Double hardening

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).

Benefits

• 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.

Process details

For both single and double hardening, intermediate annealing of the carburised components can be carried out before the final hardening. This heat treatment consists of annealing at just under Ac1, i.e. at approximately 600-650°C with a longer holding time and subsequent slow cooling.

Due to this intermediate annealing, it is possible to cut out the carbon released as surplus in the austenite carbon as cementite and avoid the risk of formation of residual austenite during the subsequent case refining process. It can also result in a reduction in distortion.