Last Updated on May 2022
Railroad joints posed rail safety and lifespan challenges that cannot be underestimated in today’s fast-growing economies. To eliminate the risks, contractors in the past joined rail ends with bolted metal brackets. Nonetheless, technology has advanced in favor of the railway industry, and specialized welding has become the best alternative for sealing the interfaces present at the rail joints.
You might be thinking about gas shielded equipment, oxy-acetylene welders, and arch welders, but that’s hardly the case when it comes to railroad welding. Instead, railroad contractors primarily use a more-than-century-old method known as thermite welding, in which a molten mixture of high-energy metal compounds is used to form a durable welded joint.
Undoubtedly, you are eager to know more about this process. Before the article comprehensively guides you on the railroad welding technique, let’s first grasp a basic understanding of the thermite reaction.
Thermite Reaction Basics
In thermite welding, also known as exothermic welding, a mixture of metal and a compound must undergo an exothermic reaction to form the molten product that acts as the metal filler. Conventionally, the mixture comprises aluminum and iron oxide, which ignite to produce molten iron and aluminum oxide, while generating extreme heat in the process.
The following equation summarizes the process;
Fe(2)O(3) + 2Al = 2Al(2)O(3) + 2Fe + Heat
Welders use a special ignition powder or rod to ignite the mixture, thereby setting the stage for the reaction. Thermite is usually in the form of powder or granules and has an ignition temperature of 1200 degrees Celsius.
It takes about 15-25 seconds for the exothermic reaction to achieve a temperature of about 4450 degrees Fahrenheit (about 2450 degrees Celsius). The ideal temperature is about 3100 degrees Celsius, but this would be unsuitable for aluminum, considering that it evaporates at 2500 degrees Celsius. Non-reacting ingredients are added to reduce the temperature to favorable limits.
The molten iron welds the metal while the aluminum oxide slug forms a protective cover over the weld. Welders must take care not to lower the temperature below 2100 degrees Celsius; Otherwise, lower temperatures would render the procedure ineffective as the aluminum oxide solidifies at 2040 degrees Celsius.
Rail Tracks Thermite Welding
Besides causing logistics problems, overly long rails would necessitate massive investment in labor and money to have them installed. Rail tracks are relatively shorter to ease on-site installation and avoid unnecessary spending.
Consequently, you will find several joints on the rails, and this is where thermite welding comes into play with one mission: to create continuous rail tracks. Thermite welding does not require an external power source and hence can be carried out in areas without power supply. Besides, the equipment is portable, allowing effortless on-site welding of rail tracks.
Let’s take a look at the procedures involved in railroad thermite welding.
Excellent preparation is necessary to ensure effectiveness. This involves cleaning the rail ends, aligning their centerlines, and setting them 23-27mm apart (about 1 inch). This gap is set to ensure consistent results as the molten thermite pours into the sand mold.
In case of failure, the workers can cut off a 3-inch piece of the rail track, remove the failed weld and damaged ends, and subsequently attempt another weld using more thermite and unique mold. It’s also essential to verify whether the rail tracks are compatible with the thermite.
A sand mold of the split-design and manufactured for welding typical rail sizes is clamped at the rail ends. It’s paramount that the center of the mold aligns with the gap between the rails. Thermite welding molds have a preheating gate, risers, and vents.
Obtaining a sound thermite weld requires preheating of the rail ends to eliminate dampness and chillness on the metals as well as increase the sand mold temperature. A specially designed welding torch operating with propane and oxygen ensures attainment of temperatures ranging between 600 and 1200 degrees Celsius. The welding professionals verify the temperature by using a special gauge or checking the color of the rails.
Having withdrawn the preheating torch, the workers place the crucible containing the thermite above the mold. The crucible has a gate at the bottom to allow the molten metal to flow into the rail gap.
The thermite mixture is ignited with a special ignition rod, lighted firecracker, or powder containing barium peroxide. With molten metal and slag having different specific gravities, the slag floats on the top, preventing the molten iron from reacting with atmospheric gases.
The automatic tapping system allows the molten metal to flow directly into the gap, with the slag remaining on the top of the weld where it solidifies. The excess slag flows into the steel catch basins at the sides. Following the solidification of the molten metal, the workers remove the mold material and excess steel using a pneumatic trimming device.
The workers chisel off debris from the cold joint and use grinders to obtain the correct rail profile. Inspection and testing are necessary to ensure that the welded joint is ready for service, with the standard tests being hardness, strength, and transverse breaking load tests. At least 30 minutes should elapse before a train passes on the welded joint.
Precautions for Thermite Welding
- Only a qualified welder with a competency certificate should thermite weld rail tracks.
- All required equipment should be available onsite and in good working condition. Most importantly, workers should wear personal protective equipment throughout the process.
- Workers should thermite weld rail joints as per prescribed procedures, paying particular attention to the vertical and horizontal alignments of the rails.
- Thermite welding should be done in an entirely moisture-free environment. A single drop of water can result in violet and hazardous reaction.
- The rail should be cleaned with a wire brush and kerosene oil before welding takes place.
- Workers should ensure that the thermite powder bags are wholly sealed and void of moisture before opening. The thermite mixture should be no more than two years old.
Thermite welding has long been the universally accepted method for joining rail tracks onsite. It doesn’t need external power, comes with a portability advantage, and forms seamless welded joints. Nonetheless, the success of the procedure lies in the adherence to stipulated guidelines and the use of the correct welding kit.