Thermit welding 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 aluminium

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 reducing agent.

• 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

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-

lined crucible. • 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 refractory sand.

• 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

materials are:

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 thermit mixture.

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 floating slag.

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 + Heat3 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 Welding

• 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 JointMetal 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 ContractionAfter 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 all points.

• 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 weld.(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 suspended. 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 each reaction.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 crucible.

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 MoldThe 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 weldAfter 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 -• ADVANTAGES

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 (building) construction.

Disadvantages of Thermit Welding

• LIMITATIONS1. Thermit welding is applicable only to ferrous metal parts of heavy sections, i.e., mill housings and heavy rail sections.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 large1. For repairing fractured rails (railway tracks).2. For butt welding pipes end to end.