Steel properties can be changed dramatically by controlling how austenite transforms during cooling and subsequent reheating.

Quenching

Steel is first austenitized by heating into a region where austenite is stable. Rapid cooling can then bypass diffusional transformations shown on TTT or CCT diagrams and form martensite, a diffusionless transformation product.

As-quenched martensite is very hard and strong but contains high internal strain and can have limited toughness. Faster quenching also creates thermal gradients and transformation stresses, increasing distortion or cracking risk.

Tempering

A quenched steel is commonly reheated below the austenitizing temperature and held for a controlled time. During tempering, carbon redistributes and the highly strained martensitic structure evolves toward more stable arrangements.

Increasing tempering severity generally reduces hardness and strength while improving ductility and toughness. The exact response depends on alloy composition, prior austenite structure, tempering temperature and time.

Quench-and-temper processing is therefore not simply 'make steel hard, then soften it'. The goal is to create martensite to obtain a strong starting microstructure and then allow enough controlled recovery and precipitation to reach the required balance of strength, toughness and dimensional stability.