Boron steel

Boron is added to steel as ferroboron (~12-24% B). As the ferroboron addition lacks protective elements it is usually added after oxygen scavengers have been added. Proprietary additives also exist with oxygen/nitrogen scavengers – one such contains 2% B plus Al, Ti, Si.^[1] Oxygen, carbon, and nitrogen react with boron in steel to form B₂O₃ (boron trioxide); Fe₃(CB) (iron boroncementite) and Fe₂₃(CB)₆ (iron boroncarbide); and BN (boron nitride) respectively.^[2]

Soluble boron arranges in steels along grain boundaries. This inhibits the γ-α transformations (austenite to ferrite transformation) by diffusion and therefore increases the hardenability, with an optimal range of ~ 0.0003 to 0.003% B.^[1] Additionally Fe₂B has been found to precipitate at grain boundaries, which may also retard the γ-α transformations .^[1] At higher B values Fe₂₃(CB)₆ is thought to form, which promotes ferrite nucleation, and so adversely affects hardenability.^[1]

Boron is effective at very low concentrations – 30 ppm B can replace an equivalent 0.4% Cr, 0.5% C, or 0.12% V.^[2] 30 ppm B has also been shown to increase depth of hardening (~ +50%) in a low-alloy steel – thought to be due to its retardation of austenite decomposition to softer bainite, ferrite, or pearlite structures on cooling from an austenitization treatment.^[2]

The presence of carbon in steel reduces the relative effectiveness of boron in promoting hardenability.^[2]

At above 30 ppm boron begins to reduce hardenability, increases brittleness, and can cause hot shortness.^[2]

The Fe-B phase diagram has two eutectic points – at 17% (mol) m.p. 1149 °C; and 63.5% boron m.p. ~1500 °C. There is a peak m.p. at 1:1 Fe:B, and an inflexion at 33% B, corresponding to FeB and Fe₂B respectively.^[1]

The solubility of boron in steel is thought to be 0.021% at 1149 °C, dropping to 0.0021% at 906 °C.^[1] At 710 °C only 0.00004% boron dissolves in γ-Fe (Austenite).^[1]