Longitudinal shrinkage (Occurs Parallel to the weld lines). Transverse shrinkage (Perpendicular). Angular shrinkage (rotation around the weld lines). Causes of Distortion: A great number of passes with small diameter electrodes. Slow arc travel speed. High residual stresses in the plate to be welded. Using improper welding sequences.

• In sand casting, solidification occurs as the molten metal is poured into the mould cavity. So, it is vital that this part of the sand casting process is carried out in a highly controlled manner to reduce shrinkage and eliminate defects. There are two main types of shrinkage in sand casting: solidification shrinkage and pattern maker’s.
• A shrinkage cavity at the end of a weld run, the main cause is shrinkage during solidification. Causes Lack of welder skill due to using processes with too high a current.

After reading this article you will learn about:- 1. Process of Thermit Welding 2. Operation of Thermit Welding 3. Application and Uses 4. Advantages 5. Disadvantages.

Process of Thermit Welding:

Thermit welding is a chemical welding process in which an exothermic chemical reaction is used to supply the essential heat energy. That reaction involves the burning of Thermit, which is a mixture of fine aluminum powder and iron oxide in the ratio of about 1:3 by weight.

Although a temperature of 3000°C may be attained as a result of the reaction, preheating of the Thermit mixture up to about 1300°C is essential in order to start the reaction.

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The mixture reacts according to the chemical reaction:

8 Al + 3 Fe₃O₄ → 9 Fe + 4 Al₂O₃ + heat (3000˚C, 35 kJ/kg of mixture)

Aluminum has greater affinity to react with oxygen; it reacts with ferric oxide to liberate pure iron and slag of aluminum oxide. Aluminum oxide floats on top of molten metal pool in the form of slag and pure iron (steel) settled below, because of large difference in densities.

Operation of Thermit Welding:

Thermit welding process is essentially a casting and foundry process, where the metal obtained by the Thermit reaction is poured into the refractory cavity made around the joint.

The various steps involved in Thermit welding are:

1. The two pieces of metal to be joined are properly cleaned and the edge is prepared.

2. Then the wax is poured into the joint so that a wax pattern is formed where the weld is to be obtained.

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3. A moulding box is kept around the joint and refractory sand is packed carefully around the wax pattern as shown in Fig. 7.40, providing the necessary pouring basin, sprue, and riser and gating system.

4. A bottom opening is provided to run off the molten wax. The wax is melted through this opening which is also used to preheat the joint. This makes it ready for welding.

Shrinkage Cavity Welding Material

5. The Thermit is mixed in a crucible which is made of refractory material that can withstand the extreme high heat and pressure, produced during the chemical reaction.

Shrinkage Cavity Welding Definition

6. The igniter (normally barium peroxide or magnesium) is placed on top of the mixture and is lighted with a red hot metal rod or magnesium ribbon.

7. The reaction takes about 30 seconds and highly super-heated molten iron is allowed to flow into the prepared mould cavity around the part to be welded.

8. The super-heated molten metal fuses the parent metal and solidifies into a strong homogeneous weld.

9. The weld joint is allowed to cool slowly.

There are different Thermit mixtures available for welding different metals, such as copper and chromium. They use different metal oxides in place of ferrous oxide. Some typical Thermit mixture reactions with their temperature obtained are given below:

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3 CuO + 2 Al → 3Cu + Al₂O₃ + Heat (4860°C, 275 Kcal)

Cr₂O₃ + 2Al → 2Cr + Al₂O₃ + Heat (3000°C, 540 Kcal)

Application and Uses of Thermit Welding:

Shrinkage Cavity Welding Equipment

Thermit welding is a very old process and now-a-days, in most cases, it is replaced by electro-slag welding. However, this process is still in use.

Some applications are:

(i) Thermit welding is traditionally used for the welding of very thick and heavy plates.

(ii) Thermit welding is used in joining rail roads, pipes and thick steel sections.

(iii) Thermit welding is also used in repairing heavy castings and gears.

(iv) Thermit welding is suitable to weld large sections such as locomotive rails, ship hulls etc.

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(v) Thermit welding is used for welding cables made of copper.

Advantages of Thermit Welding:

1. Thermit welding is a simple and fast process of joining similar or dissimilar metals.

Shrinkage Cavity Welding Techniques

2. This process is cheap, as no costly power supply is required.

3. This process can be used at the places where power supply is not available.

Disadvantages of Thermit Welding:

1. Thermit welding is essentially used for ferrous metal parts of heavy sections.

2. It is uneconomical for welding cheap metals and light parts.