Differential heat treatment (also called selective heat treatment or local heat treatment) is a technique used during heat treating of steel to harden or soften certain areas of an 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 treatment 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 is done by either of two methods. One of them is heating the steel evenly to a red-hot temperature and then cooling part of it quickly, turning that part into very hard martensite while the rest cools more slowly and becomes softer pearlite. The other is heating only part of the steel very quickly to red-hot and then rapidly cooling it by quenching, again turning that part into martensite, but leaving the rest unchanged. Conversely, one may selectively harden steel by differential tempering, that is, by heating it evenly to red-hot and then quenching it, turning it into martensite, and then tempering part of it by heating it to a much lower temperature, softening only that part.
