Exothermic welding
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Exothermic welding, also known as exothermic bonding, thermite welding (TW),[1] and thermit welding,[1] is a welding process that employs molten metal to permanently join the conductors. The process employs an exothermic reaction of a thermite composition to heat the metal, and requires no external source of heat or current. The chemical reaction that produces the heat is an aluminothermic reaction between aluminium powder and a metal oxide.
Overview
In exothermic welding, aluminium dust reduces the oxide of another metal, most commonly iron oxide, because aluminium is highly reactive. Iron(III) oxide is commonly used:
The products are
Commonly the reacting composition is five parts
Other metal oxides can be used, such as chromium oxide, to generate the given metal in its elemental form. Copper thermite, using copper oxide, is used for creating electric joints:
Thermite welding is widely used to weld railway rails. One of the first railroads to evaluate the use of thermite welding was the Delaware and Hudson Railroad in the United States in 1935[3] The weld quality of chemically pure thermite is low due to the low heat penetration into the joining metals and the very low carbon and alloy content in the nearly pure molten iron. To obtain sound railroad welds, the ends of the rails being thermite welded are preheated with a torch to an orange heat, to ensure the molten steel is not chilled during the pour.
Because the thermite reaction yields relatively pure iron, not the much stronger steel, some small pellets or rods of high-carbon alloying metal are included in the thermite mix; these alloying materials melt from the heat of the thermite reaction and mix into the weld metal. The alloying beads composition will vary, according to the rail alloy being welded.
The reaction reaches very high temperatures, depending on the metal oxide used. The reactants are usually supplied in the form of powders, with the reaction triggered using a spark from a flint lighter. 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 aluminium oxide slag that it produces is discarded.[4][5]
When welding copper conductors, the process employs a semi-permanent graphite crucible mould, in which the molten copper, produced by the reaction, flows through the mould and over and around the conductors to be welded, forming an electrically conductive weld between them.[6] When the copper cools, the mould is either broken off or left in place.[4] Alternatively, hand-held graphite crucibles can be used. The advantages of these crucibles include portability, lower cost (because they can be reused), and flexibility, especially in field applications.
Properties
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. However, the process is costly relative to other welding processes, requires a supply of replaceable moulds, suffers from a lack of repeatability, and can be impeded by wet conditions or bad weather (when performed outdoors).[4][6]
Applications
Exothermic welding is usually used for welding copper conductors but is suitable for welding a wide range of metals, including stainless steel, cast iron, common steel, brass, bronze, and Monel.[4] It is especially useful for joining dissimilar metals.[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.[4]
Because of the good electrical conductivity and high stability in the face of short-circuit pulses, exothermic welds are one of the options specified by §250.7 of the United States National Electrical Code for grounding conductors and bonding jumpers.[8] It is the preferred method of bonding, and indeed it is the only acceptable means of bonding copper to galvanized cable.[5] The NEC does not require such exothermically welded connections to be listed or labelled, but some engineering specifications require that completed exothermic welds be examined using X-ray equipment.[8]
Rail welding
History
Modern thermite rail welding was first developed by
In 1904, George E. Pellissier, an engineering student at
Process
Typically, the ends of the rails are cleaned, aligned flat and true, and spaced apart 25 mm (1 in).[9] This gap between rail ends for welding is to ensure consistent results in the pouring of the molten steel into the weld mold. In the event of a welding failure, the rail ends can be cropped to a 75 mm (3 in) gap, removing the melted and damaged rail ends, and a new weld attempted with a special mould and larger thermite charge. A two or three piece hardened sand mould is clamped around the rail ends, and a torch of suitable heat capacity is used to preheat the ends of the rail and the interior of the mould.
The proper amount of thermite with alloying metal is placed in a refractory crucible, and when the rails have reached a sufficient temperature, the thermite is ignited and allowed to react to completion (allowing time for any alloying metal to fully melt and mix, yielding the desired molten steel or alloy). The reaction crucible is then tapped at the bottom. Modern crucibles have a self-tapping thimble in the pouring nozzle. The molten steel flows into the mould, fusing with the rail ends and forming the weld.
The slag, being lighter than the steel, flows last from the crucible and overflows the mould into a steel catch basin, to be disposed of after cooling. The entire setup is allowed to cool. The mould is removed and the weld is cleaned by hot chiselling and grinding to produce a smooth joint. Typical time from start of the work until a train can run over the rail is approximately 45 minutes to more than an hour, depending on the rail size and ambient temperature. In any case, the rail steel must be cooled to less than 370 °C (700 °F) before it can sustain the weight of rail locomotives.
When a thermite process is used for
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. When rail is welded into long strings, the longitudinal expansion and contraction of steel must be taken into account. 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
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. American practice appears to be very similar, a straightforward physical restraint of the rail. The rail is prestressed, or considered "stress neutral" at some particular ambient temperature. This "neutral" temperature will vary according to local climate conditions, taking into account lowest winter and warmest summer temperatures.
The rail is physically secured to the ties or sleepers with rail anchors, or anti-creepers. If the track ballast is good and clean and the ties are in good condition, and the track geometry is good, then the welded rail will withstand ambient temperature swings normal to the region.
Remote welding
Remote exothermic welding is a type of exothermic welding process for joining two
The process incorporates either an igniter for use with standard graphite molds or a consumable sealed drop-in weld metal cartridge, semi-permanent graphite crucible
See also
- Rail lengths
References
- ^ ISBN 978-0-7506-7757-8.
- ^ "Demo Lab: The Thermite Reaction". Ilpi.com. Retrieved 2011-10-11.
- ^ "Welding Rails Together Takes Out Clicks", Popular Mechanics, October 1935.
- ^ ISBN 9780750655552.
- ^ ISBN 9780849318894.
- ^ ISBN 9781574447279.
- ^ "Why Choose Exothermically Bonded Connections?". ETS Cable Components. Archived from the original on 2014-04-26. Retrieved 2014-04-25.
- ^ ISBN 9781401859381.
- ^ a b c d Lionsdale, C. P. "Thermite rail welding: history, process developments, current practices and outlook for the 21st century" (PDF). Proceedings of the AREMA 1999 Annual Conferences. Conrail Technical Services Laboratory. Retrieved April 5, 2013.
- ^ Pellissier, George E. (1905). "Thermit Rail Joints". The Journal of the Worcester Polytechnic Institute. VIII. Worcester Polytechnic Institute: 304–321.
- ^ "Thermit Rail Welding in Holyoke". Street Railway Journal. XXV (7). New York: McGraw Publishing Company: 317–318. February 18, 1905.
G. E. Pellissier, civil engineer of the Holyoke Street Railway Company, presented on Jan. 27 a paper before the Civil Engineers' Society of Worcester Polytechnic Institute on thermit [sic] welding...When the thermit process was introduced in the United States the Holyoke Street Railway Company decided to try it on a mile of track which was about to be reconstructed, and accordingly an order for 160 joints was placed with the Goldschmidt Thermit Company...The welding was commenced on Aug. 8, 1904...The work...was the first piece of track in the United States laid with thermit joints
- ^ Pellissier, George E (1908). "Welding Entire Rail Sections at Holyoke, Mass". Electric Railway Journal. New York: McGraw Publishing Company: 1245–1246.