Standard pressure allotropes
Delta iron (δ-Fe)
As molten iron cools down, it solidifies at 1,538 °C (2,800 °F) into its δ allotrope, which has a body-centered cubic (BCC) crystal structure. δ-iron can dissolve as much as 0.09% of carbon by mass at 1,493 °C.
Gamma iron / Austenite(γ-Fe)
As the iron cools further to 1,394 °C its crystal structure 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 stainless steel.
Beta iron (β-Fe)
Beta iron (β-Fe) are obsolete terms for the paramagnetic allotrope of iron. The primary phase of low-carbon or mild steel and most cast irons at room temperature is ferromagnetic α-Fe. As iron or steel is heated above the critical temperature A2 or Curie temperature of 771 °C (1044K or 1420 °F), the random thermal agitation of the atoms exceeds the oriented magnetic moment of the unpaired electron spins. The A2 forms the low-temperature boundary of the beta iron field in the phase diagram in Figure 1. β-Fe is crystallographically identical to α-Fe, except for magnetic domains and the expanded body-centered cubic lattice parameter as a function of temperature, and is therefore of only minor importance in steel heat treating. For this reason, the beta "phase" is not usually considered a distinct phase but merely the high-temperature end of the alpha phase field.
Similarly, the A2 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 beta field and A2
critical temperature on the iron-rich side of the iron-carbon phase diagram.
A2 critical temperature and induction heating
β-Fe and the A2 critical temperature are important in induction heating of steel, such as for surface-hardening heat treatments. Steel is typically austenitized at 900–1000 °C before it is quenched and tempered. The high-frequency alternating magnetic field of induction heating heats the steel by two mechanisms below the Curie temperature: resistance or Joule (I2R) heating and ferromagnetic hysteresis losses. Above the A2, the hysteresis mechanism disappears and the required amount of energy per degree of temperature increase is substantially larger than below A2. Load-matching circuits may be needed to vary the impedance in the induction power source to compensate for the change.
Alpha iron (α-Fe)
Below 912 °C (1,674 °F) iron again adopts the BCC structure characteristic of α-iron, also called ferrite. The substance assumes a paramagnetic property. Carbon dissolves poorly in α-iron: no more than 0.021% by mass at 723 °C.
As it cools to 770 °C (1,418 °F), the Curie point (TC), the iron is a fairly soft metal and becomes ferromagnetic. As iron passes below the Curie temperature, no structural change occurs, but the magnetic properties as the magnetic domains become aligned. This form of iron is stable form at room temperature. α-Fe can be subjected to pressures up to ca. 15 GPa before transforming into a high-pressure form termed ε-iron, which crystallizes in a hexagonal close-packed (hcp) structure.
α-Fe is a component of steel and cast iron, conferring Ferromagnetism. It has a hardness of approximately 80 Brinell. the maximum solubility is about 0.02 wt% at 727 °C (1,341 °F) and 0.001% carbon at 0 °C (32 °F). 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) consist mostly of α-Fe and increasing amounts of cementite (Fe3C, an iron carbide). The mixture adopts a laminar 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.
Molar volume vs. pressure for α-Fe at room temperature.