Rail Welding Processes

Here is a detailed breakdown of the main methods used to weld railway rails.

The primary goal is to create a joint that is as strong and seamless as the rail itself. This “Continuous Welded Rail” (CWR) eliminates the clickety-clack sound of trains and provides a smoother ride, reduces wear on both the track and trains, and allows for higher speeds.

 

Method 1: Thermit Welding (Alumino-Thermic Welding) – Most Common for On-Site Welding

This is the most versatile method, often used for in-track welding, repairs, and creating insulated joints. It’s a portable process that doesn’t require external power. It’s a simple chemical reaction that produces intense heat. A mixture of aluminum powder and iron oxide powder is ignited. The aluminum aggressively binds with the oxygen in the iron oxide, producing pure molten iron and aluminum oxide slag as a byproduct. The chemical reaction is: Iron Oxide + Aluminum → Aluminum Oxide + Iron + HEAT (around 4500°F / 2500°C)

Process:

1. Preparation:

The rail ends are cut square and cleaned meticulously.

A gap of about 1 inch (25-30 mm) is left between the rail ends.

A pre-formed mold made of refractory sand is clamped tightly around the joint. The mold has a pouring gate and risers.

2. Preheating:

A propane or oxy-acetylene torch is inserted through the mold to preheat the rail ends to a dull red heat (around 1800°F / 1000°C). This is a critical step to ensure the molten steel doesn’t cool too quickly and crack.

3. Crucible Setup:

A refractory crucible (a “crucible and tap” assembly) is placed on top of the mold. The correct amount of Thermit mixture is placed inside, often with a starter powder (like magnesium) on top.

4. Ignition and Pouring:

The Thermit mixture is ignited. The reaction takes only 20-30 seconds.

Once the reaction is complete, a molten steel plug at the bottom of the crucible melts away, allowing the superheated molten steel to pour into the mold cavity, surrounding the preheated rail ends.

5. Cooling and Finishing:

The weld is allowed to cool and solidify for several minutes.

The crucible and mold are removed.

The excess steel (the “riser” and “weld collar”) is cut off using a special cutting torch.

Finally, the weld is ground smooth and flush with the rail profile using a large, rail-specific grinder. The finished weld should be virtually invisible.

 

Advantages: Portable, no external power needed, relatively quick.

Disadvantages: Quality can be operator-dependent, requires skilled labor for consistent results.

 

 

Method 2: Flash Butt Welding – The Standard for Factory Welds

This is a high-quality, automated process used primarily in welding plants to create long strings of rail (typically 1,000 ft / 300 meters long) that are then transported to the site. It is increasingly used with high-tech machinery for in-track welding as well. It uses electrical resistance to heat the rail ends and then forges them together.

Process:

1. Clamping: The two rail ends are clamped tightly into the welding machine’s jaws. One end is stationary, the other is movable.
2. Flashing (Heating): An electrical current of very high amperage (up to 100,000 amps) is passed between the two rail ends. They are brought together and pulled apart slightly in a controlled cycle. This creates a series of small arcs that heat the entire cross-section of the rail to a white heat (around 2200°F / 1200°C). The “flashing” action burns away impurities.
3. Upsetting (Forging): Once the rails reach the perfect plastic temperature, the machine rams them together with immense force (hundreds of tons). This forges the molecular structure of the steel together, pushing any remaining impurities out as “flash” or “upset metal.”
4. Trimming and Finishing:

The welding machine has built-in shears that immediately trim off the excess flash while it is still hot and soft.

After cooling, the weld area is given a final light grinding to perfect the rail profile.

 

Advantages: Extremely strong and consistent, fully automated, high production rate.

Disadvantages: Very expensive, non-portable (for factory versions), requires massive electrical power.

 

 

Method 3: Gas Pressure Welding – Less Common Now

This is an older method, similar in principle to flash butt welding but using a different heat source.

Process:

The clean, square rail ends are aligned with a small gap between them.

A large, multi-port oxy-acetylene torch is placed between the ends to heat the entire cross-section evenly to a forging temperature.

Once heated, a hydraulic force is applied to upset the ends together, similar to flash butt welding.

The resulting upset metal is then torched off and the weld is ground smooth.

 

Advantages: Can produce high-quality welds.

Disadvantages: Slower than flash butt welding, requires more operator skill, largely superseded by flash butt welding.