Iron-iron carbide phase diagram

The Fe-Fe$_3$C diagram shows the phases of plain carbon steels and cast irons against carbon content (0 to 6.67 wt% C, where Fe$_3$C cementite forms) and temperature, under slow cooling.

 T(C)   (schematic)
 1538 +   Liquid (L)
 1495 +-- delta / peritectic (0.17%C)
 1147 +--- L + g ---- eutectic (4.3%C) -- L+Fe3C
      |  Austenite (g)   g+Fe3C   Ledeburite
  912 +-- a+g
  727 +--- eutectoid (0.76%C) ------------------
      |  Ferrite+pearlite | Pearlite+Fe3C | Cast
      +-----|-------|---------|-------|-----> %C
          0.022   0.76      2.14    4.3    6.67

Phases

• Ferrite ($\alpha$): interstitial solid solution of C in BCC iron, max 0.022% C at 727 °C. Soft, ductile.
• Austenite ($\gamma$): C in FCC iron, max 2.14% C at 1147 °C. Non-magnetic, ductile, stable above 727 °C.
• $\delta$-ferrite: C in BCC iron, up to 0.09% C at 1495 °C.
• Cementite (Fe$_3$C): hard, brittle compound with 6.67% C.
• Liquid (L).

Invariant reactions

Critical lines: A$_1$ (727 °C), A$_3$ (GOS line, ferrite starts to form), A$_{cm}$ (SE line, cementite starts to form from austenite).

Classification by carbon

• Steel: up to 2.14% C (hypoeutectoid below 0.76, hypereutectoid 0.76-2.14).
• Cast iron: 2.14-6.67% C (hypoeutectic below 4.3, hypereutectic above).

Applications

• Selecting austenitising temperatures for annealing, normalising, hardening.
• Predicting microstructure and phase fractions with the lever rule.
• Understanding properties of steels and cast irons, and welding behaviour.

Limitations

• It is an equilibrium diagram (very slow cooling); real heat treatment produces non-equilibrium phases such as martensite and bainite, which do not appear on it.
• Alloying elements (Cr, Ni, Mn) shift transformation temperatures and the eutectoid composition; the diagram is valid only for plain carbon steel.
• It gives no information about the rate of transformation (TTT/CCT diagrams are needed).
• Cementite is actually metastable; graphite is the stable phase (Fe-C diagram).