Boron steel refers to

steel Steel is an alloy made up of iron with added carbon to improve its strength and fracture resistance compared to other forms of iron. Many other elements may be present or added. Stainless steels that are corrosion- and oxidation-resistan ...

alloyed with a small amount of

boron Boron is a chemical element with the symbol B and atomic number 5. In its crystalline form it is a brittle, dark, lustrous metalloid; in its amorphous form it is a brown powder. As the lightest element of the '' boron group'' it has t ...

, usually less than 1%. The addition of boron to steel greatly increases the hardenability of the resulting alloy.

Description

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 scavengersone such contains 2% B plus Al, Ti, Si. Oxygen, carbon, and nitrogen react with boron in steel to form B₂O₃ (

boron trioxide Boron trioxide or diboron trioxide is the oxide of boron with the formula . It is a colorless transparent solid, almost always glassy (amorphous), which can be crystallized only with great difficulty. It is also called boric oxide or boria. It h ...

); Fe₃(CB) ( iron boroncementite) and Fe₂₃(CB)₆ ( iron boroncarbide); and BN (

boron nitride Boron nitride is a thermally and chemically resistant refractory compound of boron and nitrogen with the chemical formula BN. It exists in various crystalline forms that are isoelectronic to a similarly structured carbon lattice. The hexagonal ...

) respectively.

Hardenability

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. Additionally Fe₂B has been found to precipitate at grain boundaries, which may also retard the γ-α transformations . At higher B values Fe₂₃(CB)₆ is thought to form, which promotes ferrite nucleation, and so adversely affects hardenability. Boron is effective at very low concentrations30 ppm B can replace an equivalent 0.4% Cr, 0.5% C, or 0.12% V. 30 ppm B has also been shown to increase depth of hardening (~ +50%) in a low alloy steelthought to be due to its retardation of

austenite Austenite, also known as gamma-phase iron (γ-Fe), is a metallic, non-magnetic allotrope of iron or a solid solution of iron with an alloying element. In plain-carbon steel, austenite exists above the critical eutectoid temperature of 100 ...

decomposition to softer

bainite Bainite is a plate-like microstructure that forms in steels at temperatures of 125–550 °C (depending on alloy content). First described by E. S. Davenport and Edgar Bain, it is one of the products that may form when austenite (the face-c ...

, ferrite, or

pearlite Pearlite is a two-phased, lamellar (or layered) structure composed of alternating layers of ferrite (87.5 wt%) and cementite (12.5 wt%) that occurs in some steels and cast irons. During slow cooling of an iron-carbon alloy, pearlite form ...

structures on cooling from an austenitization treatment. The presence of carbon in steel reduces the relative effectiveness of boron in promoting hardenability. At above 30 ppm boron begins to reduce hardenability, increases brittleness, and can cause hot shortness.

Phase diagram

The Fe-B phase diagram has two eutectic pointsat 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. The solubility of boron in steel is thought to be 0.021% at 1149 °C, dropping to 0.0021% at 906 °C. At 710 °C only 0.00004% boron dissolves in γ-Fe (

Austenite Austenite, also known as gamma-phase iron (γ-Fe), is a metallic, non-magnetic allotrope of iron or a solid solution of iron with an alloying element. In plain-carbon steel, austenite exists above the critical eutectoid temperature of 100 ...

Uses

Boron alloy steels include carbon, low alloy including HSLA, carbon-manganese and tool steels. Because of boron's high

neutron absorption Neutron capture is a nuclear reaction in which an atomic nucleus and one or more neutrons collide and merge to form a heavier nucleus. Since neutrons have no electric charge, they can enter a nucleus more easily than positively charged protons, ...

boron is added to

stainless steel Stainless steel is an alloy of iron that is resistant to rusting and corrosion. It contains at least 11% chromium and may contain elements such as carbon, other nonmetals and metals to obtain other desired properties. Stainless steel's r ...

s used in the nuclear industryup to 4% but more typically 0.5 to 1%. Boron steels find use in the car industry, typically as strengthening elements such as around door frames and in reclining seats. As of the mid 2000s it was in common use by European car manufacturers. The introduction of boron steel elements introduced issues for accident scene rescuers as its high strength and hardness resisted many conventional cutting tools ( hydraulic rescue tools) in use at that time. Flat boron steel for automotive use is hot stamped in cooled molds from the austentic state (obtained by heating to 900-950 °C). A typical steel 22MnB5 shows a 2.5x increase in tensile strength after this process, from a base of 600MPa. Stamping can be done in an inert atmosphere, otherwise abrasive scale formsalternatively a protective Al-Si coating can be used. (see

aluminized steel Aluminized steel is steel that has been plated with aluminium or aluminium- silicon alloy, in a process analogous to hot-dip galvanizing. The steel workpiece is immersed in molten aluminum to produce a tight metallic bond between the steel an ...

). Introduction of high tensile strength hot stamped mild manganese boron steel (22MnB5) (up to proof strength 1200MPa,

ultimate tensile strength Ultimate tensile strength (UTS), often shortened to tensile strength (TS), ultimate strength, or F_\text within equations, is the maximum stress that a material can withstand while being stretched or pulled before breaking. In brittle materials ...

1500MPa) allowed weight saving through down gauging in the European car industry. Boron steel is used in the shackles of some

padlock Padlocks are portable locks with a shackle that may be passed through an opening (such as a chain link, or hasp staple) to prevent use, theft, vandalism or harm. Naming and etymology The term '' padlock'' is from the late fifteenth century. ...

s for cut resistance Boron steel padlocks of sufficient shackle thickness (15mm or more) are highly hacksaw, bolt cutter, and hammer-resistant, although they can be defeated with an angle grinder.

Description

Hardenability

Phase diagram

Uses

See also

References

Further reading