Thermit Welding | Rail Alumino–Thermic Welding | Thermite Welding Procedure

Thermit welding steps:

Various steps involved in the non pressure fusion thermit welding of metal parts is explained below. The mold is non-repetitive in nature and is used for repair welds.

01-thermit welding - Rail Aluminothermic welding

1. Clean the joint:

An oxy-acetylene torch may be used to clean the metal surfaces by heating. During cleaning all dirt, grease, loose oxides, scale etc., must be removed.

2. Allow for contraction:

After cleaning, the part 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 you for

i. The contraction of the thermit steel in cooling

ii. The shrinkage of the base metal which has been heated during the welding operation

01- rail road welding crucible furnace - thermite sparking

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 constitude a replica of the eventual weld.

The molding materials should be about 100 mm thick 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.

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 mold and cause porous weld.

01-exothermic welding - Aluminum thermite

5. Crucible and its charging:

Thermit mixture is charged in the container knows 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 steel sheet. Located at the bottom of the vessel is a magnesia stone and magnesia thimble through which the tapping pin is suspended.

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.

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.

01-rail welding process - orgo thermit

7. Opening the mold:

The actual period for which the mold is left unopened depends upon the dimensions of the weld, being shorter (2 to 3 hours) for small sections and longer (about 4 hours) for heavy sections. The longer the mould can be left unopened, the better it is.

8. Finishing the weld:

After 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 and grinding.

Plasma Arc Welding | PAW Process |Transferred Plasma Arc Welding

Plasma Arc Welding:

Plasma is the name given to a high temperature stream of partially ionized gas flowing at near sonic velocity. It is a mixture of neutral atoms, free electrons that have disassociated from the gas atoms and positively charged gas ions.

01-plasma arc welding - plasma arc cutting process

Plasma arc welding is an arc welding process wherein coalescence is produced by the heat obtained from a constricted arc setup between a Tungsten alloy electrodes, Water cooled Nozzle and the job. The process employs two inert gases, one forms the arc plasma and the second shields the arc plasma. The tip of the plasma arc torch is constricted or reduced in cross sectional area. This constriction of arc considerably increases its temperature (upto 10000 deg Celsius) since it carries the same amount of current.

Types of Plasma Arc Welding:

1. Non-transferred arc process

2. Transferred arc process

01-plasma arc process - transferred arc plasma arc welding - non transferred arc plasma arc welding

Non-Transferred arc process:

The arc is formed between the electrode (negative) and the water cooled constricting nozzle (positive). Arc plasma comes out of the nozzle as a flame. The arc is independent of the workpiece and the workpiece does not form a part of the electrical circuit. Just as an arc flame, it can be moved from one place to another and it can be better controlled.

This mode is used for plasma spraying or for very low current applications as in non metals. The non-transferred arc plasma possesses comparatively less energy density as compared to transferred arc plasma and it is employed for welding and in applications involving ceramics or metal plating.

Transferred Arc Process:

Current is transferred from the tungsten electrode (negative) through the orifice to the workpiece (positive) and back to the power supply. A transferred arc possesses high energy density and plasma jet velocity.

This is the mode most commonly used for welding. So it is employed to cut and melt metals. It can also be used for welding at high arc travel speeds.

Working principle of Plasma Arc Welding:

The Plasma arc welding consists of a conventional DC power source. The ampere rating of these machines is generally higher. The plasma arc welding process is started by initiating a low current pilot arc between the electrode and the constricting arc. This ionizes the plasma gas flowing through the nozzle. The high temperature of the plasma gas provides a low resistance path to start an arc between the electrode and the workpiece.

01-plasma arc welding - PAW - plasma arc cutting

The plasma gas introduced surrounding the electrode in the plasma torch serves to shield the body of the torch from the extreme heat of the cathode. Cathodes are not affected by gas or mixture of gas other than Argon or Argon hydrogen mixture. Because of its low pressure the plasma gas cannot provide enough shielding to protect the weld.

Therefore a large volume of an inert shielding gas is supplied through the outer gas nozzle surrounding the inner nozzle. The inert gas supplied are : Argon, Helium, or mixture of hydrogen.


1. Power supply

2. High frequency generator and current limiting resistors

3. Plasma torch

4. Shielding gas or inert gas

5. Voltage control

6. Current and gas decay control

7. Fixture (for clamping workpiece)


Advantages of Plasma Arc Welding:

1. Making manual welding is easier in PAW

2. The process provides a complete penetration on a single pass (about 6 mm thickness for butt weld). So welding is faster and saving cost and time.

3. The heat affected zone is smaller and the shape of the welds is more desirable.

4. It has a parallel welds and it won’t scatter.

5. The plasma arc is more stable and the process is able to join practically all the commercially available metals

6. Greater distance between the electrode tip and the workpiece facilitates easy application of filler metal rod without contamination of the electrode

7. Filler metal required is lesser in plasma arc welding because lesser number of runs needed to complete the weld.

Disadvantages of Plasma Arc Welding:

1. Infra red and ultraviolet radiations make it necessary to provide – special protection devices

2. Higher equipment cost

3. Need to frequently replace the nozzle surrounding the electrode

4. Welders need ear plugs because of unpleasant, disturbing and damaging noise

5. More chances of electrical hazards are associated with this process

6. The process is limited to metal thickness of 25 mm and lower for Butt welds.