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The process used for relieving the internal stresses previously set up in the metal and for increasing the machinability of steel is
1. Normalising
2. Full annealing
3. Process annealing
4. Spheroidising

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Correct Answer - Option 3 : Process annealing

Normalizing: Heat the steel from 30°C to 50°C above its upper critical temp, held about fifteen minutes and then allowed to cool down in still air. Homogeneous structure provides a higher yield point, ultimate tensile strength and impact strength with lower ductility to steels.

Main objective

1. Refine grain, improve machinability, tensile strength, and structure of weld.

2. Remove cold worked stress.

3. Remove dislocations due to hot working.

Full annealing: Metal is heated above the upper critical temperature & held there until the temperature of the work piece is uniform throughout, and finally cooling the workpiece at a slow controlled rate in a furnace so that the temperature of the surface and that of the centre of the workpiece is approximately the same.

Benefits of annealing are:

• relieve stresses

• increase softness, ductility, and toughness

• produce a specific microstructure

Process annealing

After cold working, the metal can be softened by process annealing or "recrystallization“ to reduce the distortions of the crystal lattice produced by cold working.

It is also known as low-temperature annealing or sub-critical annealing. This process is used for relieving the internal stresses previously set up in the metal and for increasing the machinability of the steel.

Spheroidising

Heat them slightly above the critical temperature, hold them at this temp for a period of time, and then letting them cool in the furnace. Spheroidizing produces a rounded or globular form of carbide. It improves abrasion resistance.

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