Name the heat treatment process in which after hardening a tool, it is reheated and cooled to get the required property of toughness.

Explanation:

Tempering:

• Tempering is a heat-treatment process consisting of reheating the hardened steel to a temperature below 400°C, followed by cooling.

Purpose of tempering the steel:

• Steel in its hardened condition is generally too brittle to be used for certain functions.
• Therefore, it is tempered.
• The aims of tempering are:
  • to relieve internal stresses -
  • to regulate the hardness and toughness
  • to decrease the brittleness
  • to restore some ductility
  • to induce shock resistance.

Normalizing:

• The process of removing internal defects or refining the structure of steel components is called normalizing.

Purpose:

• To produce fine grain size in the metal.
• To remove stresses and strains formed in the internal structure due to repeated heating and uneven cooling
• hammering.
• To reduce ductility.
• To prevent warping.

Case hardening:

• Parts to be hardened by this process are made from steel with a carbon content of 0.15% so that they will not respond to direct hardening.
• The steel is subjected to treatment in which the carbon content of the surface layer is increased to about 0.9%.
• When the carburized steel is heated and quenched, only the surface layer will respond, and the core will remain soft and tough as required.
• Case hardening takes place in two stages.
  1. Carburising in which the carbon content of the surface is increased.
  2. Heat treatment in which the core is refined and the surface hardened.

Induction hardening:

• This is a production method of surface-hardening in which the part to be surface-hardened is placed within an inductor coil through which a high-frequency current is passed.
• The depth of penetration of the heating becomes less, as the frequency increases.
• The depth of hardening for high-frequency current is 0.7 to 1.0 mm.
• The depth of hardening for medium frequency current is 1.5 to 2.0 mm.
• Special steels and unalloyed steels with a carbon content of 0.35 to 0.7% are used.