Quenching and tempering (tough-hardening) to achieve good toughness entails hardening with subsequent tempering at relatively high temperatures, often in the 550 – 700 ºC range and hardnesses between about 250 HB and 350 HB. The process is used to create a good combination of strength and toughness. Tough-hardening is applied to finished parts as well as to semi-finished products in the form of bar, tube and forging. There are a number of standardised strength classes for tough-hardened steel with hardnesses that provide good machinability.

Parts produced in tough-hardened steel are often subjected to a final and complementary surface hardening process such as nitriding, nitrocarburizing or induction hardening.

Ref. Steel and its Heat Treatment – A Handbook (8.3.4)