Boron

A metalloid that is extracted from its metal salts, such as borax, by conversion using hydrochloric acid and subsequent reduction using magnesium.

SymbolB
Atomic number5
Atomic weight10.82
Specific weight at 20 °C2.3 g/cm3
Melting point2050 °C
Boiling point2550 °C
Melting heat268 kJ/kg


Boron in steel

Solubility of boron in γ-iron is 0.0021 % at 906 °C and 0.0210 % at 1140 °C. α-iron dissolves 0.004 % B at 710 °C and 0.0082 % at 906 C. Boron is similar to carbon in that it forms interstitial solid-solutions with the γ-iron. Its speed of diffusion in γ-iron is also comparable with that of carbon so that it is possible to boronize and deboronize iron relatively easily.

In the iron-boron system, the γ-phase is constricted. At 3.8 % B and 1174 °C an eutectic occurs that comprises γ-iron and the intermetallic compound Fe2B, comparable with the cementite Fe3C in the iron-carbon system (s. Iron-carbon phase diagram). Iron-boron alloys with 0.1 % B that feature this eutectic with low melting point, are prone to hot rupture.

In regions with low boron contents within the iron-boron-carbon ternary system various types of boroncarbide may develop. Carbon only has little effect on the solubility and diffusion speed of boron in austenite.

A distinguishable increase of hardenability of steel is achieved by boron additions of up to 0.007%. The prevailing commercially available steels are low-alloy steels with 0.0005 and 0.007 % B. In heat-treated steel it is possible to replace a significant percentage of other alloy elements by boron without compromising any of the mechanical properties.

Boron displays great affinity towards oxygen and nitrogen. Therefore it is required to remove almost all of the oxygen and nitrogen present in a steel material by means of aluminum, titanium, or other deoxidizers in order to produce a steel type with the required content of reactive boron.

Boron in cast iron
Pig iron rarely contains more than 0.005 % B. In cast iron, boron counteracts the graphitization process and increases the chill depth. With addition of more than 0.01 % B, boron acts as a powerful carbide stabilizer and can therefore be used for improvement of wear resistance in chilled cast iron.

Nickel- and boron-containing white cast iron was used as surfacing alloy for parts under great wear loads. Cast iron for rollers (see Roller casting) may contain up to 0.02 to 0.1 % B for increased surface hardness and improved chill.

In gray cast iron, boron contents of approx. 0.1 % or more cause undesired hardness increase and cracking. Remelted enamel-containing scrap is a source for boron additions to gray cast iron, however, the largest part of the boron taken up in that way is removed during melting.

Boron additions between 0.001 and 0.005 % B to melleable cast iron promote formation of nodular graphite while simultaneously increasing the number of graphite particles and improving their distribution pattern. This effect facilitates annealing treatment, probably due to the boron binding the nitrogen. For production of melleable cast iron, small amounts of boron are often added to counteract the detrimental influence of elements, such as chromium, introduced with the scrap.

The element reacts with carbon and silicon to form borides, which are used as abrasion agents.

 

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