Iron-carbon phase diagram

Processed iron, such as steel and cast iron, always contains a proportion of carbon. This amount of carbon is decisive for the quality and properties of the steel. The iron-carbon phase diagram is an equilibrium diagram of the two elements. With its help, the relationship between the carbon content and the temperature is derived. On this basis, the phase composition can be determined.

Carbon is the most important alloying element in iron. For this reason, even the smallest changes in carbon content can have massive changes in the characteristics of the material. However, the importance of the iron-carbon phase diagram decreases rapidly if the material is rapidly cooled or heated. The diagram is also less meaningful if the proportion of other alloying elements increases.

Carbon occurs in two different forms: firstly in bound form and secondly as elementary carbon in the form of graphite. This is why the iron-carbon phase diagram appears in two forms. A stable system with Fe-Graphite diagram and a metastable with a Fe-Fe3C diagram. Both systems can be represented in one diagram, although in practice the metastable Fe-Fe3C system is mostly used.

How are phases represented in the iron-carbon phase diagram?

The x-axis of the diagram represents the mass percent of carbon. The temperature is plotted on the y-axis. To make the diagram clearer, only the technically interesting carbon content from 0 to 6.67% is shown. Alloys that contain more than 6.67% of carbon form a phase of 100% cementite

The phase fields are delimited by lines representing breakpoints shifted to the other temperatures. For better understanding, the relevant points are marked with letters. Note that some diagrams show point I as J. One of the most important lines is the liquidus line, represented by the ABCD polyline. Above this line, the alloy is present in liquid form. The AHIECF polyline is called the solidus line. Below this line the alloy is completely rigid. If the temperature is in between, the alloy has a mushy consistency. The alloy consists of residual melt, δ-iron, γ-iron and cementite (Fe3C). The proportions are fluid and change depending on the temperature. As soon as the temperature falls below the liquidus line during cooling of the alloy, primary crystallization from the melt starts.

Iron has various allotropic modifications. Thus, different phases are formed depending on the carbon content and temperature. The intercalation mixed crystals formed by iron δ-, γ- and α-solid solutions have different solubilities for carbon. The variations are caused by the different spatial lattices and lattice constants. 

What is the metallographic designation?

In metallography, the mixed crystals are referred to as δ ferrite, austenite for γ mixed crystals and ferrite for α mixed crystals. Here is an overview of the carbon content of the individual phases:

Designation

Max C-content

Metallographic designation

δ-solid solution

0,10 % bei 1493° C

δ-Ferrite

γ-solid solution

2,06 % bei 1147° C

Austenite

α-solid solution

0,02 % bei 723° C

Ferrite

Cementite (Fe3C) is an iron-carbon compound which is also a phase. However, cementite is an intermediate phase which should not be confused with iron mixed crystals. The chemical composition of cementite is always the same, although it occurs in three different forms:

  • Primary cementite: primary crystallization from the melt (corresponds to the line CD)
  • Secondary cementite: precipitation from the austenite (corresponds to the ES line)
  • Tertiary cementite: precipitation from the ferrite (corresponds to the PQ line)

The secondary cementite is present at a carbon content between 2.06 and 4.3 % C, but is not shown in the diagram. This is because it cannot be detected metallographically.

In addition to the phases, phase mixtures also occur:

Designation

Consists of

Area of existence

Perlite

88 % Ferrite / 12 % cementite

0,02 % - 6,67 % bei T ≤ 723° C

Ledeburite I

51,4 % Austenite / 48,6 % cementite

2,06 % - 6,67 % bei 723° C ≤ 1147° C

Ledeburite II

51,4 % Perlite / 48,6 % cementite

2,06 % - 6,67 % bei T ≤ 723° C

What isothermal reactions does the iron carbon phase diagram show?

Three isothermal reactions are shown in the iron-carbon phase diagram. The line HIB represents a peritectic, the line ECF an eutectic and the left PSK an eutectoid reaction. 

When steel is heated or cooled, transformations occur on the lines. These are marked by breakpoints. Here are the most important ones:

  • At the P-S-K line, austenite decays to pearlite if the carbon content is less than 0.02 % (A1).
  • Ferrite loses ferromagnetism at the M-O line when heated above 769° C (A2)
  • If the temperature falls below the G-O-S line during cooling, low-carbon ferrite is formed. During this process, austenite accumulates with the released carbon until the temperature rises to 723°C and it reaches an eutectoid concentration (A3).

How is the iron-carbon diagram applied?

The iron-carbon phase diagram helps for better understanding the behavior of cast iron and steel. Steel, for example, is easy to form in the austenite range and can therefore be forged.  Cast iron on the other hand has a higher proportion of carbon, which is present in the form of graphite and ledeburite. This significantly limits the malleability. 

For this reason, the iron-carbon diagram becomes an important tool for evaluating steel and cast iron.