Alpha iron (α-Fe) 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 (
TC or
A2) 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 A2 line forms the boundary between the beta iron and alpha fields in the
phase diagram in Figure 1. Similarly, the A2 boundary is of only minor importance compared to the A1 (
eutectoid), A3 and Acm critical temperatures. The Acm, 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 A2. 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 (Fe3C, 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.
A2 critical temperature and induction heating changes to a
face-centered cubic (fcc) crystalline structure. In this form it is called gamma iron (γ-Fe) or
austenite. γ-iron can dissolve considerably more carbon (as much as 2.04% by mass at 1,146 °C). This γ form of carbon saturation is exhibited in
austenitic stainless steel.
Delta iron (δ-Fe) Peculiarly, above 1,394 °C (2,541 °F), iron changes back into the bcc structure, known as δ-Fe. δ-iron can dissolve as much as 0.08% of carbon by mass at 1,475 °C. It is stable up to its melting point of 1,538 °C (2,800 °F). δ-Fe cannot exist above 5.2 GPa, with austenite instead transitioning directly to a molten phase at these high pressures. ==High pressure allotropes==