Tempering

Tempering is used to increase the toughness of iron alloys, particularly steel. Untempered steel is very hard but is too brittle for most applications. Tempering is commonly done after hardening to reduce excess hardness.

Tempering is used to alter:

• Hardness
• Ductility
• Toughness
• Strength
• Structural stability

Tempering involves heating the metal to a precise temperature below the critical point, and is often done in air, vacuum or inert atmospheres.

It is essential to heat the metal gradually to avoid the steel being cracked. The metal is then held at this temperature for a fixed period. A rough guideline is one hour per inch of thickness. During this time the internal stresses in the metal are relieved. The metal is then cooled in still air.