Boriding, also called boronizing, is the process by which

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 ...

is added to a

metal A metal () is a material that, when polished or fractured, shows a lustrous appearance, and conducts electrical resistivity and conductivity, electricity and thermal conductivity, heat relatively well. These properties are all associated wit ...

alloy An alloy is a mixture of chemical elements of which in most cases at least one is a metal, metallic element, although it is also sometimes used for mixtures of elements; herein only metallic alloys are described. Metallic alloys often have prop ...

. It is a type of

surface hardening Case-hardening or carburization is the process of introducing carbon to the surface of a low-carbon iron, or more commonly a low-carbon steel object, in order to harden the surface. Iron which has a carbon content greater than ~0.02% is known ...

. In this process

atoms are diffused into the surface of a metal component. The resulting surface contains metal

boride A boride is a compound between boron and a less electronegative element, for example silicon boride (SiB3 and SiB6). The borides are a very large group of compounds that are generally high melting and are covalent more than ionic in nature. Some b ...

s, such as iron borides, nickel borides, and cobalt borides. As pure materials, these borides have extremely high hardness and wear resistance. Their favorable properties are manifested even when they are a small fraction of the bulk solid.Helmut Kunst, Brigitte Haase, James C. Malloy, Klaus Wittel, Montia C. Nestler, Andrew R. Nicoll, Ulrich Erning and Gerhard Rauscher "Metals, Surface Treatment" in Ullmann's Encyclopedia of Industrial Chemistry, 2006, Wiley-VCH, Weinheim. Boronized metal parts are extremely wear-resistant and will often last two to five times longer than components treated with conventional

heat treatment Heat treating (or heat treatment) is a group of industrial, thermal and metalworking processes used to alter the physical, and sometimes chemical, properties of a material. The most common application is metallurgical. Heat treatments are a ...

s such as hardening, carburizing, nitriding, nitrocarburizing, or induction hardening. Most borided steel surfaces will have iron boride layer hardnesses of 1200–1600 HV. Nickel-based

superalloy A superalloy, or high-performance alloy, is an alloy with the ability to operate at a high fraction of its melting point. Key characteristics of a superalloy include mechanical strength, thermal creep deformation resistance, surface stability, ...

s such as

Inconel Inconel is a nickel-chromium-based superalloy often utilized in extreme environments where components are subjected to high temperature, pressure or Mechanical load, mechanical loads. Inconel alloys are oxidation- and corrosion-resistant. When he ...

and Hastalloys will typically have nickel boride layer hardnesses of 1700–2300 HV.

Methods

Boriding can be achieved in several ways, but commonly the metal piece is packed with a boriding mixture and heating at 900 °C. A typical boriding mixture consists of

boron carbide Boron carbide (chemical formula approximately B4C) is an extremely hard boron–carbon ceramic, a covalent material used in tank armor, bulletproof vests, engine sabotage powders, as well as numerous industrial applications. With a Vickers har ...

powder diluted with other refractory materials. The process converts some of the iron to iron boride, consisting of two phases: FeB concentrated near the surface, and diiron boride (Fe₂B). Boride layer depths can range from depending on base material selection and treatment.

Materials

It is often used on

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 ...

, but is applicable to a variety of alloys and cermet materials. A wide range of materials suitable for treatment including plain carbon steels, alloy steels, tool steels, nickel-based super alloys, cobalt alloys, and stellite.

Properties conferred

Boriding gives the material the following desirable properties: wear resistance, improved hardness (1300–2000HV is possible), thermal stability, resistance to

corrosion Corrosion is a natural process that converts a refined metal into a more chemically stable oxide. It is the gradual deterioration of materials (usually a metal) by chemical or electrochemical reaction with their environment. Corrosion engine ...

by acids, reduced coefficient of friction, and increased galling/cold-welding resistance. It is possible to combine boriding with other heat treatments such as carburizing, hardening, or induction hardening to create deeper wear layers or high core hardness.

Methods

Materials

Properties conferred

See also

References

Further reading