We continue to review some of the most important materials in heat treatment and metallurgy.
Boron (Chemical symbol: B)
Although not identified as an element until 1808 by French chemists Joseph L. Gay-Lussac and L.J. Thénard, and independently by Sir Humphry Davy in London, boron compounds such as borax (sodium tetraborate) had been known and used by ancient cultures for thousands of years. Borax’s name comes from the Arabic word buraq, meaning “white.” It was not until 1909 that American chemist Ezekiel Weintraub produced 99% pure boron by reducing boron halides with hydrogen. The previous efforts had yielded only 60%-pure samples. Pure boron (Fig. 1) is very rarely found in nature. It is more commonly found in the compounds borax, boric acid, colemanite, kernite, ulexite and borates.
Boron is considered a semi-metallic element, with its place on the periodic table between the metal beryllium and carbon, a nonmetal. It is a very hard element, second only to carbon in the form of diamond. Boron is, however, more temperature-resistant than diamond.
Boron is an important alloying element of high-strength steel. It significantly increases the hardenability of steel without loss of ductility. Boron steel has a yield strength four times that of other high-strength steels. Originally used in high-performance European sports cars, it is now used by nearly every automotive manufacturer for certain strengthening bars, gussets and pillars (Fig. 2). Boron alloys with steel under high temperatures by forming a molecular bond with the metal. Its effectiveness is most evident at lower carbon levels, and it is used in amounts ranging from 5-15 parts per million.
Steel and other metals can be surface hardened with boron, a process called boriding or boronizing. In this process, boron atoms are diffused into the surface of the metal. The resulting surface contains metal borides, such as iron borides, nickel borides and cobalt borides. These borides have extremely high hardness and wear resistance, even at small concentrations. Boronized metal parts are extremely wear-resistant and will often last two to five times longer than components treated with conventional heat treatments such as hardening, carburizing, nitriding, nitrocarburizing or induction hardening. Agricultural parts are prime candidates for boronizing.
Boron is also used in NIB (neodymium/iron/boron) magnets. NIB magnets (Fig. 3) are the most powerful of the rare-earth magnets and were invented in 1982 by General Motors and Sumitomo Special Metals. They are used in computers, cell phones, medical equipment, toys, motors and wind turbines, among others. Boron is a superb neutron absorber, and it is used in control rods for nuclear reactors when alloyed with steel or reacted with carbon, titanium or zirconium.
Here are a few important facts about boron.
- Atomic Number: 5
- Atomic Weight: 10.81
- Density (g/cc): 2.34
- Melting Point: 2075°C
- Specific Heat (@20°C J/g mole): 1.025
- Fusion Heat (kJ/mole): 23.60
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