Steel Tempering Colors

Steels containing more than 0.3% carbon are intended for heat treatment. Lower values of carbon makes the steel more tough than hard and suitable for applications where strength are important - like springs and high tensile bolts. Steels with carbon content in excess of 0.7% can be hardened to a higher degree and are suitable for applications where hardness rather than toughness is important. Tool steels make excellent cutting and drilling equipment as they contain tungsten, molybdenum, cobalt, and vanadium to increase heat resistance and durability.

Tempering steel is a two-step process where the

1. The tool is forged and hardened

• tool end is heated to bright red color and forged

• tool quenched in cold water and cooled until touchable
• tool is sharpened and polished

2. The tool is tempered

• tool heated to tempering temperature as indicated below

• tool quenched in cold water

Note that oil baths and salt bats are also used extensively for tempering steel tools. Lead baths and sand baths are also used. 

If steel is heated in an oxidizing atmosphere - like air - a film of oxide forms on the surface and the color changes as the temperature increases. The colors are affected to some extent by the composition of the steel and the method may not be dependable.

Melting Points of Heat-Treating Baths