A treatment in which a part is subjected to two complete hardening operations, or a first annealing step followed by a hardening step. Generally conducted at the same temperature, but not always, in order to refine the grain size of the steel after a first long treatment of austenitisation, or after a long carburising step for a high case depth.
Sometimes, due to misuse of language, double hardening means long duration of austenitisation or long carburising time, followed by a soft hardening or a slow cooling outside the heating chamber (like an annealing step) and re-austenitisation followed by a hardening step (quench).
Double hardening also involves hardening a carburised part twice whereby the first hardening is carried out from the hardening temperature of the core part, and the second from the hardening temperature of the case (see DIN 17014).
-Refined grain size and microstructure of the core of the part, grown during long duration at high temperature
-Avoids surplus/retained austenite content in the case depth
-Reduces or limits the distortion level of parts with complex shapes
-Adjusts more precisely the hardness of the core and the case
Application & materials
Double hardening is generally used in the case of long duration carburising needed to obtain a high case depth or after a long duration austenitisation step. Typical applications can be critical gears in the aerospace industry, large gears used in wind turbines or construction vehicles or other large forged parts.