airhardening

Air hardening is a heat treatment process in which steel is heated into the austenitic range and then allowed to cool in still air, rather than being quenched in oil, water, or other media. It relies on steels with high hardenability, typically alloyed with chromium, vanadium, molybdenum, and nickel, along with sufficient carbon, to permit a martensitic transformation during relatively slow cooling. The result is a hardened surface and core with improved dimensional stability for certain cross-sections and geometries.

The process generally involves heating to the austenitizing temperature specific to the alloy (often roughly 800–1050°C), holding to dissolve carbides, followed by uninterrupted air cooling. For larger or more complex parts, controlled air cooling or specialized ovens are used. After quenching, tempering is commonly performed to achieve the desired balance of hardness and toughness and to relieve residual stresses. Some steels may experience retained austenite, which can be addressed by additional tempering or cryogenic treatment.

Common examples of air-hardening tool steels include A2-type grades, and other high-hardenability alloys designed for minimal distortion upon quenching. In practice, air hardening reduces distortion and eliminates the need for oil or water quenchants, making it suitable for tools, dies, and forming components where precise tolerances are important. Limitations include potential retained austenite, slower cooling, and the need for careful temperature control to avoid cracking or warping.