Hardening by rapid cooling followed by reheating to a lower temperature for toughness.
Quenching traps a hard, brittle martensitic structure. Tempering reheats it below the critical temperature to trade some hardness for toughness — the higher the tempering temperature, the tougher and softer the result.
Almost nothing is used in the as-quenched condition. When a steel is specified as "Q&T" or given as "quenched and tempered to 1,020 MPa", it has been through both steps and the properties quoted are the tempered ones.
Reading a hardness figure without the tempering condition. The same steel at 30 HRC and 55 HRC has completely different toughness.
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