Features
of Exothermic Weld:
·
Superior electrical
conductivity to the conductors themselves.
·
Does not corrode, oxidize
or degrade with time and is resistant to galvanic coupling.
·
Able to withstand repeated
electrical discharges.
·
Never increases its
resistance.
·
Has greater mechanical and
squeezing resistance than the conductors themselves.
Exothermic Weld
Source:
Wikipedia
Exothermic weld
is a welding process for joining two electrical conductors that employs
superheated copper alloy to permanently join the conductors. The process
employs an exothermic reaction of a copper thermite composition to heat the
copper, and requires no external source of heat or current. The chemical
reaction that produces the heat is an alumina thermic reaction between
aluminium powder and a metal oxide.
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.
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. When the copper cools, the mould is
either broken off or left in place. 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.
The
weld formed has higher mechanical strength than other forms of weld, and
excellent corrosion resistance. 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).
Overview:
In
exothermic weld, aluminium dust
reduces the oxide of another metal, most commonly iron oxide, because aluminium
is highly reactive. Iron(III) oxide is commonly used:
Fe2O3
+ 2 Al → 2 Fe + Al2O3
The
products are aluminium oxide, free elemental iron, and a large amount of heat.
The reactants are commonly powdered and mixed with a binder to keep the
material solid and prevent separation. Commonly the reacting composition is 5
parts iron oxide red (rust) powder and 3 parts aluminium powder by weight,
ignited at high temperatures. A strongly exothermic (heat-generating) reaction
occurs that via reduction and oxidation produces a white hot mass of molten
iron and a slag of refractory aluminium oxide. 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.
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 in a process called Cad welding:
3
CuO + 2 Al → 3 Cu + Al2O3
Thermite welding is widely used to weld railway rails. 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 rail road 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.
Process
Typically,
the ends of the rails are cleaned, aligned flat and true, and spaced apart 25
millimetres (0.98 in). This gap between rail ends for welding is to ensure
consistent results in the pouring of the molten steel into the weld mould. In
the event of a welding failure, the rail ends can be cropped to a 75
millimetres (3.0 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 (698 °F) before it can sustain the weight of
rail locomotives.
When
a thermite process is used for track signals – the bonding of wires to the
rails with a copper alloy, a graphite mould is used. The graphite mould is
reusable many times, because the copper alloy is not as hot as the steel alloys
used in rail welding. In signal bonding, the volume of molten copper is quite
small, approximately 2 cubic centimetres (0.12 cu in) and the mould is lightly
clamped to the side of the rail, also holding a signal wire in place. In rail
welding, the weld charge can weigh up to 13 kilograms (29 lb). 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 is to use
a sliding joint of some sort, to allow this movement. American practice is very
often a straightforward physical restraint of the rail. The rail is pre stressed,
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 weld is a type of exothermic weld process for joining two
electrical conductors from a distance. The process reduces the inherent risks
associated with exothermic welding and is used in installations that require a
welding operator to permanently join conductors from a safe distance of the
superheated copper alloy.
The
process incorporates either igniters for use with standard graphite moulds or a
consumable sealed drop in weld metal cartridge, semi-permanent graphite
crucible mould, and an ignition source that tethers to the cartridge with a cable
that provides the safe remote ignition.
Amiable Impex supply Exothermic Weld
Accessories all over India mainly at Ahmadabad,
Pune, Delhi, Mumbai, Bangalore, Chennai, Kolkata, Surat, Pune, Lucknow, Kanpur,
Nagpur, Indore, Bhopal, Vadodara(Baroda) etc.
For
more details or any queries in terms for Exothermic
Weld Accessories please visit us at http://www.amiableimpex.com or
mail us at info@amiableimpex.com
Our Other products in Exothermic
Weld Accessories
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