In their simplest form, steels are alloys of Iron (Fe) and Carbon (C). The study of the constitution and structure of iron and steel start with the iron carbon phase diagram. It is also the basic understanding of the heat treatment of steels.

Iron Carbon phase diagram

iron carbon phase diagram

On this diagram, the carbon percentage is shown on the x-axis and temperature on the y-axis. This figure shows the Iron Carbon Equilibrium Diagram. In this diagram, the lines indicate the boundaries where the alloy changes its phase. The different phases or mixture of phases occur in different areas enclosed by these curves. Pure iron exists in two allotropic forms, ∝-iron, γ-iron, both in the solid state. The ∝-iron exists between 910°C, and also above 1392°C, and its crystal lattice body-centred cubic. The ∝-iron which exists above 1392°C is also called δ-iron. The γ-iron exists in the range 910°C to 1392°c, and its crystal is face-centred cubic. The melting point of iron is 1539°C.

In Fe-C system is in the solid state, the different phases which are present are Ferrite(Solid solution), Austenite, Cementite(Chemical compound iron Carbide ), and free carbon in the allotropic form of graphite.

Steel is an alloy of carbon and iron and other alloying elements (e.g. Mn, Si) with carbon content up to 2% intended for wrought products or semi-products. Cast iron is an alloy of carbon and iron and other alloying elements (e.g. Mn, Si) with carbon content over 2% intended for castings. Now, we consider only a part of Fe-Fe3C diagram referring to steel. Perlite is a structure (i.e. consists of two phases) consists of alternate layers of ferrite and cementite in the proportion 87:13 by weight. Perlite is formed from austenite at the eutectoid temperature (A1) 727°C upon slow cooling. There are three groups of steels according to carbon content: – hypereutectoid steels containing less than 0.76% C – eutectoid steel with carbon content about 0.76% – hypereutectoid steels contain more than 0.76% C (up to 2% C).


The austenite-ferrite transformation

Under equilibrium conditions, pro-eutectoid ferrite will form in iron-carbon alloys containing up to 0.8 percent carbon. The reaction occurs at 910 Deg. C in pure iron, but takes place between 910 Deg. C and 723 Deg. C in iron-carbon alloys.
However, by quenching from the austenitic state to temperatures below the eutectoid temperature Ae1, ferrite can be formed down to temperatures as low as 600 Deg. C. There are pronounced morphological changes as the transformation temperature is lowered, which it should be emphasized apply in general to hypo-and hyper-eutectoid phases, although in each case there will be variations due to the precise crystallography of the phases involved. For example, the same principles apply to the formation of cementite from austenite, but it is not difficult to distinguish ferrite from cementite morphologically.
The austenite-cementite transformation
The Dube classification applies equally well to the various morphologies of cementite formed at progressively lower transformation temperatures. The initial development of grain boundary allotriomorphs is very similar to that of ferrite, and the growth of side plates or Widmanstaten cementite follows the same pattern. The cementite plates are more rigorously crystallographic in form, despite the fact that the orientation relationship with austenite is a more complex one.
As in the case of ferrite, most of the side plates originate from grain boundary allotriomorphs, but in the cementite reaction more side plates nucleate at twin boundaries in austenite.

Iron Carbon phase diagram

iron carbon phase diagram
The austenite-pearlite reaction

Pearlite is the most familiar microstructural feature in the whole science of metallography. It was discovered by Sorby over a century ago, who correctly assumed it to be a lamellar mixture of iron and iron carbide.
Pearlite is a very common constituent of a wide variety of steels, where it provides a substantial contribution to strength. Lamellar eutectoid structures of this type are widespread in metallurgy, and frequently pearlite is used as a generic term to describe them.
These structures have much in common with the cellular precipitation reactions. Both types of reaction occur by nucleation and growth, and are, therefore, diffusion controlled. Pearlite nuclei occur on austenite grain boundaries, but it is clear that they can also be associated with both pro-eutectoid ferrite and cementite. In commercial steels, pearlite nodules can nucleate on inclusions.

It may be seen that the normal Iron-carbon equilibrium diagram represents the metastable equilibrium between iron and iron carbide. Cementite is metastable as the true equilibrium is between iron and graphite. Although graphite occurs extensively in cast irons (2 to 4 wt percent carbon), it is usually difficult to obtain this equilibrium phase in steel (0.03 to1.5 wt percent carbon). Therefore, the metastable equilibrium between iron and iron carbide is normally considered, since it is relevant to the behavior of a variety of steels in practice.
On comparing austenite (?-iron) with ferrite (?-iron) it is noticed that solubility of carbon is more in austenite with a maximum value of just over 2 wt percent at 1147 Deg. C. This high solubility of carbon in austenite is extremely important in heat treatment when solution treatment in the austenite followed by rapid quenching to room temperature allows the formation of a supersaturated solid solution of carbon in iron.
The ferrite phase is restricted with a maximum carbon solubility of 0.02 wt percent at 723 Deg. C. Since the carbon range available in common steels is from 0.05 to 1.5 wt percent, ferrite is normally associated with cementite in one or other form. Similarly, the ?-phase is very restricted and is in the temperature range between 1390 and 1534 Deg. C and disappears completely when the carbon content reaches 0.5 wt percent.