Transformations in steel

The susceptibility of iron-carbon alloys to solid-state transformations enables their structure to be altered by heat treatment.

There are two crystal structures that pure iron forms at atmospheric pressure, each can be described by a repeating unit cell. The crystal structures are the body-centred cubic (alpha-iron or delta-iron), and face-centred cubic (austenite or gamma-iron) forms. At ambient temperatures, and at pressures in excess of 130 kbar, hexagonal close-packed becomes the stable form (epsilon-iron).

The addition of solute elements can change the relative stabilities of the austenite and ferrite. Alloying elements can either substitute for an atom in the lattice or when the solute atoms are small enough, they may occupy the interstitial sites between the solvent atoms. Carbon is an important addition to steel; because it has a relatively small size it occupies the interstitial sites in the lattice. The solubility of carbon is much higher in austenite than ferrite and although graphite forms in cast irons (2-4 wt% carbon), it is difficult to obtain in steels (0.03-1.5 wt% carbon). The metastable equilibrium structure at lower temperatures when carbon is present in concentrations greater that its solubility in ferrite is therefore usually a mixture of ferrite and iron-carbides.

A metastable equilibrium structure containing cementite (instead of graphite) is more easily approached during slow cooling. An intimate mixture of ferrite and cementite lamellae is formed by diffusion, a microstructure first identified by Sorby who named it `pearlite'. With sufficiently fast cooling the austenite transforms without diffusion, into a super-saturated solution known as martensite. The parent lattice is deformed with respect to that of the product, so the transformation is often called `displacive'. The martensite e forming in steels is frequently known as alpha' , it has a body-centred tetragonal structure, similar to the body-centred cubic structure of ferrite, but distorted by the misfitting carbon atoms. Displacive transformations like the martensitic transformation in steels occur in many different materials.