hardened arms

Differential heat treatment (also called selective heat treatment or local heat treatment) is a technique used during heat treating to harden or soften certain areas of a steel object, creating a difference in hardness between these areas. There are many techniques for creating a difference in properties, but most can be defined as either differential hardening or differential tempering. These were common heat treating techniques used historically in Europe and Asia, with possibly the most widely known example being from Japanese swordsmithing. Some modern varieties were developed in the twentieth century as metallurgical knowledge and technology rapidly increased.
Differential hardening consists of either two methods. It can involve heating the metal evenly to a red-hot temperature and then cooling it at different rates, turning part of the object into very hard martensite while the rest cools slower and becomes softer pearlite. It may also consist of heating only a part of the object very quickly to red-hot and then rapidly cooling (quenching), turning only part of it into hard martensite but leaving the rest unchanged. Conversely, differential tempering methods consist of heating the object evenly to red-hot and then quenching the entire object, turning the whole thing into martensite. The object is then heated to a much lower temperature to soften it (tempering), but is only heated in a localized area, softening only a part of it.

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