Page 231 - CITS - Welder - Trade Theory

WELDER - CITS

Thermit welding

Objectives : At the end of this lesson you shall be able to
• describe the principal of thermit welding
• explain the equipments of thermit welding
• explain the thermit welding process.

Thermit welding
Thermit is a trade name for a mixture of finely divided metal oxide (usually iron oxide) and a metal reducing agent.
(almost always aluminum). The thermit mixture may consist of about five parts of aluminum and eight parts of iron
oxide, and the weight of thermit used will depend on the size of the parts of to be welded. The ignition powder
usually consists of powdered magnesium or a mixture of Aluminum and Barium Peroxide.
Principle of thermit welding
The heat necessary for joining in the thermit welding process is obtained from a chemical reaction that takes
place between a metal oxide (Iron oxide) and a metal reducing agent. (Aluminum) When ignited by using a
burning magnesium ribbon in one spot of thermit mixture. The reaction spreads throughout the mixture. The
tremendous heat re- leased approximately 2760°C (5000°F) causes the iron to change to a liquid state within 25
to 30 seconds. As the aluminum in the mixture combines with the oxygen from the iron oxide, it forms Alumina
oxide, which serves as slag and float to the top. Thermit reaction is an exothermic process. There are two types
of Thermit Welding:
1 Plastic or Pressure Thermit Welding

2 Fusion of Non-pressure Thermit Welding
Plastic or Pressure thermit welding
This type is used mainly for butt welding of thick pipes of rails. In this weld, pressure is used to join the metal. The
work pieces are clamped into C.I moulds and are forced together when the desired temperature is achieved. The
thermit is heated in a crucible above the work. While heat- ing, the lighter aluminium oxide slag rises to the top.
When pouring temperature is reached, the thermit solution is poured into the mould. When the thermit mixture has
heated the work pieces sufficiently, the work pieces are forced together forming a pressure butt weld. The whole
welding process takes 45 to 90 seconds for welding of thick walled pipe.
Fusion of non-pressure thermit welding
In this process, the work pieces are lined up, leaving a space between the ends the are to be welded. Wax is
placed between the joint. The whole frame is suspended in a mould, and then the molten metal is poured. The
first step in preparing a non-pressure thermit weld is the cleaning of the joint.
Equipments, materials and supplies
The thermit welding process requires an adequate sup- ply of

1 Thermit mixture
2 Thermit Ignition powder and a
3 Device (Flint Gun, Hot Iron Rod etc…)
Thermit mixture
The most commonly used types of thermit for welding the various ferrous metals are:

1 Plain Thermit
2 MS Thermit or Forging Thermit
3 Cast Iron Thermit
4 Steel Mill Wobblers
5 Rail welding Thermit

6 Thermit for welding electric connections

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