The Iron-Carbon phase diagram (or more simply: Fe-C diagram) represents the microstructural evolution of ferrous alloys (steels and cast irons) as a function of the percentage of Carbon present and the temperature. The diagram is essential in evaluating, knowing and describing the technological and usage characteristics of ferrous alloys.

The various regions in the diagram make it possible to identify for which combinations of temperature and composition the alloys are in the liquid state, or have coexisting liquid and solid phases, or develop the different possible microstructures (austenite, cementite, graphite, ferrite, pearlite).

The part of the diagram used to study steels is the part where the carbon content varies between 0 and about 2%, while the cast irons area is associated with a variable carbon content between 2% and 6.67% (the area highlighted in yellow in the Fe-C diagram, simplified and illustrated in the following figure).

With regard to cast irons, coexistence (or rather, competition) between the systems should also be noted

  • Iron-cementite, at the base of white cast irons,
  • Iron-graphite, at the base of grey and spheroidal graphite irons.

By focusing on the system of greater interest, that is, iron-graphite, it is possible to distinguish between:

  • Eutectic cast irons: the solidification and cooling path is that indicated by the red line: liquid (1); eutectic austenite-graphite (2); transformation of austenite into ferrite/pearlite (3) and permanence of graphite;
  • Hypo-eutectic cast iron: the solidification and cooling path is that indicated by the green line: liquid (1); liquid + primary austenite (2); primary austenite + eutectic austenite-graphite (3); transformation of austenite into ferrite/pearlite (4) and permanence of graphite;
  • Hypereutectic cast iron: the solidification and cooling path is that indicated by the blue line: liquid (1); liquid + primary graphite (2); primary graphite + eutectic austenite-graphite (3); transformation of austenite into ferrite/pearlite (4) and permanence of graphite.
    In general, graphitic cast irons of practical interest are eutectic or slightly hypo-eutectic.

It should be noted, however, that the Fe-C diagram is nonetheless a simplification, since:

  • Cast irons contain other elements (silicon in particular) that influence the transformation temperatures and existence ranges of the various phases;
  • It is a “thermodynamic” diagram that, therefore, does not take into account the time variable: the study of the real microstructural evolution of a ferrous alloy is made by introducing the isothermal (TTT) and non-isothermal (CCT) transformation curves.

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