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 i