12 May 2017

Thermit welding

Introduction :

Thermit welding is a process, which was traditionally used for the welding of very thick plates. Though this was used for welding large sections such as locomotive rails, ship hulls and broken large castings, its use has decreased nowadays because of the availability of other simpler methods such as submerged arc welding. 

A thermit mixture that uses for welding steels is aluminium and iron oxide. One question arises in your mind that what is thermit mixture?
The heat source utilized for fusion in this welding process is the exothermic reaction of the thermit mixtures. The thermit reaction starts when the mixed thermit powder is brought to its ignition temperature of 1200 0C. 

How it works?

The thermit welding in which the molten metal obtained by the thermit reaction is poured into the refractory cavity made around the joint. It is a similar process as casting. The two pieces to be joined are properly cleaned and the edge is prepared. Then wax is poured into the joint so that a pattern is formed where the weld is to be obtained. Around the joint moulding, a flask is kept and sand is rammed carefully around the wax patterns as shown in below figure providing necessary pouring basin, risers and sprue. For the run off the molten wax, the bottom opening is also provided. The wax is melted through this opening which is also used to preheat the joint and make it ready for welding.

Thermit welding

The thermit mixture which is mixed with fluxes is filled into a ladle through the bottom opening. The opening is initially closed. The igniting mixture which is normally barium peroxide or magnesium is placed at the top of the thermit mixture. The igniting mixture is lighted by means of a heated metal rod, whereby the complete reaction takes place and molten metal is produced. The bottom plug of the ladle is opened and the metal is allowed to flow into the mould prepared. The weld joint is allowed to cool slowly. Thus weld is formed. If we making a fast weld, thermit welding also provides a reasonably strong weld. The strength if thermit welded joint reaches that of a forged metal without any defects.

Applications :

The main application of this welding technique is in the repair works of rails in railways.

Advantages of thermit welding :

  • Very large and heavy parts are also joined.
  • No external power source is required it just use the heat of the chemical reaction.
  • It is also used for building up large wobblers.
Disadvantages of thermit welding :

  • Weld may contain gas mainly hydrogen so that the slag inclusion problem occurs.
  • The metal which has low melting point can't weld by this process.
  • Low deposition rate with operating factor.
  • Only ferrous parts may be weld by this process.
  • It is the high-temperature process so that cause distortion and changes in Grain structure in the weld region.