temperinghardening

Tempering and hardening are heat treatment methods used to modify the mechanical properties of ferrous alloys, most commonly steel. Hardening increases hardness by producing a martensitic microstructure, typically through heating into the austenite range followed by rapid quenching to room temperature. The rapid cooling traps carbon in solution as martensite, which is very hard but brittle. The required cooling rate depends on carbon content and alloying elements; higher-carbon steels generally require faster quenching and are often quenched in oil or water, while low-carbon steels may be air-quenched or not hardened at all.

Tempering is performed after hardening. The quenched steel is reheated to a moderate temperature well below the austenite range and held there for a period before cooling. Tempering reduces hardness somewhat but increases toughness, ductility, and resistance to brittle failure, while relieving residual stresses. The tempering temperature and time determine the final hardness and toughness; typical tempering ranges are approximately 150 to 650°C, with higher temperatures yielding lower hardness and greater ductility.

Variations and related processes include surface or case hardening, such as carburizing or nitriding, which harden only the outer layer while preserving a softer interior; induction hardening, which locally hardens a surface via rapid heating and quenching; austempering and martempering, which modify the cooling path to influence ductility and strength. Applications span tools, cutting blades, gears, springs, and shafts, with cycle selection tailored to the desired balance of hardness, toughness, and resistance to cracking.