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. It can also be achieved by carefully pouring water (perhaps already heated) onto the edge of a blade as is the case with the manufacture of some kukri.

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

Another process, often referred to as differential hardening, but in reality differential tempering 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]