Full definition
Controlled heating and cooling processes applied to metals (primarily steel) to alter their microstructure and achieve desired mechanical properties without changing shape. Primary processes: (1) Quenching/hardening — heat to austenitizing temperature (800-900°C for carbon steel), rapid cool in oil, water, or air; produces martensite, maximum hardness (50-65 HRC) but brittle. (2) Tempering — reheat quenched steel to 150-650°C; reduces brittleness while retaining most hardness; temper temperature determines final hardness/toughness balance. (3) Annealing — slow cooling from austenitizing temperature; maximum ductility and machinability; softest condition. (4) Normalizing — air cooling from austenitizing; uniform fine-grained structure. (5) Case hardening — carburizing (carbon diffusion at 900-950°C) or nitriding (nitrogen diffusion at 500-550°C); hard surface (58-62 HRC) with tough core. Per ASTM A255 (hardenability), SAE AMS 2750 (pyrometry). Applications: shaft hardening, gear teeth, bearing races, spring tempering, and tool steel processing. Selection depends on steel grade, required surface and core properties, and part geometry.