Phases and the carbon composition window
The iron-carbon phase diagram maps the equilibrium phases (austenite, ferrite, cementite, pearlite) across composition (0 to 6.67 percent carbon by weight) and temperature (20 to 1600+ degrees Celsius). Pure iron is ferrite at room temperature and transforms to austenite at 912 degrees C. Carbon expands the austenite region, forming steel alloys that cool into pearlite or martensite depending on cooling rate and carbon content.
Pearlite, cementite, and the eutectoid reaction
At 0.77 percent carbon (the eutectoid composition), austenite transforms to pearlite, a layered mixture of ferrite and cementite (iron carbide, Fe3C), at 727 degrees C. This microstructure gives moderate strength and toughness. Hypoeutectoid steels (less than 0.77 percent carbon) form ferrite plus pearlite; hypereutectoid steels (more than 0.77 percent) form cementite networks around pearlite. Cast irons (2 to 6 percent carbon) separate graphite or cementite depending on cooling rate and alloying.
Design implications and rapid cooling
Engineers select carbon content and cooling rate to achieve desired hardness and toughness. Low-carbon steels (0.1 to 0.3 percent) remain soft and ductile; medium-carbon steels (0.3 to 0.7 percent) offer a balance suitable for machinery; high-carbon steels (0.7 to 1.2 percent) harden and hold an edge for tools. Rapid cooling bypasses the diagram's slow-equilibrium predictions, forming martensite and enabling precise control through tempering.