Thermit welding comprises a group of welding
processes where in coalesence is produced by
heating with superheated liquid metal and slag
resulting from chemical reaction between a metal
oxide and, aluminium, with or without the application
of pressure. The liquid metal acts as filler metal too.
Thermit welding is a chemically reaction welding process.
The weld joint is produced by pouring of superheated molten
metal around the joint to be welded, applying with or without
of pressure. Thermit welding basically called a mixture of
finely divided metal oxide and a metal reducing agent as
Thermit welding principle:
• Thermit welding is based on casting and foundry practice, and
consists essentially of providing, by means of a chemical (thermit)
reaction, a volume of molten weld metal which is poured into the
joint to be welded.
• The necessary heat for joining metal of thermit welding is
obtained from chemical reaction of metal oxide and metal
• Usually iron oxide is used as a metal oxide and aluminium or
magnesium is used as metal reducing agent.
• The strong chemical attraction of aluminium for oxygen is the
basis for thermit process. First the thermit mixture is ignited by a
burning magnesium ribbon.
• The ignited temperature of thermit is about 1200ºC. When ignited
in one spot of mixture, the heat reaction spreads through the
mass. The aluminium merging with the oxygen of metal oxide and
setting free the iron, which is deposited on joint portion into the
mold as a highly superheated liquid metal.
• If theoretical temperature is about 3000ºC of thermit, due to
chilling effect of crucible the temperature is reduced about
2500ºC. So it is sufficient for welding temperature.
Thermit welding: (1) Thermit ignited; (2) crucible tapped,
superheated metal flows into mold; (3) metal solidifies
to produce weld joint.
Thermit welding working
• Thermit material is a mechanical mixture of metallic
aluminum and processed iron oxide.
• Molten steel is produced by the reaction in a magnesite-
• At the bottom of the crucible, a magnesite stone is
burned, into which a magnesite stone thimble is fitted.
• This thimble provides a passage through which the
molten steel is discharged into the mold.
• The hole through the thimble is plugged with a tapping
pin, which is covered with a fire-resistant washer and
• The crucible is charged by placing the correct quantity of
thoroughly mixed material in it.
• In preparing the joint for welding, the parts to be welded
must be cleaned, alined, and held firmly in place.
Thermit welding working
• If necessary, metal is removed from the joint to
permit a free flow of the metal into the joint.
• A wax pattern is then made around the joint in
the size and shape of the intended weld.
• A mold made of refractory sand is built around
the wax pattern and joint to hold the molten
metal after it is poured.
• The sand mold is then heated to melt out the
wax and dry the mold.
• The mold should be properly vented to permit
the escape of gases and to allow the proper
distribution of the metal at the joint.
• welding crucible and mold is shown in figure
Thermit Welding Mixtures
Thermit mixtures most commonly used for the welding of ferrous
1. Plain Thermit is a mixture of iron oxide and finely divided
aluminum. It is the basis of most thermit mixtures and yields one of
the highest temperatures for thermit welding.
2. Mild Steel Thermit is plain thermit with the addition of mild steel
punchings to augment (i.e., increase) the metal produced. Carbon
and manganese are also added to adjust the chemistry of the
3. Cast Iron Thermit
(a) A plain thermit is available with additions of ferrosilicon and mild
steel punching and is used for welding cast iron. Unless the weld
area is post heat treated, this weld metal is generally not
machinable. This thermit is used where the length of the cast iron
part is less than 8 times its width because of the differential in
contraction between the mixture and the cast iron parent metal.
(b) Another thermit produces an average cast iron weld metal
analysis and gives machinable weld metal. It is used when the ratio
between the length of weld and its width is greater then 8 : 1.
Thermit Welding Mixtures
4. Thermit for Welding Rails consists of plain
thermit with additions of carbon and manganese
to adjust the hardness of the deposited metal to
the hardness of the rail being welded. Alloying
elements may be added to act as grain refiners
and to control resistance to abrasion.
5. Thermit for welding electric connections
consists of copper oxide and aluminium.
Thermit welding reactions
• 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
produces through reduction and oxidation 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
Thermit welding reactions
The chemical or thermit reaction takes place between a
metal oxide (usually iron oxide) and a metal reducing
agent (usually aluminium but sometimes magnesium
also). The chemical affinity of aluminium for oxygen is the
basis for the thermit process.
Thermit reaction is an exothermic one. A few typical
thermit reactions are given below:
3 Fe3O4 + 8 Al -> 9 Fe + 4 Al203 + Heat
3 FeO + 2 Al -> 3 Fe + Al203 + Heat
Fe203 + 2 Al -> 2 Fe + Al203 + Heat
• (i) 3Fe3O4 + 8Al--> 9Fe + 4Al2O3 (3088°C) 719.3 kcal
(ii) Fe2O3 + 2Al--> 2Fe + Al2O3 (29600C) 181.5 kcal
(iii) 3CuO + 2Al -->3Cu + Al2O3 (4865°C) 275.3 kcal
Various Methods of Thermit
• The heat of the thermit reaction may be utilised in the
following ways to join metal sections.
1. It may heat and fuse the metal parts to be welded.
The thermit mixture acts as the filler metal also.
This process is called fusion welding and has been
discussed in this chapter.
2. It may heat the metal parts to be welded and raise
them to the forging temperature, when the weld faces
are forced together to forge a bond of the heated parts.
This process is known as pressure welding.
Heat of the thermit reaction may be utilised for the
purpose of brazing also.
Procedure for Thermit Welding
• The various steps involved in the non-pressure fusion
thermit welding of metal parts are given below. The mold
is non-repetitive in nature and is used for repair welds.
1. Clean the Joint
Metal surfaces to be joined are cleaned thoroughly in
order to obtain a strong weld.
A length of 125 to 150 mm back from the ends to be
welded must be cleaned thoroughly to expose bright
metal on each side. The adjacent ends may be cleaned
by a sand blast. An oxyacetylene torch may be used to
clean the metal surfaces by heating Doing cleaning, all
dirt, grease, loose oxides, scale, etc., must be removed.
2. Allow for Contraction
After cleaning, the parts to be welded are to be lined up
with a space of about 1.5 to 6 mm between the ends,
depending upon the size of the parts to be joined. This
space makes up for (i) the contraction of the thermit steel
in cooling and (ii) the shrinkage of the base metal which
has been heated during the welding operation.
3. Construct the Mold
• After the parts have been cleaned and spaced properly,
the next stage is the making of the wax pattern from
which the mould will be formed and which must in shape
constitute a replica of the eventual weld.
The wax is placed in a container and heated until it
reaches its plastic state. The wax is then shaped around
the parts that are being welded together. A hole or vent
is made in this wax from the heating gate to the riser to
enable gases to escape when the weld is begun. It is
usually made by forming the wax round a cord of about
6.4 mm diameter and withdrawing the cord when the
pattern has been made.
• A molding box is then placed around the portion to be
welded and a molding material* is rammed into the box.
Ramming should ensure a tight contact between the
molding sand and the wax.
• The sand mixture must fill the mold completely and be
rammed hard. The molding material should be about 100
mm tick between the wax pattern and the molding box at
• The mold should be provided with the necessary number of
pouring gates, heating gates and risers depending on the
size of the weld.
• After the ramming has been done, the molder should lightly
rap the gate, riser and preheat opening patterns and draw
them out carefully.
• Wipe away any loose sand that might tend to fall into the
holes]he mould walls and recesses will then need be
trimmed, so that any broken surfaces may be patched,
obstructions in the pouring and other gates removed and
the surfaces efficiently smoothed.
4. Preheating the Mold
• The mold prepared as above is then preheated
in order to:
(i) Melt away and remove the wax thereby
leaving a mold cavity in the exact shape of the
(ii) Dry the mold thoroughly otherwise the
superheated molten metal will form steam within
the hold and cause porous weld.
(iii) Bring the parts to be welded to a desired
temperature* * in order to prevent chilling of the
hot thermit metal.
5. Crucible and its Charging• Thermit mixture is charged * in the container known as
crucible or reaction vessel. This vessel is of conical shape
and is lined with magnesia tar lining.
• The outside shell of this vessel is made up, of sheet steel.
Located at the bottom of the vessel is a magnesia stone and a
magnesia thimble through which the tapping pin is
First of all, the thimble is inserted in the stone.
• The thimble provides a channel through which the liquid
thermit steel is poured. Every time a new thimble is used for
The thimble is plugged by suspending the tapping pin through
the thimble and placing a metal disc above the pin. This disc
is then covered with refractory sand.
After drying the crucible, a small quantity of the thermit
powder is first introduced, the object being to avoid damage to
the refractory sand layer and to cushion off the plugging
material in the bottom of the crucible from the impact of the
full weight of the thermit charge.
The rest of the thermit is then carefully mixed and put into the
6. Igniting the Thermit Mixture
• A low ignition point thermit in the form of a powder is
placed on the top of the thermit in the crucible. To initiate
the reaction, the low ignition temperature thermit is
contacted with a hot rod.
This ignition immediately starts the reaction in the main
thermit charge. The reaction is violent enough to be
readily audible, so that as soon as the noise ceases the
reaction can be safely regarded as at an end.
• The chemical reaction may last up to 60 seconds.
The crucible should be tapped only after making sure
that the reaction has been completed.
• The crucible works as a bottom pour ladle and allows
fast removal of the molten metal with no danger of slag
entering the mold.
• The intense heat of the molten metal melts the
preheated ends of the parts to be welded and complete
fusion takes place.
• 7. Opening the Mold
The actual period for which the mold is left
unopened depends upon the dimensions of the
weld, being shorter (two or three hours) for
small sections and longer (about four hours) for
heavy sections. The longer the mould can be
left unopened, the better it is.
8. Finishing the weld
After removing the mold, the risers and gates
are cut away with a cutting torch. In case of
shafts or parts requiring specific finished
contour the same can be given by either
machining or grinding.
Advantages and Disadvantages of Thermit Welding -
1. The heat necessary for welding is obtained from a chemical
reaction and thus no costly power supply is required. Therefore
broken parts (rails etc.) can be welded on the site itself.
3. For welding large fractured crankshafts.
4. For welding broken frames of machines.
5. For building up worn wobblers.
6. For welding sections of castings where size prevents there
being cast in one piece.
7. For replacing broken teeth on large gears.
8. Forgings and flame cut sections may be welded together to
make huge parts.
9. For welding new necks to rolling mill rolls and pinions.
10. For welding cables for electrical conductors.
11. For end welding of reinforcing bars to be used in concrete
Disadvantages of Thermit Welding
1. Thermit welding is applicable only to ferrous metal
parts of heavy sections, i.e., mill housings and heavy rail
2. The process is uneconomical if used to weld cheap
metals or light parts.
• Uses and Applications of Thermit Welding - Thermit
welding is used chiefly in the repair or assembly of large
1. For repairing fractured rails (railway tracks).
2. For butt welding pipes end to end.