P.O. Box 33604, Jeppestown 2043
416 Heidelberg Road, Tulisa Park
Johannesburg
Tel : +27 11 907 1355
Fax: +27 11 907 1433
email [email protected]
www.exoweld.co.za
the exothermic welding process
Introduction
Welding by exothermic chemical reaction was originally devised in
about 1897. In a typical process, an exothermic chemical reaction
is achieved with heavy metal oxides, using aluminium as a reducing
agent, for example, the reaction between aluminium powder and
iron oxide. When ignited, these reactants produce the necessary
welding heat within a few seconds. The resultant superheated,
molten metal is so hot (between 2500-2960°C for steel) that the
contacting surface layers of the substrate or joint melt. These
layers, together with the liquid weld metal, coalesce to form a
welded joint or clad layer.
Exothermic welding is a commercially established technique for
joining steel railway track, reinforcing bar, copper bus bars and
earth and lightning conductors.
The exothermic welding process
The exothermic welding process is simple as it is effective. The
process is a method of making strong and low resistance electrical
connections of various metals such as copper to copper, copper to
steel and steel to steel without the need of an external source of
heat or power.
The weld metal powder,
consisting
of
powdered
aluminium and copper oxide, is
poured into a graphite mould (as
shown in figure 2.) following by
ignition powder and ignited by
means of an ordinary flint gun.
The resultant exothermic (heat
giving) reaction produces high
temperature molten copper and
aluminium oxide slag.
Figure 2. Typical exothermic weld mould
in resistance. Increase in resistance causes heat which in turn
accelerates the corrosion, etc. The ultimate result is premature
failure. Exothermic connections avoids these problems.
Exothermically welded connections produce a permanent joint
[connection) superior in performance to any known mechanical or
pressure type of surface to surface contact connections. By virtue
of its molecular bond, an exothermically bonded connection will not
degrade or increase in resistance over life of the installation.
Additional benefits include:
Figure 1. Machine cross
section of weld shows
excellent fusion of
conductors
The molten copper melts the steel retaining disc and flows down the
Tap hole and over the conductors in the weld chamber, melting and
welding them into a solid homogeneous joint. The whole process
takes no more than a few seconds.
The step by step process of operating a vertical type mould is
illustrated on the next page.
Benefits of the exothermic weld
Mechanical connections can result in the following: Increase in
time causes corrosion. Increase in corrosion causes increase
1. Has a current carrying capacity greater than that of the
conductors (see figure 2 & Table 1)
2. Will last indefinitely and will not age.
3. Has a lower electrical resistance than a mechanical joint.
4. Will not corrode or loosen as it is a permanent atomic bond.
5. Is time saving, with no external source of heat or power required
and lends itself to field work.
6. Low labour costs (no special skills, minimal training required.)
7. As opposed to a compression joint, an exothermic weld
connection may be tested by any of the NDT techniques
applicable to other welding processes.
Table 1. Temperature limitations of various connection
Type of connection
Max temp limit
Mechanical and pressure type connections
250OC
Brazed connections
450OC
Exothermic connections
800OC
NB: Copper melting point
Figure 3. An exothermic weld has a greater current carrying capacity than of the conductor
1083OC
P.O. Box 33604, Jeppestown 2043
416 Heidelberg Road, Tulisa Park
Johannesburg
Tel : +27 11 907 1355
Fax: +27 11 907 1433
email [email protected]
www.exoweld.co.za
Exothermic process for operating a vertical type mould
Please note that it is important for safety reasons that the correct personal protective equipment is used when undertaking this process.
1. Place conductors into the mould.
Ensure that these items are clean
and dry.
2. Close and lock mould handle
clamp.
3. Place steel retaining disc in mould
cup over the tap hole.
4. Pour weldmetal powder into the
mould cup.
5. Pour some starting powder on the
mould lip, sprinkle remainder on
weldmetal powder in mould cup.
6. Close lid on the mould and from
the side ignite the starting powder
with the flint gun.
7. Ignite starting powder. This in turn
ignites the weldmetal powder.
8. After +/- 30 seconds, remove
conductors from the mould.
9. Clean mould with poster paintbrush
before next connection.