At room temperature, the most stable form of iron is the body-centered cubic (bcc) structure (Fig. 1) known as ferrite or α-Fe (alpha iron). In this state, iron is relatively soft and can dissolve only a small amount of carbon – no more than 0.021 wt% at 910°C (1670°F).
Perhaps the most important polymorphic form of iron occurs when, in the form of steel, it is converted from austenite to martensite, a metastable structure with about four to five times the strength of ferrite. In fact, martensite is often referred to as supersaturated ferrite.
A minimum of 0.4 wt% of carbon is needed to form martensite. When austenite is rapidly quenched to form martensite, the carbon is "frozen" in place as the crystal structure attempts to change from fcc to bcc. The carbon atoms being too large to fit in the interstitial vacancies of the crystal lattice distort the structure into a body-centered tetragonal (bct) structure (Fig. 3). Martensite and austenite have an identical chemical composition. As such, martensite requires extremely little thermal activation energy to form. Austenite is transformed to martensite on quenching at approximately the speed of sound - too fast for the carbon atoms to come out of solution. The resulting transformation of the unit cell results in numerous lattice dislocations in each crystal, which consists of millions of unit cells. These dislocations make the crystal structure extremely resistant to shear stress and extremely strong.