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Guide to Thermit® Welding


Guide to Thermit® Welding


Summary: The thermit® welding process is the result of fusion created by a chemical reaction that occurs due to the difference of free energy between aluminum and metal oxide.This difference produces enough heat to produce liquid steel or any other metal and allow without applying outside energy.

It occurs without or with the application of pressure. The use of a filler metal is optional.  All components for the aluminothermic process are used as powders or granules. Once the reaction is initiated by an external heat source, the aluminum reduces the oxygen from the metal-oxide, which will result in a liquid superheated aluminum-oxide (Al(2)O(3). Since these two components are different in density, they separate automatically and the liquid metal can be used for the production of alloys, special metals or for multiple welding applications. (see video below)

The process was discovered by Hans Goldschmidt in 1898. Since the turn of the century the thermit process has been used to join train rails as well as other applications in the transit industry.

Thermit Welding Video

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Key Definitions:

  • crucible: the thermit chemical reaction takes place in a vessel called a crucible
  • mixture: A mixture of finely divided aluminum, metal oxide with the addition of any required alloying metals
  • mold: This is a mold that is formed around parts that need to be welded. The mode receives the molten metal.
  • reaction: The chemical reaction between aluminum and metal oxide. The reaction produces an aluminum oxide slag and superheated molten metal.


Thermit Welding Process

Thermit material is a mechanical mixture of metallic aluminum and
processed iron oxide. Molten steel is produced by the thermit reaction
in a magnesite-lined crucible. At the bottom of the crucible, a
magnesite stone is burned, into which a magnesite stone thimble is
fitted. This thimble provides a passage through which the molten steel
is discharged into the mold. The hole through the thimble is plugged
with a tapping pin, which is covered with a fire-resistant washer and
refractory sand. The crucible is charged by placing the correct quantity
of thoroughly mixed thermit material in it.


In preparing the joint for
thermit welding, the parts to be welded must be cleaned, alined, and
held firmly in place. If necessary, metal is removed from the joint to
permit a free flow of the thermit metal into the joint. A wax pattern is
then made around the joint in the size and shape of the intended weld. A
mold made of refractory sand is built around the wax pattern and joint
to hold the molten metal after it is poured. The sand mold is then
heated to melt out the wax and dry the mold. The mold should be properly
vented to permit the escape of gases and to allow the proper
distribution of the thermit metal at the joint. A thermit welding
crucible and mold is shown in figure 5-41 below.

Welding Process for Rail: (see video above)

  1. Prepare rail to be welded
  2. Set the weld gap
  3. Clamp
  4. Apply molds
  5. Place Thermit portion into the crucible
  6. Preheat ends of the rails
  7. Ignite and pour steel
  8. Demolding
  9. Shear excess head metal
  10. Rough grinding
  11. Final grinding

Thermit Welding Mold and Crucible Diagram - Figure 5-41

Thermit Welding Process Animation Video
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Thermit Welding Kit for Startups

Shown: Startup Tool Kit from Orgo Thermit

Kits can vary based on the required welding process:

  • Inch SKV Process: Common approach for joining two train rails
  • 1.5 Inch SKV Process: new process that reduces the risk of internal inclusions.
  • 2.75 Inch Wide Gap Weld: Used when replacing a defective flashbutt or Thermit weld.
  • Full Head Repair Weld: Used to fill in after a damaged area at the rail head is removed.

Thermit Crane Rail Tool Kit


Welding of chormium-vanadium rail steel by H.D. Fricke

Orgo Thermit

(Free PDF Download)Goldschmidt Group