Beta ferrite
Beta ferrite (β-Fe) and beta iron (β-iron) are obsolete terms for the paramagnetic form of alpha ferrite (α-Fe). The primary phase of low-carbon or mild steel and most cast irons at room temperature is ferromagnetic ferrite (α-Fe). As iron or ferritic 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 in the 3d shell. The A2 forms the low-temperature boundary of the beta iron field in the phase diagram in Figure 1. Beta ferrite is crystallographically identical to alpha ferrite, 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: alpha ferrite (α-Fe), beta ferrite (β-Fe), austenite (γ-Fe), high-temperature delta ferrite (δ-Fe) and high-pressure hexaferrum (ε-Fe).
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