β铁是一种已过时的名词，以前用来称呼钢铁的一种显微组织，后来发现其实就是温度超过临界温度A₂，由铁磁性变为顺磁性的肥粒铁。

低碳钢、中碳钢及大部份铸铁在室温下的主要相都是肥粒铁（α铁），当铁或肥粒铁钢加热超过临界温度A₂，也称为居里温度771 °C（1044K或1420 °F），，其原子因热的随机运动已超过3d轨域未成对自旋电子的磁矩，A₂是图1相图中β铁的温度下界，β铁在晶体学上α铁完全相同，只有磁畴及和温度有关的延伸立方晶系晶格参数不同，因此在热处理上的重要性不大。因为这个原因，β相半多不会视为一个独立的相，会视为是α铁在高温下的情形，而the A₂温度的重要性也比A₁（共晶温度）、A₃、A_cm临界温度要低。A_cm是奥氏体和碳化三铁+γ-Fe平衡的状态，已超过图1的右边界。在技术上来说，α相 + γ相的区域在超过A₂之后即为β相 + γ相。β铁的命名维持用希腊字母为铁及钢的相命名的习惯：肥粒铁（α-Fe）、β铁（β-Fe）、奥氏体（γ-Fe）、高温的δ铁（δ-Fe）及高压下的Ε铁（ε-Fe）。

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).