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Nitriding is a
heat treating 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 ...
process that diffuses
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 se ...
into the surface of a
metal A metal (from ancient Greek, Greek μέταλλον ''métallon'', "mine, quarry, metal") is a material that, when freshly prepared, polished, or fractured, shows a lustrous appearance, and conducts electrical resistivity and conductivity, e ...
to create a case-hardened surface. These processes are most commonly used on low-alloy steels. They are also used on
titanium Titanium is a chemical element with the symbol Ti and atomic number 22. Found in nature only as an oxide, it can be reduced to produce a lustrous transition metal with a silver color, low density, and high strength, resistant to corrosion i ...
,
aluminium Aluminium (aluminum in American and Canadian English) is a chemical element with the symbol Al and atomic number 13. Aluminium has a density lower than those of other common metals, at approximately one third that of steel. It ha ...
and
molybdenum Molybdenum is a chemical element with the symbol Mo and atomic number 42 which is located in period 5 and group 6. The name is from Neo-Latin ''molybdaenum'', which is based on Ancient Greek ', meaning lead, since its ores were confused with lead ...
. Typical applications include
gear 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 ...
s,
crankshaft A crankshaft is a mechanical component used in a piston engine to convert the reciprocating motion into rotational motion. The crankshaft is a rotating shaft containing one or more crankpins, that are driven by the pistons via the connecti ...
s,
camshaft A camshaft is a shaft that contains a row of pointed cams, in order to convert rotational motion to reciprocating motion. Camshafts are used in piston engines (to operate the intake and exhaust valves), mechanically controlled ignition systems ...
s, cam followers,
valve A valve is a device or natural object that regulates, directs or controls the flow of a fluid (gases, liquids, fluidized solids, or slurries) by opening, closing, or partially obstructing various passageways. Valves are technically fitting ...
parts, extruder screws,
die-casting Die casting is a metal casting process that is characterized by forcing molten metal under high pressure into a mold cavity. The mold cavity is created using two hardened tool steel dies which have been machined into shape and work similarly ...
tools,
forging Forging is a manufacturing process involving the shaping of metal using localized compressive forces. The blows are delivered with a hammer (often a power hammer) or a die. Forging is often classified according to the temperature at which ...
dies,
extrusion Extrusion is a process used to create objects of a fixed cross-sectional profile by pushing material through a die of the desired cross-section. Its two main advantages over other manufacturing processes are its ability to create very complex c ...
dies, firearm components,
injector An injector is a system of ducting and nozzles used to direct the flow of a high-pressure fluid in such a way that a lower pressure fluid is entrained in the jet and carried through a duct to a region of higher pressure. It is a fluid-dynamic ...
s and plastic-
mold A mold () or mould () is one of the structures certain fungi can form. The dust-like, colored appearance of molds is due to the formation of spores containing fungal secondary metabolites. The spores are the dispersal units of the fungi. Not ...
tools.


Processes

The processes are named after the medium used to donate. The three main methods used are: ''gas nitriding'', ''salt bath nitriding'', and ''plasma nitriding''.


Gas nitriding

In gas nitriding the donor is a nitrogen-rich gas, usually
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 ...
(NH3), which is why it is sometimes known as ''ammonia nitriding''. When ammonia comes into contact with the heated work piece it dissociates into nitrogen and hydrogen. The nitrogen then diffuses onto the surface of the material creating a nitride layer. This process has existed for nearly a century, though only in the last few decades has there been a concentrated effort to investigate the thermodynamics and kinetics involved. Recent developments have led to a process that can be accurately controlled. The thickness and phase constitution of the resulting nitriding layers can be selected and the process optimized for the particular properties required. The advantages of gas nitriding over other variants are: * Precise control of chemical potential of nitrogen in the nitriding atmosphere by controlling gas flow rate of nitrogen and oxygen. *All round nitriding effect (can be a disadvantage in some cases, compared with plasma nitriding) *Large batch sizes possible - the limiting factor being furnace size and gas flow *With modern computer control of the atmosphere the nitriding results can be closely controlled *Relatively low equipment cost - especially compared with plasma The disadvantages of gas nitriding are: *Reaction kinetics heavily influenced by surface condition - an oily surface or one contaminated with cutting fluids will deliver poor results *Surface activation is sometimes required to treat steels with a high chromium content - compare sputtering during plasma nitriding *Ammonia as nitriding medium - though not especially toxic it is harmful when inhaled at a high concentration. Also, care must be taken when heating in the presence of oxygen to reduce the risk of explosion


Salt bath nitriding

In salt bath nitriding the nitrogen donating medium is a nitrogen-containing salt such as cyanide salt. The salts used also donate carbon to the workpiece surface making salt bath a nitrocarburizing process. The temperature used is typical of all nitrocarburizing processes: 550 to 570 °C. The advantages of salt nitriding is that it achieves higher diffusion in the same period of time compared to any other method. The advantages of salt nitriding are: *Quick processing time - usually in the order of 4 hours or so to achieve *Simple operation - heat the salt and workpieces to temperature and submerge until the duration has transpired. The disadvantages are: *The salts used are highly toxic - Disposal of salts are controlled by stringent environmental laws in western countries and has increased the costs involved in using salt baths. This is one of the most significant reasons the process has fallen out of favor in recent decades. *Only one process possible with a particular salt type - since the nitrogen potential is set by the salt, only one type of process is possible


Plasma nitriding

Plasma nitriding, also known as ''ion nitriding'', ''plasma ion nitriding'' or ''glow-discharge nitriding'', is an industrial surface hardening treatment for metallic materials. In plasma nitriding, the reactivity of the nitriding media is not due to the temperature but to the gas ionized state. In this technique intense electric fields are used to generate ionized molecules of the gas around the surface to be nitrided. Such highly active gas with ionized molecules is called plasma, naming the technique. The gas used for plasma nitriding is usually pure nitrogen, since no spontaneous decomposition is needed (as is the case of nitriding with ammonia). There are hot plasmas typified by plasma jets used for metal cutting,
welding Welding is a fabrication process that joins materials, usually metals or thermoplastics, by using high heat to melt the parts together and allowing them to cool, causing fusion. Welding is distinct from lower temperature techniques such as b ...
,
cladding Cladding is an outer layer of material covering another. It may refer to the following: *Cladding (boiler), the layer of insulation and outer wrapping around a boiler shell *Cladding (construction), materials applied to the exterior of buildings ...
or spraying. There are also cold plasmas, usually generated inside
vacuum A vacuum is a space devoid of matter. The word is derived from the Latin adjective ''vacuus'' for "vacant" or " void". An approximation to such vacuum is a region with a gaseous pressure much less than atmospheric pressure. Physicists often ...
chambers, at low
pressure Pressure (symbol: ''p'' or ''P'') is the force applied perpendicular to the surface of an object per unit area over which that force is distributed. Gauge pressure (also spelled ''gage'' pressure)The preferred spelling varies by country a ...
regimes. Usually steels are beneficially treated with plasma nitriding. This process permits the close control of the nitrided microstructure, allowing nitriding with or without compound layer formation. Not only is the performance of metal parts enhanced, but working lifespans also increase, and so do the strain limit and the fatigue strength of the metals being treated. For instance, mechanical properties of austenitic stainless steel like resistance to wear can be significantly augmented and the surface hardness of tool steels can be doubled. A plasma nitrided part is usually ready for use. It calls for no machining, or polishing or any other post-nitriding operations. Thus the process is user-friendly, saves energy since it works fastest, and causes little or no distortion. This process was invented by Bernhardt Berghaus of Germany who later settled in Zurich to escape Nazi persecution. After his death in late 1960s the process was acquired by Klockner group and popularized globally. Plasma nitriding is often coupled with
physical vapor deposition Physical vapor deposition (PVD), sometimes called physical vapor transport (PVT), describes a variety of vacuum deposition methods which can be used to produce thin films and coatings on substrates including metals, ceramics, glass, and polym ...
(PVD) process and labeled Duplex Treatment, with enhanced benefits. Many users prefer to have a plasma oxidation step combined at the last phase of processing to produce a smooth jetblack layer of oxides which is resistant to wear and corrosion. Since nitrogen ions are made available by ionization, differently from gas or salt bath, plasma nitriding efficiency does not depend on the temperature. Plasma nitriding can thus be performed in a broad temperature range, from 260 °C to more than 600 °C. For instance, at moderate temperatures (like 420 °C), stainless steels can be nitrided without the formation of
chromium nitride Chromium nitride is a chemical compound of chromium and nitrogen with the formula CrN. It is very hard, and is extremely resistant to corrosion. It is an interstitial compound, with nitrogen atoms occupying the octahedral holes in the chromium latt ...
precipitates and hence maintaining their corrosion resistance properties. In the plasma nitriding processes, nitrogen gas (N2) is usually the nitrogen carrying gas. Other gasses like hydrogen or
argon Argon is a chemical element with the symbol Ar and atomic number 18. It is in group 18 of the periodic table and is a noble gas. Argon is the third-most abundant gas in Earth's atmosphere, at 0.934% (9340 ppmv). It is more than twice a ...
are also used. Indeed, argon and H2 can be used before the nitriding process during the heating of the parts to clean the surfaces to be nitrided. This cleaning procedure effectively removes the oxide layer from surfaces and may remove fine layers of solvents that could remain. This also helps the thermal stability of the plasma plant, since the heat added by the plasma is already present during the warm up and hence once the process temperature is reached the actual nitriding begins with minor heating changes. For the nitriding process H2 gas is also added to keep the surface clear of oxides. This effect can be observed by analysing the surface of the part under nitriding (see for instance ).


Materials for nitriding

Examples of easily nitridable steels include the
SAE SAE or Sae may refer to: Science and technology : * Selective area epitaxy, local growth of epitaxial layer through a patterned dielectric mask deposited on a semiconductor substrate * Serious adverse event, in a clinical trial * Simultaneous Aut ...
4100, 4300, 5100, 6100, 8600, 8700, 9300 and 9800 series, UK aircraft quality steel grades BS 4S 106, BS 3S 132, 905M39 (EN41B), stainless steels, some tool steels (H13 and P20 for example) and certain cast irons. Ideally, steels for nitriding should be in the hardened and tempered condition, requiring nitriding to take place at a lower temperature than the last tempering temperature. A fine-turned or ground surface finish is best. Minimal amounts of material should be removed post nitriding to preserve the surface hardness. Nitriding alloys are alloy steels with nitride-forming elements such as aluminum,
chromium Chromium is a chemical element with the symbol Cr and atomic number 24. It is the first element in group 6. It is a steely-grey, lustrous, hard, and brittle transition metal. Chromium metal is valued for its high corrosion resistance and hard ...
, molybdenum and titanium. In 2015, nitriding was used to generate a unique duplex
microstructure Microstructure is the very small scale structure of a material, defined as the structure of a prepared surface of material as revealed by an optical microscope above 25× magnification. The microstructure of a material (such as metals, polymers ...
in an iron-manganese alloy (
martensite Martensite is a very hard form of steel crystalline structure. It is named after German metallurgist Adolf Martens. By analogy the term can also refer to any crystal structure that is formed by diffusionless transformation. Properties M ...
-
austenite Austenite, also known as gamma-phase iron (γ-Fe), is a metallic, non-magnetic allotrope of iron or a solid solution of iron with an alloying element. In plain-carbon steel, austenite exists above the critical eutectoid temperature of 100 ...
,
austenite Austenite, also known as gamma-phase iron (γ-Fe), is a metallic, non-magnetic allotrope of iron or a solid solution of iron with an alloying element. In plain-carbon steel, austenite exists above the critical eutectoid temperature of 100 ...
- ferrite), known to be associated with strongly enhanced mechanical properties.


History

Systematic investigation into the effect of nitrogen on the surface properties of steel began in the 1920s. Investigation into gas nitriding began independently in both Germany and America. The process was greeted with enthusiasm in Germany and several steel grades were developed with nitriding in mind: the so-called nitriding steels. The reception in America was less impressive. With so little demand the process was largely forgotten in the US. After WWII the process was reintroduced from Europe. Much research has taken place in recent decades to understand the thermodynamics and kinetics of the reactions involved.


See also

*
Boriding Boriding, also called boronizing, is the process by which boron is added to a metal or alloy. 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 ...
*
Carburization Carburising, carburizing (chiefly American English), or carburisation is a heat treatment process in which iron or steel absorbs carbon while the metal is heated in the presence of a carbon-bearing material, such as charcoal or carbon monoxide. ...
*
Carbonitriding Carbonitriding is a metallurgical surface modification technique that is used to increase the surface hardness of a metal, thereby reducing wear. During the process, atoms of carbon and nitrogen diffuse interstitially into the metal, creati ...
*
Ferritic nitrocarburizing Ferritic nitrocarburizing or FNC, also known by the proprietary names Tenifer, Tufftride and Melonite as well as ARCOR,Other trade names include Tuffride/ Tuffrider, QPQ, Sulfinuz, Sursulf, Meli 1, and Nitride, among others is a range of proprie ...
* Surface finishing


References


Further reading

* * *


External links

*
An Introduction to Nitriding
{{Authority control Metal heat treatments Plasma processing Nitrogen et:Nitriitimine