Question

Which of the following statement is wrong?
1. The spheroidising process is usually applied to high carbon tool steels which are difficult to machine
2. In spheroidising process, the cementite in the granular form is produced in the structure of steel
3. The annealing process causes complete recrystallisation in steels which have been severely cold worked and a new grain structure is formed
4. None of the above

Answer 1

Correct Answer - Option 4 : None of the above
Explanation:

Heat treatment

It is defined as an operation of heating and cooling of metals in solid-state to induce certain desired properties into them.

Various types of heat treatment processes are:

Annealing

• The specimen is heated beyond upper critical temperature and held it there for some time and then cooled slowly in furnace.
• It is used to refine grain size due to phase recrystallisation and produce uniformity.
• After Annealing structure became large-grained pearlite.
• We will be able to improve the properties of cast and forged steels before machining.

Spherodizing

• The specimen is heated slightly above the lower critical temperature and held at this temperature for some time and then cooled slowly in the furnace.
• It produces round and globular form of carbides (cementite).
• It is mainly used for low carbon steels to improve its strength.
• High carbon steels can also be spherodized to have improved machinability during cutting operations.
• By using this process, abrasive resistance is also improved and the material can be easily cold worked.

Normalising

• The specimen is heated beyond upper critical temperature and is cooled in still air.
• The structure will now become fine equiaxial pearlite.
• It is used to restore the ductility of cold and hot worked material without appreciable alteration in their properties.

Tempering

• The specimen is reheated to temperature below lower critical temperature followed by any desired rate of cooling.
• In this process, martensite is produced in which iron carbide will be present in the matrix of ferrite.

Case hardening:

• The surface of the steel will be made hard and the core remains soft and tough.
• The structure also becomes wear-resistant.