heattreatment

Heat treatment is a controlled heating and cooling process applied to metals and certain other materials to alter their microstructure and properties. The goal is to achieve a desired combination of hardness, strength, toughness, ductility, wear resistance, and dimensional stability, while relieving internal stresses and improving machinability.

Common heat-treating processes include: annealing, which involves heating to a specific temperature, holding, and slow cooling to soften the metal and refine its structure; normalizing, which heats above the critical range and air-quench to produce a uniform grain and higher toughness; hardening, which heats to form austenite and then quenches rapidly to create a hard, brittle martensitic structure; tempering, which re-heats a quenched metal to a lower temperature to reduce brittleness while maintaining hardness; case hardening, which enriches the surface with carbon or other elements (carburizing, nitriding, carbonitriding, etc.) and then quenches to produce a hard surface with a tougher core; and precipitation or age hardening, which dissolves alloying elements and then ages at elevated temperature to form strengthening precipitates. Stress relieving heat treatment reduces residual stresses without substantially changing properties, and is widely used for complex parts.

Process outcomes depend on alloy composition, geometry, and service requirements. Critical variables include temperature, time at temperature, and cooling rate or medium. Quality control typically involves hardness testing, microstructural examination, and residual-stress assessment. Heat treatment is applied across industries to components such as gears, bearings, springs, tools, and aerospace fasteners.