mechanism, collect the opinions and suggestions of rail welding technicians, summarize the application experience and shortcomings of rail welding technology, continuously innovate rail welding technology and rail welding construction management schemes, in order to improve the quality of rail welding comprehensively.

III. Conclusion

The application characteristics and applicable scopes of rail welding technologies, such as aluminothermic welding, pressure welding, and flash welding, are different. Technicians should clearly understand the advantages and disadvantages of common high-speed railway rail welding technologies, master the application key points of aluminothermic welding, pressure welding, and flash welding, as well as the fundamental causes of problems such as shrinkage cavities, gas pores, and slag inclusion in rail welding. They should strictly control the selection of rail welding technology and implement the inspection work after welding, strengthen the comprehensive quality training of welding technicians, fully consider the insulation performance of the insulation joints in the early stage of rail welding, implement the management of rail welding for high-speed railways, standardize the application of overall rail welding technology, and improve the safety and stability of high-speed railway operation.

Supplier

Luoyang Fonyo Heavy Industries Co., Ltd,founded in 1998,is a manufacturer in railway casting parts.Our factory covers an area of 72,600㎡, with more than 300 employees, 32 technicians, including 5 senior engineers, 11 assistant engineers, and 16 technicians.Our production capacity is 30,000 tons per year. Currently, we mainly produce casting, machining, and assembly for locomotive,railcar,high-speed trains, mining equipment,wind power,etc.
We are the railway parts supply to CRRC(including more than 20 branch companies and subsidiaries of CRRC), Gemac Engineering Machinery, Sanygroup, Citic Heavy Industries, etc. Our products have been exported to Russia, the United States, Germany, Argentina, Japan, France, South Africa, Italy and other countries all over the world.
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