IRON–CARBON PHASE DIAGRAM

Iron–carbon (Fe–C) phase diagram shows the solubility of carbon in iron at different temperatures and corresponding structure of the steel. For describing the Fe–C phase diagram, the equilibrium between Fe and Fe–C is considered as metastable.

The larger phase field of γ-iron (austenite) compared with that of α-iron (ferrite) reflects the greater solubility of carbon, with a maximum value of just over 2% at 1,147°C (E) as shown in Figure 24.1. This high solubility of carbon in γ-iron is of extreme importance in heat treatment, when solution treatment in the γ-region followed by rapid quenching to room temperature allows a supersaturated solid solution of carbon in iron to be formed.

Figure 24.1 Fe–C Phase Diagram

The α-iron phase field is severely restricted, with a maximum carbon solubility of 0.02% at 723°C (P), so over the carbon range encountered in steels from 0.05 to 1.5 %, α-iron is normally associated with iron carbide in one form or another. Similarly, the δ-phase field is very restricted between 1,390°C and 1,534°C and disappears completely when the carbon content reaches 0.5 % (B).

The great difference in carbon solubility between γ– and α-iron leads normally to the rejection of carbon as iron carbide at the boundaries of the γ phase field. The transformation of γ to α-iron occurs via a eutectoid reaction, which plays a dominant role in heat treatment. The eutectoid temperature is 723°C while the eutectoid composition is 0.80% C. On cooling alloys containing less than 0.80% C slowly, hypoeutectoid ferrite is formed from austenite in the range 910–723°C with enrichment of the residual austenite in carbon, until at 723°C the remaining austenite, now containing 0.8% carbon transforms to pearlite (a lamellar mixture of ferrite and cementite). In austenite with 0.80–2.06% carbon, on cooling slowly in the temperature interval 1,147–723°C, cementite first forms progressively depleting the austenite in carbon, until at 723°C, the austenite contains 0.8% carbon and transforms to pearlite.

Steels with less than about 0.8% carbon are thus hypoeutectoid alloys with ferrite and pearlite as the prime constituents, the relative volume fractions being determined by the lever rule which states that as the carbon content is increased, the volume percentage of pearlite increases, until it is 100% at the eutectoid composition. Above 0.8% C, cementite becomes the hyper-eutectoid phase, and a similar variation in volume fraction of cementite and pearlite occurs on this side of the eutectoid composition.

There are several temperatures or critical points in the diagram, which are important, both from the basic and from the practical point of view.

• Firstly, there is the A₁, temperature at which the eutectoid reaction occurs, which is 723°C in the binary diagram.
• Secondly, there is the A₃, temperature when α-iron transforms to γ-iron. For pure iron this occurs at 910°C, but the transformation temperature is progressively lowered along the line GS by the addition of carbon.
• The third point is A₄ at which γ-iron transforms to δ-iron, 1,390°C in pure iron, but this is raised as carbon is added. The A₂ point is the Curie point when iron changes from the ferro-to the paramagnetic condition. This temperature is 769°C for pure iron, but no change in crystal structure is involved. The A₁, A₃, and A₄ points are easily detected by thermal analysis during cooling or heating cycles.