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Quenching and tempering
When steel is rapidly cooled (quenched) from austenite, martensite forms — a hard and brittle structure because the carbon gets trapped in the lattice. Afterwards the steel is tempered by moderate heating, which makes it less brittle with slightly lower hardness, giving a better compromise between toughness and hardness.
quenching and tempering in two steps
Example
A hand tool is quenched and tempered to balance hardness (wear resistance) against toughness (resistance to brittle fracture).
Quenched but untempered steel is often too brittle for practical use — tempering is rarely optional.
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