civil Medium

What is Flash Butt Welding

Flash butt welding is a highly effective resistance welding process employed in railway construction to connect rails seamlessly. By utilizing an electric current, this technique generates intense heat at the connecting ends, which are then bonded together under high pressure. While manual flash butt welding is possible, most welding machines used in rail construction are automatic or semi-automatic. This improves precision and efficiency.

Flash Butt welding for Railway line construction

Railway line construction aims to enhance the comfort of train travel, and one key aspect is the connection between rails. When the rail line is smooth and devoid of irregularities, it significantly reduces noise and vibrations. As a result, overall comfort of the ride is enhanced . Various welding methods are employed during railway line construction, and flash butt welding stands as a prominent choice alongside other methods like Alumino Thermic welding, Gas Pressure welding, and Electric arc welding.

In the following sections, we will delve deeper into the intricacies of flash butt welding, exploring the specialized equipment involved, the welding process itself, and the advantages it offers in railway construction. We will also discuss the challenges associated with this technique.

During a recent railway line construction project in Sri Lanka, I had the opportunity to be actively involved in both Flash Butt welding and Alumino Thermic welding processes. These welding techniques were employed to connect the rails and ensure the smooth operation of the railway line. Being part of this project provided me with valuable firsthand experience and insights into the application and effectiveness of Flash Butt welding and Alumino Thermic welding in real-world construction scenarios.

Equipment used for the flash butt welding

Flash butt welding requires specialized equipment, such as the flash butt welding machine, which plays a central role. This machine is designed to smoothly move along the rail track, making the welding process easier. It has the ability to produce a strong electric current, which creates the heat needed to melt the rail ends, joining them together seamlessly. The machine has clamps that securely hold the rail ends in place during welding, ensuring stability and accuracy. The electrodes, important parts of the machine, carry the electric current to generate the heat required for welding. To ensure a smooth and controlled process, the machine includes an advanced control system. This supervises and manages the entire operation, optimizing the welding settings for the best outcomes. The combination of these elements in the flash butt welding machine enables efficient and dependable rail connections.

Process

The process begins by placing the rails on sleepers and carefully grinding the connecting faces and sides. Prior to welding, the rails need to be aligned, which we done manually by skilled workers. The alignment process involves checking the straightness of the rails from the top and both sides of the rail flange. We maintained a gap of ¼ to ½ inch between them. Since the rail head is thick and cooled down slowly, slight depression can appear after 100% cooling. Therefore a slight upward inclination is introduced at the two rail ends.

Once the alignment is complete, the operator proceeds to lower the heating chuck of the welding machine and connects the rails, ensuring proper contact. The heating process is initiated by applying an electric current. It typically takes around 180 seconds for the temperature to reach the forging temperature of 1250 to 1400 degrees Celsius. As the current passes through the joint, continuous sparking occurs, creating high current density. This causes the material resistance to heat up, melt, and blow out of the joint in a shower of melted particles, resulting in the characteristic flashing action.

Stages

The process consists of four main stages, each playing a crucial role in achieving a strong and reliable rail connection:

1. Preheating: In this stage, high amperage ranging from 15000 A to 20000 A and low voltage between 5 V to 20 V are used. The energy supplied to the joint for heating purposes can be calculated using the formula (current² × resistance × time, I²Rt). The welding machine brings the rails together, allowing the current to flow, which gradually increases the temperature at the rail ends.
2. Flashing: During this stage, the welding machine slowly separates the two rails, triggering arcing and flashing. This flashing action plays a critical role in cleaning the rail ends before they are connected. It effectively removes impurities and oxides through either evaporation or melting, ensuring a clean surface for a solid joint.
3. Forging/Upsetting Stage: Once the rail ends have reached the desired temperature and are glowing red-hot, the machine automatically pushes one rail into the other, creating the welded joint. Immediately after the joint is forged, the machine checks the connectivity between the rails. If the welding connection is satisfactory, the machine displays an “OK” indication.
4. Hold: In the final stage, the welding machine maintains pressure on the joint to allow it to harden and solidify over a specified period of time. This step is crucial in ensuring the strength and durability of the welded joint, as it allows the metal to cool and set properly.

Once the connection has sufficiently cooled down, the next step involves the smoothening of the rail connection through grinding. This process utilizes a small petrol engine equipped with a grinding wheel to carry out the task. The grinding action helps to refine the surface of the rail connection, ensuring a smooth and even finish. By carefully removing any imperfections or rough edges, the grinding process contributes to the overall quality and integrity of the welded joint.

By carefully progressing, flash butt welding achieves a reliable and robust rail connection that meets the required standards for railway construction.

Advantages:

• Flash butt welding produces precise welds without the need for filler metal, resulting in a weld with a minimal heat-affected zone and consistent hardening progression, leading to a favorable crystal-line structure.
• The strength of the flash butt joint is nearly equivalent to that of the parent rail.
• These connections are is less susceptible to corrosion compared to other welding methods.
• The failure rate of flash butt welding is typically less than 10%, indicating a high level of reliability.
• The welding process ensures excellent weld quality by eliminating defects such as porosity, inclusions, and lack of fusion.
• Flash butt welding allows for the joining of various section sizes and complex shapes.
• The process is stable, thanks to the control provided by the processor, ensuring consistent and reliable results.
• The flashing action in this welding also helps remove impurities from the ends, resulting in high material quality in the welded connection.
• Flash butt welding is known for its efficiency in terms of time and cost. It is a relatively quick process, enabling efficient welding of long rail sections. Additionally, the resulting weld is often smoother, requiring less post-weld grinding compared to other welding methods. The availability of mobile and automatic machines further enhances productivity, allowing for a high output rate. Even with manual rail handling, we completed one joint within 20 minutes.

Disadvantages:

• The quality of the flash butt weld heavily relies on precise control of temperature and pressure. If any of these parameters are not maintained at the required level, the weld may not be successful. Proper monitoring and adherence to welding parameters are crucial to ensure satisfactory results.

Conclusion:

Flash butt welding is a reliable and efficient method for connecting rails in railway construction. Its advantages, such as accurate welds, high strength, and efficient operation, make it a preferred choice for many projects. Understanding the process and considering the challenges involved can lead to successful rail connections, ensuring a comfortable and reliable train journey for passengers.