Iron-carbon diagram

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Processed iron ( steel and cast iron ) always contains a certain amount of carbon , the proportion of which determines the properties of the steel and cast iron. The iron-carbon diagram (EKD) is an equilibrium diagram for the binary iron-carbon system, from which the phase composition can be read off depending on the carbon content and the temperature .

So-called time-temperature conversion diagrams are used if the temporal structure development is of interest with cooling at different speeds .

Carbon is the most important alloy component of steel, as even the smallest changes in the carbon content have a major impact on the properties of the material. However, the informative value of the iron-carbon diagram becomes less the faster it is cooled or heated or the greater the content of other alloying elements. The EKD is represented in two forms: the metastable system (Fe-Fe 3 C), in which the carbon occurs in bound form, and the stable system (Fe-C) with elemental carbon in the form of graphite . The two systems are usually shown in a diagram and marked accordingly. In practice, however, the metastable Fe-Fe 3 C system is mainly used.

Representation of the phases in the iron-carbon diagram

Iron-carbon diagram (black: metastable, red: stable) with structure description

The mass percent of carbon is plotted on the x-axis and the temperature on the y-axis . The diagram only shows the technically interesting carbon content of 0 to 6.67%. The latter corresponds to a cementite content of 100%.

The lines represent the breakpoints or breakpoints shifted to other temperatures and delimit the individual phase fields from one another. The significant points are marked with letters. In some representations, point I is referred to as point J.

The ABCD line represents the liquidus line, above this the alloy is liquid, the AHIECF line corresponds to the solidus line below which the alloy has completely solidified . In the temperature interval between the liquidus and solidus temperature, the alloy has a pulpy consistency and consists of residual melt, δ-iron, γ-iron and cementite (Fe 3 C) in varying concentrations and proportions. If the temperature falls below the liquidus line when the alloy cools, primary crystallization from the melt begins .

Due to the various allotropic modifications of iron, different phases are formed depending on the carbon content. The iron forms different intercalation mixed crystals (δ-, γ- and α- mixed crystals ) with different solubilities for carbon. The reasons for the different solubility of the individual mixed crystals are the different spatial lattices and lattice constants . The metallographic designations of mixed crystals are δ- ferrite for δ-mixed crystals, austenite for γ-mixed crystals and ferrite for α-mixed crystals.

Rough subdivision

  • Melt refers to the liquid iron-carbon alloy. Above the liquidus line, the entire alloy is present as a melt, between the liquidus and solidus line as a mixed crystal component with melt and below the solidus line as mixed crystals.
  • δ ferrite (delta ferrite): body-centered cubic crystal structure
  • Austenite (γ mixed crystals): face-centered cubic crystal structure
  • α-Ferrite (Alpha-Ferrite): body-centered cubic crystal structure
  • Graphite (stable system) or cementite (Fe 3 C; metastable system)

When perlite and Ledeburit is not about phases, but special phase mixtures ( structure ). These arise only in a stable or metastable system, i.e. with slow cooling. On the other hand, with rapid cooling (e.g. quenching in water), austenite becomes martensite . Martensite is a hard and brittle structure. In the metastable system there are the following characteristic points, lines and phenomena:

  • Points
    • A: (0% / 1536 ° C) B: (0.53% / 1493 ° C) C: (4.3% / 1147 ° C) D: (6.67% / 1320 ° C) E: (2nd , 06% / 1147 ° C)
    • F: (6.67% / 1147 ° C) G: (0% / 911 ° C) H: (0.1% / 1493 ° C) I: (0.16% / 1493 ° C) K: (6 , 67% / 723 ° C)
    • N: (0% / 1392 ° C) P: (0.022% / 723 ° C) S: (0.8% / 723 ° C) Q: (0.002% / 20 ° C) M: (0% / 769 ° C)
    • S ': (0.69% / 738 ° C) E': (2.03% / 1153 ° C) C ': (4.25% / 1153 ° C)
  • Lines
  • Phenomena

Metallographic designation

The metallographic designations of mixed crystals are δ-ferrite for δ-mixed crystals, austenite for γ-mixed crystals and ferrite for α-mixed crystals.

designation Max. C content Metallographic designation
δ mixed crystal 0.10% at 1493 ° C δ ferrite
γ mixed crystal 2.06% at 1147 ° C Austenite
α mixed crystal 0.02% at 723 ° C ferrite

The iron-carbon compound Fe 3 C or cementite is also a phase, but this should not be confused with mixed iron crystals, it is an intermetallic / intermediate phase. Cementite occurs in three different forms, whereby the chemical composition of the cementite is always the same:

designation Emergence
Primary cementite primary crystallization from the melt (line CD)
Secondary cementite Precipitation from the austenite (line ES)
Tertiary cementite Precipitation from the ferrite (line PQ)

In addition to the pure phases, phase mixtures also occur:

designation Consists Realm of existence
Perlite 88% ferrite and 12% cementite 0.02% to 6.67% at T≤723 ° C
Ledeburit I 51.4% austenite and 48.6% cementite 2.06% to 6.67% at 723 ° C≤ T ≤1147 ° C
Ledeburit II 51.4% pearlite and 48.6% cementite 2.06% to 6.67% at T ≤723 ° C

Isothermal reactions

The iron-carbon diagram shows three isothermal reactions, one peritectic (line HIB), one eutectic (line ECF) and one eutectoid (line PSK). Point H: Maximum C solubility in the δ ferrite. Point J: peritectic δ + S → γ.

When the steel is heated or cooled, breakpoints are created on some lines that mark the individual transformations. The most important are:

  • A 1 - PSK line, at 723 ° C the austenite disintegrates into pearlite, at a carbon content> 0.02%
  • A 2 - line MO, denotes the loss of ferromagnetism of ferrite when heated above 769 ° C ( Curie point ).
  • A 3 - line GOS, if this line is not reached during cooling, low-carbon α-mixed crystals (ferrite) form, the released carbon accumulates in the austenite until it has reached the eutectoid concentration at 723 ° C.

The line ECF is called the eutectic line or also eutectical, because this is where the eutectic arises. If the melt has the eutectic concentration of 4.3% C, it transforms isothermally at 1147 ° C (point C) into a mixed structure of austenite and cementite, the so-called ledeburite.

The line HJB is called the peritectic line or peritectical, here the δ mixed crystals with 0.1% C and residual melt with 0.51% C settle isothermally at 1493 ° C to γ ​​mixed crystals with 0.16% C (point I) .


With the help of the iron-carbon diagram, for example, some questions about the different behavior of steel (forgeable iron alloy, usually with <2.06% C) and cast iron (non-forgeable iron alloy, usually with> 2.06% C) can be explained:

  • Steel can be forged because it can be easily deformed in the wide, homogeneous austenite range. This behavior does not occur with cast iron because the larger proportions of carbon in the form of graphite or ledeburite make deformability more difficult and the transition into the melt is abrupt.
  • The melting temperature of pure iron is 1536  ° C , the temperatures of complete solidification (or beginning of melting) of steel (AHIE line) and cast iron (ECF line at 1147 ° C) can also be read. The lower melting point of cast iron is one of the reasons why it is better and easier to cast than steel.

The graphical representation of these properties as a function of the carbon content makes the iron-carbon diagram an important tool for assessing and understanding the various iron alloys that make up one of the most commonly used groups of materials.


  • Hermann Schumann, Heinrich Oettel: Metallography . 14th edition. Wiley-VCH Verlag.
  • Hans-Jürgen Bargel, Günter Schulze: Material technology . 8th edition. Springer Verlag Berlin.
  • Volker Läpple: Heat treatment of steel . 9th edition. Publishing house Europa-Lehrmittel.

Individual evidence

  1. a b Christoph Broeckmann: Material Science 1. Aachen 2015, p. 277 ff.
  2. Dr. Volker Läpple: Heat treatment of steel basics, processes and materials. 8th edition. Europa-Lehrmittel Nourney, Vollmer, Haan 2003, ISBN 3-8085-1308-X .
This version was added to the list of articles worth reading on September 28, 2005 .