Thermite Welding For Railroad Tracks

Thermite welding for railroad track

As we all know, railroad tracks are produced with certain lengths, such as 12.5m, 25m, and the lengthen ones from 50m to 100m. No matter how long the railroad rail is, they need to be joined end to end to make a continuous railroad. This can be realized by rail joints (joint bars) or welding methods. At this moment, we focus on welding railroad tracks. 

There are four common methods for welding railroad tracks. They are the flash-butt welding, the gas-pressure welding, the enclosed-arc welding, and the thermite welding. What’re the differences between these methods? We have summarized the application of them under different construction condition for your comparison.

Construction conditionthermite weldingenclosed-arc weldingflash-butt weldinggas-pressure welding
Fixed venueYesNoApplicableApplicable
Fixed venueNot for massive use For massive useFor massive use
On railway lineApplicableNoNoNo
Specially shaped rail endApplicableNoNoNo
During short driving intervalsApplicableNoNoNo
No stop (Tramway only)ApplicableNoNoNo

In a word, thermite welding is the most widely used method for welding railroad tracks. It is simple to operate, need no electric power. The equipment and materials required are easy to get. 

The principle of railroad thermite welding

Railroad thermite welding is a welding method that uses the heat of chemical reaction as the heat source. When welding, the ends of the two rails are fixed in the mold in advance, and then the mixture of aluminum powder and iron oxide powder (called thermite) is heated in the crucible. An exothermic reaction is producing liquid metal (Iron) and molten slag which are injected into the mold. Liquid metal flows into the joint gaps to form weld metal, while slag floats on the surface. To adjust the temperature of the melt and the chemical composition of the weld metal, appropriate additives and alloys are often added to the thermite.

The process of welding railroad rail

  1. Preparation

Equipment and auxiliary material preparation
Consuming material: thermite,sand mould, crucible, sealing mud, high-temperature matches, grinding wheel, propane, oxygen, hose.
Auxiliary equipment: mould, slag tray, crucible fork, sand mould fixture, sand mould cover, 1m leaning ruler, rail thermometer, clamping device, crowbar, pre-heat frame, steel wedge.
Auxiliary machinery: rail saw, polisher, portable grinder, etc.

Railroad track preparation
Unload the 2-4 pairs of rail fasteners on both sides of welding end.

  1. Rail end cleaning and drying
    Use the portable grinder to remove rust and decontamination of rail end. Make sure the face of rail ends to be welded is dry and clean.
  2. Rail end alignment
    The width between the rail end face needs to be 27mm-30m. The vertical and horizontal alignment of the railhead, rail web and rail base is required.
  3. Fixture installation
    The fixture should be installed in the upwind direction.
  4. Sand mold installation
    The pouring hole is slightly trimmed to prevent sand particles from falling off during preheating and pouring, resulting in welding slag.
    Install the bottom plate first, then install the side mold, finally use the clamp to fix the sand mould.
  5. Sealing
    Apply sealing mud to the outlet of the sand mold and the thread of the fixture to make the sand mold and the rail tightly sealed.
  6. Preheating
    Firstly, regulate the working pressure of propane and oxygen. Secondly, install the heater on the frame, adjust the nozzle to the center of the sand mold and place the shunt plug on the edge of the sand mold, remove the heater from the support, and ignite the nozzle. Thirdly, regulate the pressure and mixing ratio of propane and oxygen to get the best flame. Finally, preheat for 5 minutes.
  7. Crucible installation
    This step is prepared at the same time as the preheat. Mix the thermite evenly, pour them into the crucible. Insert the high-temperature match and then cover the crucible.
  8. Ignition
    Put the crucible on the sand mold right after the preheating is finished. Verify the position and light the match quickly.
  9. Thermite Reaction
    Observe the chemical reaction. When there comes the open flame, it is the starting time for the thermite reaction. As soon as the flame fades out, the chemical reaction stopped. Normally, the time for thermite reaction is 8-15 seconds.
  10. Demoulding
    Remove the sand mold and the slag tray 5 minutes after the pouring of molten iron.
  11. Cut off excess thermite
    Cut off the extra thermite and bend the ones protruding on both sides of the rail base.
  12. Grinding
    This is the final step of welding railroad rail. This is to make the surface of joined rails flat and smooth.

After the 13 steps are finished, check the welding joints to make sure they are up to the standard. Restore the railroad tracks to its original place. Also, there is still some cleanup job to do. Workers need to take away the dust, slag, scrap, sand mold and the crucible.

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