Exothermic welding
The molten iron is the actual welding material; the aluminium oxide is much less dense than the liquid iron and so floats to the top of the reaction, so the set-up for welding must take into account that the actual molten metal is at the bottom of the crucible and covered by floating slag.
The activation energy for this reaction is very high however, and initiation requires either the use of a "booster" material such as powdered magnesium metal or a very hot flame source.
The advantages of these crucibles include portability, lower cost (because they can be reused), and flexibility, especially in field applications.
An exothermic weld has higher mechanical strength than other forms of weld, and excellent corrosion resistance[7] It is also highly stable when subject to repeated short-circuit pulses, and does not suffer from increased electrical resistance over the lifetime of the installation.
[5] The process is marketed under a variety of names such as AIWeld, American Rail Weld, AmiableWeld, Ardo Weld, ERICO Cadweld, FurseWeld, Harger Ultrashot, Quikweld, StaticWeld, Techweld, Tectoweld, TerraWeld, Thermoweld and Ultraweld.
[9] In 1904, George E. Pellissier, an engineering student at Worcester Polytechnic Institute who had been following Goldschmidt's work, reached out to the new company as well as the Holyoke Street Railway in Massachusetts.
Pellissier oversaw the first installation of track in the United States using this process on August 8, 1904,[11] and went on to improve upon it further for both the railway and Goldschmidt's company as an engineer and superintendent, including early developments in continuous welded rail processes that allowed the entirety of each rail to be joined rather than the foot and web alone.
[12] Although not all rail welds are completed using the thermite process, it still remains a standard operating procedure throughout the world.
The mould is removed and the weld is cleaned by hot chiselling and grinding to produce a smooth joint.
When a thermite process is used for track circuits – the bonding of wires to the rails with a copper alloy, a graphite mould is used.
The hardened sand mould is heavy and bulky, must be securely clamped in a very specific position and then subjected to intense heat for several minutes before firing the charge.
British practice sometimes uses a sliding joint of some sort at the end of long runs of continuously welded rail, to allow some movement, although by using a heavy concrete sleeper and an extra amount of ballast at the sleeper ends, the track, which will be prestressed according to the ambient temperature at the time of its installation, will develop compressive stress in hot ambient temperature, or tensile stress in cold ambient temperature, its strong attachment to the heavy sleepers preventing sun kink (buckling) or other deformation.
Current practice is to use welded rails throughout on high speed lines, and expansion joints are kept to a minimum, often only to protect junctions and crossings from excessive stress.
