Heat treatment changes a metal's microstructure — and with it hardness, strength and toughness — without changing the part's shape. For steel it's the difference between a part that wears out in a week and one that lasts for years, so it's worth knowing what the common processes actually do.
Hardening: quench to trap carbon
Heat a medium- or high-carbon steel above its critical temperature (~800–900 °C) so it becomes austenite, then quench it fast. The carbon is trapped and the steel forms martensite — very hard and strong, but brittle. How hard depends on carbon content; how deep the hardening reaches depends on hardenability (alloying) and quench severity — thin sections in oil, heavy sections in water or polymer.
Tempering: buy back toughness
As-quenched martensite is too brittle to use. Tempering — reheating to 150–650 °C and holding — relieves quench stresses and precipitates carbides, trading a little hardness for a large gain in toughness. The higher the tempering temperature, the softer and tougher the result, so tempering temperature is the knob you turn to hit a target hardness.
Annealing and normalising
Annealing (slow furnace cool) makes steel as soft and machinable as possible and removes internal stress. Normalising (air cool) is faster, refines the grain and leaves a more uniform, slightly harder structure — often the starting point before machining and final hardening.
Case hardening
When you want a hard, wear-resistant skin over a tough core — gears, shafts, cams — harden only the surface. Carburising diffuses carbon into a low-carbon steel's surface; nitriding and induction/flame hardening harden a thin case while leaving the core ductile and shock-tolerant.
Reading hardness
Hardness is the quick proxy for strength and wear resistance. Rockwell C (HRC) is standard for hardened steel;
Brinell (HB) and Vickers (HV) suit softer or thin parts. They interconvert approximately, and for steel
tensile strength (MPa) ≈ 3.45 × HB. Convert between scales with the
Hardness Converter, look up alloys in the
material reference, and remember hardness feeds straight into
fatigue life.