Boron steel refers to

steel Steel is an alloy of iron and carbon that demonstrates improved mechanical properties compared to the pure form of iron. Due to steel's high Young's modulus, elastic modulus, Yield (engineering), yield strength, Fracture, fracture strength a ...

alloyed with a small amount of

boron Boron is a chemical element; it has 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 three ...

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

hardenability Jominy test dimensioning Jominy test apparatus Used Jominy test-piece Hardenability is the depth to which a steel is hardened after putting it through a heat treatment process. It should not be confused with hardness, which is a measure of a s ...

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 B N. It exists in various crystalline forms that are isoelectronic to a similarly structured carbon lattice. The hexago ...

) 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

, 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 1000 ...

decomposition to softer bainite, ferrite, or pearlite 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 1000 ...

Uses

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

stainless steel Stainless steel, also known as inox, corrosion-resistant steel (CRES), or rustless steel, is an iron-based alloy that contains chromium, making it resistant to rust and corrosion. Stainless steel's resistance to corrosion comes from its chromi ...

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 tool Hydraulic rescue tools, also known as jaws of life, are used by emergency rescue personnel to assist in the extrication of victims involved in vehicle accidents or railway accidents and cutting large-sized debris of mild metal structures int ...

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

ultimate tensile strength Ultimate tensile strength (also called UTS, tensile strength, TS, ultimate strength or F_\text in notation) is the maximum stress that a material can withstand while being stretched or pulled before breaking. In brittle materials, the ultimate t ...

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 Lock (security device), locks with a shackle that may be passed through an opening (such as a chain, chain link, or hasp staple) to prevent wikt:use, use, theft, vandalism or harm. Naming and etymology The term '':wikt: ...

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. Boron steel flats, typically 30MnB5 modified with an addition of 0.5%

chromium Chromium is a chemical element; it has Symbol (chemistry), symbol Cr and atomic number 24. It is the first element in Group 6 element, group 6. It is a steely-grey, Luster (mineralogy), lustrous, hard, and brittle transition metal. Chromium ...

are used in the manufacture of fork arms for forklift trucks.

Description

Hardenability

Phase diagram

Uses

See also

References

Further reading