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 A_cm critical temperatures. The A_cm, 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).