Page 232 - CITS - Welder - Trade Theory

WELDER - CITS

Plain thermit

A mixture of finely divided Aluminium and Iron Oxide. This is the basic of most thermit mixtures and yields maxi-
mum temperature.
MS thermit
This is a plain thermit with the addition of manganese and mild steel punching, and is used in welding steel.
Manganese is added to adjust the chemistry of thermit mixture (Carbon may also be added, the mild steel
punching are used to augment the metal content)
Cast iron thermit

Cast iron Thermit consists of plain thermit with addition of Ferro-silicon and Mild Steel punching and is used for
welding cast iron. The mild steel punching augments the total metal content. Unless the weld area is post heat
treated, this weld metal is generally not Mechinical and, because of the different contraction between it and the
cast iron parent metal. It is limited to use where the maximum weld dimension is less than 8 times its width.

Thermit for steel mill wobblers
This consists of plain thermit with additions of manganese and carbon to provide a hard, wear resistant, machinable
steel especially adopted to the building up of worn wobbler ends of steel mill rolls.
Thermit for welding rail

These mixtures usually consist of plain thermit with additions of carbon and manganese to adjust hardness of
the deposited metal to the hardness of the rail being welded. Alloys are also added for controlling resistance to
abrasion and to act as grain refiners.
Thermit for welding electric connections
This consists of Copper Oxide and Aluminium.
Thermit ignition powder

There are a number of different ignition powders available. Barium per oxide is frequently used. The thermit
ignition powder is ignited directly with a match. This should be done by partly burying the match head in the
mixture and igniting the match with a thin red hot rod. This avoids the danger of burns injuries to the finger or hand
due to the sudden flare in igniting. The thermit ignition powder can also be ignited with a spark from a flint gun or
by using a burning magnesium ribbon.
Thermit welding procedure
The ends are to be welded are thoroughly cleaned of scale and rust. After cleaning, the parts to be welded are to
be lined up with a gap of 1.5 to 6mm depending upon the size of parts. This gap compensates the contraction of
thermit steel and the shrinkage of the base metal. The next stage is making wax pattern of the weld. A refractory
sand mould is rammed up around the wax joint and necessary gates and risers provided. Ramming should be
light between the moulding sand and the wax. When ramming is completed, the pattern may be drawn and loose
sand may be wiped out. Then, the heat is given to the wax pattern through the heating gate to melt and burn out
wax. The heating is continued until the ends to be welded are at a red heat. This prevents the thermit steel being
chilled, as it would be if it came into contact with cold metal. The preheating gate is now sealed off with sand. Now,
charge the thermit in the crucible. The approximate weight of thermit is 12 to 14 kgs. against one kg. of wax. The
outside shell of crucible is made by steel and is lined with manganese tar lining. At the bottom, a magnesia stone
and a thimble is provided through which the tapping pin works. The thimble is inserted in the stone which provides
a channel through which the molten metal is poured for each reaction a new thimble is used. The thimble is
plugged by suspending the tapping pin and placing a metal disc above pin. The metal disc is lined with refractory
sand. At the top of the thermit, low ignition temperature thermit is placed in the crucible. When ignited in one spot
of thermit mixture, the reaction spreads throughout the mixture. The reacted can be heard, as soon as the noise of
reaction stops, the crucible should be tapped. The intense heat of thermit melts the preheated ends of the parts to
be welded and the fusion welding takes place. Then the mould is allowed to cool overnight. If possible or at least
for 12 hours and cut away the gates and risers with a cutting torch and finish the weld. (Fig 1)

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CITS : C G & M - Welder - Lesson 77 - 82 CITS : C G & M - Welder - Lesson 77 - 82