Hardening steel is a heat treatment that has been used for centuries to increase the strength of steel. By heating steel above the critical temperature the inherent microstructure changes to a face-centred cubic formation in the form of Austenite. The properties of the steel will then depend on the cooling rate of the quenching media. For steel that has been swiftly quenched a hard but brittle product is formed due to the formation of Martensite. To alleviate some of this excessive hardness hardening is usually followed by a tempering treatment.
As per the technical bulletin on torque Technical Bulletin: Torque hardening plays a critical role in the integrity Donhad offers in its products.