Heat Treatment

From SwordWiki

The temper of a blade is the result of heat treatment, by which the blade is first annealed to remove internal stresses, and then quenched to rapidly cool the metal. This rapid cooling prevents large crystal structures from forming, and results in a differential hardening that makes the thinner areas (such as the edges) and surface of the blade quite rigid, while leaving the core soft and flexible. These features are important for maintaining a resilient sharp edge, while allowing the blade to flex rather than shatter or split under the stresses of impact.