Thermite-welding,

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Thermite-welding is a process that realizes joining of two separate bars, typically rails, end to end.

It fuses them together with the heat obtained from superheated molten metal of exothermic reactions, poured in a mold assembled around the joint.

The process is not new, having been patented in Germany around the end of the nineteenth century.

Although basically low-tech, it has been incrementally improved in time and it is still widely used today.

For specific jobs like field welding of track or other thick section parts, it is widely applied for its intrinsic benefits.

Thermite-welding Advantages

The major advantage of Thermite-welding is its portability from place to place.

It is also economic as it does not need costly equipment.

It is performed by relatively unskilled workers who are required to follow a very precise simple routine.

The aluminothermic reaction of Thermite-welding provides, in its basic configuration, both heat and filler metal without additional external power source, except possibly for Preheating when needed.

The process therefore is suitable for welding in remote areas where common supplies are not available.

The general form of the thermochemical reaction is the following:

Metal oxide + Aluminum (reducing agent) >> aluminum oxide + metal + heat

Note that in common parlance these are generally called aluminothermic reactions even if in certain cases, although for specific applications, aluminum is substituted by copper, magnesium, calcium or silicon.

An example of the several formulations available is reported hereafter:

3Fe3O4 + 8Al >> 9Fe + 4 Al2O3

The heat evolving from this exothermic reaction is H = 3350kJ

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The proportions of the components in this and similar mixtures are about three parts by weight of iron oxide to one part of aluminum in powder, thoroughly intermixed.

The reaction of the Thermite-welding powder mixture is started at about 1200 °C (2200 °F) using a special ignition powder or an ignition rod to be lighted with the flame of a burning match.

Although not explosive, the reaction is extremely violent taking less than one minute to complete irrespective of the total weight of the reacting materials.

Care must be used to assure that all materials and molds are completely dry to prevent the formation of steam under pressure that could eject molten metal.

The maximum reachable temperature is 3090 °C (5600 °F) that has to be reduced quickly to about 2480 °C (4500 °F) by the addition of non reacting pellets of ferroalloy, because at 2500 °C (4530 °F) the aluminum would vaporize.

However the Thermite-welding temperature cannot be much lower, because the alumina slag (Al2O3), that has to remain liquid to float and separate from the metal, would solidify at 2040 °C (3700 °F) and less.

Of course all factors contributing to the heat balance have to be taken into account, but large quantities of the reactive mixture react more completely because heat losses per unit weight result lower than for smaller charges.

The addition to the mix of non reacting materials provides the opportunity to adjust the metal chemistry as required.

The workpieces, typically rail sections, must be cut square, cleaned of all contaminants and then aligned with the proper gap. A mold of suitable shape has to be assembled around the joint prepared for Thermite-welding with proper sealing of all clearances between mold and workpieces.

Preheating of the joint faces by external means like a gas flame is usually required to assure elimination of all moisture and to promote complete fusion.

Thermite-welding is similar to casting processes and therefore suitable gates and risers are necessary

  • to provide the molten metal needed to fill shrinkage cavities developing upon cooling
  • and to contribute to its laminar flow.

To reduce the need for separate preheating, special mold configurations can be used that provide a portion of the molten metal to preheat the joint ends.

After the weld metal is solid and cooled down, the mold is removed and the joint is finished by grinding or otherwise.

A different thermochemical reaction using copper oxide and aluminum is used to weld electrical conducting joints, forming a metallurgical joint between steel rail and copper conductors.

Other applications are used to repair heavy sections of steel, mostly with variations of the basic Thermite-welding process to suit the particular requirements of the items to be repaired.

Concrete reinforcing bars can also be usefully spliced in place (when partially buried in concrete columns or beams) by suitable variants of this process.

In conclusion, far from being an obsolete process Thermite-welding is still used successfully and economically for those applications where its benefits are most appreciated.

You may wish to download:

Aluminothermic Welding Manual RTS 3602 (81 pages)
http://extranet.artc.com.au/docs/eng/track-civil/workinstruct/rail/RTS3602.pdf

See the following document:

ANSI/AWS D15.2:2003
Recommended Practices for the Welding of Rails
and Related Rail Components for Use by Rail Vehicles

American Welding Society / 07-Oct-2003 / 49 pages

An Article on Filler Metals for Thermite Welding was published (4) in Issue No. 68 of Practical Welding Letter for April 2009.
Click on PWL#068 to see it.

A link to a Presentation (18 frames) on Porosity in Thermite Welds was published in the Mid July 2011 Bulletin 63.
Click on PWL#095 to see it.

An Article on Rail Welding Processes was published (7) in Issue 107 of Practical Welding Letter for July 2012.
Click on PWL#107 to see it.

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Watch the following Video Animation of the Thermite-welding process
by Thermitrex.

* * *

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