Differential hardening

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Differential hardening is a method used in forging swords and knives to increase the hardness of the edge without making the whole blade brittle. To achieve this, the edge is cooled more rapidly than the spine by adding a heat insulator to the spine before quenching. Clay or another material is used for insulation.

This technique is mainly used in katana, the traditional Japanese swords and khukuri, traditional Nepalese knives. Most pieces made with this technique have visible temper line/styles.

Differential hardening can also be obtained by quenching the object uniformly, then differentially tempering one part of it with a torch or some other directed heat source. The heated portion of the metal is softened by this process. [1]