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Hardened
From WikiPedalia
Steel can be made to have different characteristics by various heat treating processes. These involve heating steel to various temperatures, then cooling it down either rapidly or gradually. Depending on the specific heat treatment chosen, steel may be either very hard and brittle or soft and flexible.
This is a very complicated technology, with very ancient roots. For many applications, such as sprockets and bearings you want a very hard surface to the part, but the inside of the part should be treated to a lesser hardness so as to avoid excessive brittleness and breakage.
