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Carbonitriding is a metallurgical surface modification technique that is used to increase the surface
hardness In materials science, hardness (antonym: softness) is a measure of the resistance to localized plastic deformation induced by either mechanical indentation or abrasion. In general, different materials differ in their hardness; for example hard ...
of a metal, thereby reducing
wear Wear is the damaging, gradual removal or deformation of material at solid surfaces. Causes of wear can be mechanical (e.g., erosion) or chemical (e.g., corrosion). The study of wear and related processes is referred to as tribology. Wear in ...
. During the process, atoms of carbon and
nitrogen Nitrogen is the chemical element with the symbol N and atomic number 7. Nitrogen is a nonmetal and the lightest member of group 15 of the periodic table, often called the pnictogens. It is a common element in the universe, estimated at sevent ...
diffuse interstitially into the metal, creating barriers to
slip Slip or SLIP may refer to: Science and technology Biology * Slip (fish), also known as Black Sole * Slip (horticulture), a small cutting of a plant as a specimen or for grafting * Muscle slip, a branching of a muscle, in anatomy Computing an ...
, increasing the hardness and modulus near the surface. Carbonitriding is often applied to inexpensive, easily machined low carbon steel to impart the surface properties of more expensive and difficult to work grades of steel. Surface hardness of carbonitrided parts ranges from 55 to 62 HRC. Certain pre-industrial
case hardening Case-hardening or surface hardening is the process of hardening the surface of a metal object while allowing the metal deeper underneath to remain soft, thus forming a thin layer of harder metal at the surface. For iron or steel with low carbon ...
processes include not only carbon-rich materials such as charcoal, but nitrogen-rich materials such as
urea Urea, also known as carbamide, is an organic compound with chemical formula . This amide has two amino groups (–) joined by a carbonyl functional group (–C(=O)–). It is thus the simplest amide of carbamic acid. Urea serves an important ...
, which implies that traditional surface hardening techniques were a form of carbonitriding.


Process

Carbonitriding is similar to
gas Gas is one of the four fundamental states of matter (the others being solid, liquid, and plasma). A pure gas may be made up of individual atoms (e.g. a noble gas like neon), elemental molecules made from one type of atom (e.g. oxygen), or ...
carburization with the addition of
ammonia Ammonia is an inorganic compound of nitrogen and hydrogen with the formula . A stable binary hydride, and the simplest pnictogen hydride, ammonia is a colourless gas with a distinct pungent smell. Biologically, it is a common nitrogenous wa ...
to the carburizing atmosphere, which provides a source of nitrogen. Nitrogen is absorbed at the surface and diffuses into the workpiece along with carbon. Carbonitriding (around 850 °C / 1550 °F) is carried out at temperatures substantially higher than plain
nitriding Nitriding is a heat treating process that diffuses nitrogen into the surface of a metal to create a case-hardened surface. These processes are most commonly used on low-alloy steels. They are also used on titanium, aluminium and molybdenum. T ...
(around 530 °C / 990 °F) but slightly lower than those used for carburizing (around 950 °C / 1700 °F) and for shorter times. Carbonitriding tends to be more economical than carburizing, and also reduces distortion during
quenching In materials science, quenching is the rapid cooling of a workpiece in water, oil, polymer, air, or other fluids to obtain certain material properties. A type of heat treating, quenching prevents undesired low-temperature processes, such as phas ...
. The lower temperature allows oil quenching, or even gas quenching with a protective atmosphere.


Characteristics of carbonitrided parts

Carbonitriding forms a hard, wear-resistant case, is typically 0.07 mm to 0.5 mm thick, and generally has higher hardness than a carburized case. Case depth is tailored to the application; a thicker case increases the wear life of the part. Carbonitriding alters only the top layers of the workpiece; and does not deposit an additional layer, so the process does not significantly alter the dimensions of the part. Maximum case depth is typically restricted to 0.75 mm; case depths greater than this take too long to diffuse to be economical. Shorter processing times are preferred to restrict the concentration of nitrogen in the case, as nitrogen addition is more difficult to control than carbon. An excess of nitrogen in the work piece can cause high levels of
retained austenite In the United Kingdom and Ireland, a retained firefighter, also known as an RDS Firefighter or on-call firefighter, is a firefighter who does not work on a fire station full-time but is paid to spend long periods of time on call to respond to eme ...
and porosity, which are undesirable in producing a part of high hardness.


Advantages

Carbonitriding also has other advantages over carburizing. To begin, it has a greater resistance to softening during tempering and increased
fatigue Fatigue describes a state of tiredness that does not resolve with rest or sleep. In general usage, fatigue is synonymous with extreme tiredness or exhaustion that normally follows prolonged physical or mental activity. When it does not resolve ...
and impact strength. It is possible to use both carbonitriding and carburizing together to form optimum conditions of deeper case depths and therefore performance of the part in industry. This method is applied particularly to steels with low case hardenability, such as the seat of the valve. The process applied is initially carburizing to the required case depth (up to 2.5 mm) at around 900-955°C, and then carbonitriding to achieve required carbonitrided case depth. The parts are then oil quenched, and the resulting part has a harder case than possibly achieved for carburization, and the addition of the carbonitrided layer increases the residual compressive stresses in the case such that the contact fatigue resistance and strength gradient are both increased. Studies are showing that carbonitriding improves corrosion resistance.Yazıcı, A., Zeybek, M.S., Güler, H., Koç, M., Pekitkan, F. G., 2017 Effect of Carbonitriding on Corrosion Resistance of Steel 30MnB5 in Two Acidic Environments. Metal Science and Heat Treatment, 59 (3-4), 190-193 (Doi: 10.1007/s11041-017-0127-8).


Applications

Typical applications for case hardening are
gear teeth A gear is a rotating circular machine part having cut teeth or, in the case of a cogwheel or gearwheel, inserted teeth (called ''cogs''), which mesh with another (compatible) toothed part to transmit (convert) torque and speed. The basic p ...
, cams, shafts, bearings,
fastener A fastener (US English) or fastening (UK English) is a hardware device that mechanically joins or affixes two or more objects together. In general, fasteners are used to create non-permanent joints; that is, joints that can be removed or disman ...
s,
pin A pin is a device used for fastening objects or material together. Pin or PIN may also refer to: Computers and technology * Personal identification number (PIN), to access a secured system ** PIN pad, a PIN entry device * PIN, a former Dutch ...
s, hydraulic piston rods, automotive clutch plates,
tool A tool is an object that can extend an individual's ability to modify features of the surrounding environment or help them accomplish a particular task. Although many animals use simple tools, only human beings, whose use of stone tools dates bac ...
s, dies and tillage tools.


See also

*
Differential hardening Differential heat treatment (also called selective heat treatment or local heat treatment) is a technique used during heat treating to harden or soften certain areas of a steel object, creating a difference in hardness between these areas. There ar ...
*
Nitridization Nitriding is a heat treating process that diffuses nitrogen into the surface of a metal to create a case-hardened surface. These processes are most commonly used on low-alloy steels. They are also used on titanium, aluminium and molybdenum. T ...
* Quench polish quench *
Surface engineering Surface engineering is the sub-discipline of materials science which deals with the surface of solid matter. It has applications to chemistry, mechanical engineering, and electrical engineering (particularly in relation to semiconductor manufactur ...


References

{{Iron and steel production Metal heat treatments ru:Нитроцементация сталей