Agricultural tools such as scythes and axes, and weapons such as swords were found to have a carburised layer. Early blacksmiths discovered that to leave forged blades in the hot coals followed by quenching in water would harden the surface layers.
Later processes involved putting such implements into cast iron boxes with charcoal and bone ash. These were heated to "red heat" (around 900 °C) and left for several hours depending on thickness of the layer required. Then they were quenched and tempered.
Cooling the forged tool in water (quenching) forms martensite. This is a hard, brittle microstructure, in which the carbon is held in a solution in a highly stressed 'tetragonal' microstructure. In order to make the tools useful, they have to be re-heated to a lower temperature than that used for the forging temperature. This tempers the steel and whilst reducing the hardness, improves its toughness.
During the tempering treatment, the fine carbides precipitate within the martensite, thus reducing hardness and improving ductility. Further tempering would revert the martensite to carbides in a ferrite matrix but at the same time removes all the "as quenched" hardness.
The residual colour of the oxide is related to the tempering temperature. It is therefore a skill to judge "the temper" by the eye.
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Authors/Contributors:
- Graham Haywood, Bergsman UK
