2. HISTORY
• In 1893 Hans Goldschmidt of Germany began to
experiment with aluminothermic reactions.
• Aluminothermic reactions are highly exothermic
processes involving reactions of metallic oxides
with aluminum powders.
• This work led to a patent application for the
Thermit process in 1895.
• Due to the large amount of heat released by
exothermic chemical reactions and the versatility
of the thermit process, other applications were
quickly found and Goldschmidt started a
corporation in 1897.
3. • By the end of the 19th Century, the thermit
process had been successfully used to make
repairs to large cast and forged steel parts.
• In 1904 the Goldschmidt Thermit Company was
founded in New York and extensive use of thermit
welding on street railways in the United States
started.
4.
5. INTRODUCTION
• It is a process in which heat for coalescence is
produced by superheated molten metal from the
chemical reaction of thermite.
• Thermite is mixture of Al and Fe3O4 fine powders
that produce an exothermic reaction when ignited.
• Also used for incendiary bombs.
• Filler metal obtained from liquid metal.
• Process used for joining, but has more in common
with casting than welding .
6. PRINCIPLE OF
OPERATION
• In thermit welding the heat is produced by highly
exothermic reactions between metal oxides
(usually iron oxides) and a metal reducing agent
(usually aluminium but magnesium).
• The chemical affinity of aluminium for oxygen is
the basis for the thermit process.
• Some of these reactions are
Eqn. 1 Fe2O3 + 2Al = 2Fe + Al2O3 + 181.5 kcal
Eqn. 2 3Fe3O4 + 8Al = 9Fe + 4Al2O3 + 719.3
kcal
• These reactions produces temp up to 3000 Degree
Celsius.
7. • The volume of molten weld metal is also produced by
the chemical reactions involved.
• Now this molten metal is used to weld the joint inside
a mould.
•For maximum efficiency, the magnetite thermite
mixture should contain 23.7% aluminium and 76.3%
iron oxide (mass percent).
8. THERMIT WELDING
MIXTURES
Thermit mixtures most commonly used for the
welding of ferrous materials are-
• Plain Thermit: mixture of iron oxide and finely
divided aluminium. It produces highest
temperature for thermit welding.
• Mild steel Thermit: plain thermit with the addition
of mild steel punchings to increase the metal
products. Carbon and manganese are also added to
balance reactions.
• Thermit for welding rails: plain thermit with
addition of carbon and manganese to increase
hardness of rail joints.
9. METHODS OF THERMIT
WELDING
The heat of thermit welding may be utilized in two
ways-
Fusion welding: It may heat and fuse the metal
parts, thermit mixture acts as the filler metal
also.
Pressure welding: It may heat the metal parts
and raise them to forging temperature, and
forging force is applied to join them.
10.
11. PROCEDURE OF
THERMIT WELDING
Various steps involved in fusion thermit welding of
metal parts are-
• Cleaning the joint: Joints to be welded should
be cleaned and free from dust, grease, loose
oxides, etc before welding.
• Allow for contraction: A space of 1.5 to 6 mm
is given between ends as contraction allowance.
• Construction of mould: A mould is the part in
which the metal parts to be joint lies and molten
metal is poured in it. It is 100 mm thick between
wax pattern and molding box and should provide
necessary pouring gates, heating gates and riser.
12. • Preheating the mould: Mould is preheated to
remove wax pattern, dry the mould thoroughly and
to bring the parts to be welded at desired
temperature, i.e. 816oC-982oC.
• Crucible and it’s charging: crucible is a conical
shaped vessel, it is filled with required amount of
thermite mixture to joint the metal parts.
• Igniting the thermite mixture: ignition of thermite
mixture lasts up to 60 seconds. After the ignition is
completed crucible is tapped to release molten
metal mould where fusion takes place.
• Opening the mold: mold is opened after 2-4
hours. The longer the mould is left unopened, the
better it is.
13. • Finishing the weld: after removing the mould
riser and gate are cut away by cutting torch. And
joint is prepared
18. Advantages
• It is very portable process.
• No external power supply required.
• It is very cheap process for repairing broken parts
of large metal structures such as rail lines, large
parts of ships.
• On site welding can be done for railways.
19. LIMITATIONS
• Can only be used for ferrous metals.
• It is uneconomical when used for welding cheap
metals or light parts.
• Thermite mixtures can not be stored due to safety
hazards and should be used as soon as prepared.
20. Thermit Applications
• Repairing fractured rails
• For butt welding pipes end to end
• For welding large fractured crankshafts
• For welding broken frames of machines
• Welding of sections of casting where size
prevents there being caste in one piece
• Replacing broken pieces or large gears
• End welding of reinforcing bars used in huge
concrete constructions
21. Military Applications
• Thermate -TH3 is a mixture of thermite and
pyrotechnic additives for incendiary purposes.
Its composition by weight is generally thermite
68.7%, Ba(NO3)2 29.0%, S 2.0% and binder
0.3%.
• Ba(NO3)2 increases its thermal effect, creates
flame in burning and significantly reduces the
ignition temperature.
• Ames Process – an adaptation of the thermite
reaction for obtaining pure Uranium (as part of
the Manhattan Project at Ames Laboratory).
22. Hazards
• Thermite usage is hazardous due to extremely high
temperatures produced and the extreme difficulty
in smothering a reaction once initiated.
• The thermite reactions releases dangerous ultra
violet (UV) light.
• Mixing water with thermite or pouring water onto
burning thermite can cause a steam explosion,
spraying hot fragments in all directions.
