Below 912 °C (1,674 °F), iron has a body-centered cubic (bcc) crystal structure and is known as α-iron or ferrite. It is thermodynamically stable and a fairly soft metal. α-Fe can be subjected to pressures up to ca. 15 GPa before transforming into a high-pressure form termed ε-Fe discussed below.
Magnetically, α-iron is paramagnetic at high temperatures. However, below its Curie temperature (T or A) of 771 °C (1044K or 1420 °F), it becomes ferromagnetic. In the past, the paramagnetic form of α-iron was known as beta iron (β-Fe). Even though the slight tetragonal distortion in the ferromagnetic state does constitute a true phase transition, the continuous nature of this transition results in only minor importance in steel heat treating. The A line forms the boundary between the beta iron and alpha fields in the phase diagram in Figure 1.
Similarly, the A boundary is of only minor importance compared to the A (eutectoid), A and A critical temperatures. The A, where austenite is in equilibrium with cementite + γ-Fe, is beyond the right edge in Fig. 1. The α + γ phase field is, technically, the β + γ field above the A. The beta designation maintains continuity of the Greek-letter progression of phases in iron and steel: α-Fe, β-Fe, austenite (γ-Fe), high-temperature δ-Fe, and high-pressure hexaferrum (ε-Fe).
The primary phase of low-carbon or mild steel and most cast irons at room temperature is ferromagnetic α-Fe.  It has a hardness of approximately 80 Brinell. The maximum solubility of carbon is about 0.02 wt% at  and 0.001% at . When it dissolves in iron, carbon atoms occupy interstitial "holes".  Being about twice the diameter of the tetrahedral hole, the carbon introduces a strong local strain field.
Mild steel (carbon steel with up to about 0.2 wt% C) consists mostly of α-Fe and increasing amounts of cementite (FeC, an iron carbide).  The mixture adopts a lamellar structure called pearlite. Since bainite and pearlite each contain α-Fe as a component, any iron-carbon alloy will contain some amount of α-Fe if it is allowed to reach equilibrium at room temperature. The amount of α-Fe depends on the cooling process.