This portion of the iron–carbon phase diagram shows the rich complexity
of carbon steel's crystalline composition. Below 912 °C, the stable form of pure iron is ferrite,
a body-centered cubic lattice. Above 912 °C, its stable form is face-centered cubic austenite.
Adding carbon stabilizes the austenite phase relative to the ferrite phase and therefore depresses
the transition temperature until it reaches a minimum of 727 °C for iron containing 0.77%
carbon by weight.
Carbon is almost insoluble in ferrite; its maximum concentration peaks
at only 0.0218% by weight at a temperature of 727 °C. But austenite containing 0.77% dissolved
carbon by weight is stable at that temperature. That strong dependence of solubility on crystal
structure leads to interesting phase separations in steel.
Below 727 °C, all but the smallest quantity of carbon is accommodated
as cementite (iron carbide) so that at equilibrium, cool carbon steel consists of phase-separated
ferrite and cementite. Above 727 °C, some or all of the carbon can be accommodated in austenite.
Steel with the eutectoid composition, 0.77% carbon by weight, becomes pure austenite above 727 °C.
Carbon steel with less than the eutectoid composition—hypoeutectoid
steel—has too little carbon to become pure austenite at 727 °C. So it forms a phase-separated
mixture of ferrite (the carbon-poor phase) and austenite (the carbon-rich phase). Above the eutectoid
composition, hypereutectoid steel has too much carbon to become pure austenite at 727 °C,
so it phase separates into austenite (carbon poor) and cementite (carbon rich).
To harden steel, it must be heated until it is fully austenized. That transformation
is completed just above 727 °C for eutectoid steel, while both hypo- and hypereutectoid steels
require higher temperatures. Quenching then destabilizes the austenite and leads to steel consisting
of nonequilibrium composition; in particular, quenching allows martensite to form.
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