Boriding, also called boronizing, is the process by which
boron is added to a
metal
A metal (from Greek μέταλλον ''métallon'', "mine, quarry, metal") is a material that, when freshly prepared, polished, or fractured, shows a lustrous appearance, and conducts electricity and heat relatively well. Metals are typi ...
or
alloy
An alloy is a mixture of chemical elements of which at least one is a metal. Unlike chemical compounds with metallic bases, an alloy will retain all the properties of a metal in the resulting material, such as electrical conductivity, ductilit ...
. It is a type of
surface hardening. In this process
boron atoms are diffused into the surface of a metal component. The resulting surface contains metal
borides, 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 al ...
s such as hardening, carburizing, nitriding, nitrocarburizing or induction hardening. Most borided steel surfaces will have iron boride layer hardnesses ranging from 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. Several key characteristics of a superalloy are excellent mechanical strength, resistance to thermal creep deformation, ...
s such as
Inconel
Inconel is a registered trademark of Special Metals Corporation for a family of austenitic nickel-chromium-based superalloys.
Inconel alloys are oxidation- corrosion-resistant materials well suited for service in extreme environments subjected ...
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. 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 ha ...
powder diluted with other refractory materials. The process converts some of the Fe to
iron boride, consisting of two phases: FeB concentrated near the surface, and diiron boride (Fe
2B).
[ Boride layer depths can range from 0.001 - 0.015 inches (25.4 - 38 µm) depending on base material selection and treatment.
]
Materials
It is often used on steel, but is applicable to a variety of alloys and cermet materials.[https://www1.eere.energy.gov/manufacturing/intensiveprocesses/pdfs/ultra-fast_boriding.pdf ] 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 engi ...
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.
See also
*Boron steel
Boron steel refers to steel alloyed with a small amount of boron, 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 th ...
References
Further reading
*
Metal heat treatments
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