§1The iron-carbon idea
Heat treatment works because steel changes its internal structure with temperature. Heated past a critical point it becomes austenite, in which carbon dissolves freely; how that austenite is then cooled decides everything.
The key landmark is the eutectoid: at 0.77 % carbon and 727 °C, austenite transforms on slow cooling directly into pearlite, a fine layered mix of soft ferrite and hard iron carbide. Cool slowly and you get this soft, layered structure; cool fast and the carbon is trapped before it can separate, producing hard martensite instead. Every heat treatment on this page is a way of steering that cooling — slow to soften, fast to harden, then a reheat to balance. The partial iron-carbon diagram in the hero shows the austenite field and the eutectoid point that anchors it.
Contents§2The lever rule
Below the eutectoid, a steel is a mix of two constituents, and their proportions are read off the diagram by the lever rule — a simple weighted balance across the composition.
A medium-carbon steel at 0.40 % C, cooled slowly, is pro-eutectoid ferrite plus pearlite. The pearlite fraction = (0.40 − 0.022)/(0.77 − 0.022) = 0.378/0.748 = 50.5 %, so the steel is very nearly half ferrite, half pearlite — soft ferrite for toughness, hard pearlite for strength, in balance. Drop to 0.20 % C and the pearlite falls to 23.8 % (softer, tougher); rise to the eutectoid 0.77 % C and it is 100 % pearlite. The lever rule turns the carbon figure straight into the structure, and the structure into the properties.
§3Hardening by quench
To harden a steel, heat it into the austenite field and then cool it fast enough to trap the carbon — forming martensite, the hard, brittle structure that gives quenched steel its edge.
The sequence is heat, soak, quench. Heating dissolves the carbon into austenite; quenching in water, oil or air (the medium set by the steel’s hardenability, per the tool-steel page) cools it so fast that the carbon cannot diffuse out, leaving a strained, extremely hard martensite. The catch is that as-quenched martensite is also brittle — hard enough to be useful, but too brittle to trust under shock — and the quench itself sets up internal stresses that can distort or crack the part. A freshly hardened steel is therefore rarely used as-is; it is almost always tempered next.
Contents§4Tempering
Tempering reheats the hardened steel to a moderate temperature to trade a little hardness for a large gain in toughness — the essential second step of hardening.
By warming quenched steel to somewhere between about 150 °C and 650 °C and holding, some of the trapped carbon precipitates and the martensite relaxes: hardness falls a little, brittleness falls a great deal, and the internal quench stresses are relieved. The temperature is the control — a low temper keeps most of the hardness for a knife edge or a cutting tool, a high temper gives up more hardness for the toughness a shaft or a gear needs. “Hardened and tempered” is thus a single decision about where on the hardness-toughness line the part should sit, and it is why every quench is followed by a temper. Carbon steels tempered much above 200 °C begin to lose their edge — the very limit that red-hardness tool steels are designed to beat.
Contents§5Annealing and normalising
The opposite of hardening: heat treatments that soften steel, relieve stress and refine its grain, by cooling slowly rather than fast.
Annealing heats the steel into the austenite range and cools it very slowly, usually in the furnace, producing the softest, most ductile, most machinable and stress-free condition — the state to work a steel in before a final hardening. Normalising does much the same but cools in still air, a little faster, giving a slightly harder and stronger result with a fine, uniform grain — often the condition a part is supplied and used in. Stress-relieving uses a lower temperature simply to relax the internal stresses left by welding, machining or forming, without changing the structure. Where hardening traps and strains, these treatments release and refine — the same transformations run gently in reverse.
Contents§6Surface hardening
Many parts need a hard, wear-resisting skin over a tough, shock-absorbing core — a gear tooth, a cam, a shaft journal. Surface hardening delivers exactly that split.
Two families do it. Case hardening (carburising, nitriding) diffuses carbon or nitrogen into the surface of a low-carbon steel at high temperature, so only the enriched skin hardens on quenching. The depth follows a diffusion law — case depth grows with the square root of time.
If a process reaches a 0.5 mm case in 4 hours, the depth constant is 0.5/√4 = 0.25 mm/√h. To double the case to 1.0 mm needs t = (1.0/0.25)² = 16 hours — four times as long for twice the depth, the hallmark of a √t diffusion process.
Selective hardening (flame, induction) instead heats just the surface of an already medium-carbon steel fast enough to austenitise the skin alone, then quenches — hardening the surface by where the heat goes rather than by adding carbon. Both leave the core soft and tough, which is precisely what a loaded gear or shaft needs.
Contents§7Quick reference
The working core of the page on one card rack.
Eutectoid
0.77 % C · 727 °C
→ pearlite (slow cool)
Lever rule
%pearlite = (C−0.022)/(0.77−0.022)
0.40 % C → ~50/50
Harden
austenitise + quench
→ hard, brittle martensite
Temper
reheat 150–650 °C
↓ hardness, ↑ toughness
Soften
anneal (furnace) · normalise (air)