airquench

Air quenching is a heat treatment method in which a heated metal, typically steel, is cooled primarily by ambient air rather than immersion in water, oil, or other liquids. It is used to minimize distortion, reduce residual stresses, and avoid cracking in components with complex geometries or large cross-sections. The process relies on steels with high hardenability or those specifically designed to harden in air.

Cooling behavior during air quenching depends on alloy composition, thickness, surface condition, and whether forced air is employed. Natural convection cooling relies on still air, while forced-air quenching uses fans, blowers, or air jets to increase the heat-transfer rate. The resulting cooling rate is generally slower than traditional liquid quenching, which limits the extent of martensitic transformation in many steels but can be sufficient for air-hardenable alloys.

Materials commonly suited to air quenching include steels formulated for high hardenability or those designated as air-hardening steels. Thick or very high-carbon sections may not achieve desired hardness with air quenching alone, and alternative quenching media or subsequent processing may be required. Post-quench tempering is typically performed to achieve the intended combination of hardness and toughness and to relieve residual stresses.

Advantages of air quenching include reduced distortion and cracking, lower environmental and handling hazards, and simpler equipment needs compared with liquid quenchants. Limitations include slower cooling rates, limited hardness for many steels, and potential formation of bainite or pearlite in sections that do not cool rapidly enough. Proper selection of alloy, thickness, and cooling method is essential for successful outcomes. See also quenching, air-hardening steels, and tempering.