Nitriding

Last updated August 20, 2024

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.

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 (NH₃), which is why it is sometimes known as ammonia nitriding.^[2] 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.^{[ citation needed ]} The salts used also donate carbon to the workpiece surface making salt bath a nitrocarburizing process.^{[ citation needed ]} The temperature used is typical of all nitrocarburizing processes: 550 to 570 °C.^{[ citation needed ]} Unfortunately, since the salts used are extremely toxic, modern environmental and safety regulation have caused this process to fall out of favor.^{[ citation needed ]}

The advantages of salt nitriding are:

Quick processing time – Usually in the order of 4 hours or so to achieve desired diffusion, where other methods take longer.^{[ citation needed ]}
Simple operation – The salt bath and workpieces are heated to the desired temperature, and the workpieces are submerged for a given period of time.^{[ citation needed ]}

The disadvantages are:

The salts used are highly toxic^{[ citation needed ]} – Disposal is controlled by stringent environmental laws in western countries.^{[ citation needed ]}
Cost – These regulations have increased the costs involved in using salt baths.
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.^{[ citation needed ]}

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, cladding or spraying. There are also cold plasmas, usually generated inside vacuum chambers, at low pressure 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.^[3]^[4]

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 a physical vapor deposition (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.^[4] For instance, at moderate temperatures (like 420 °C), stainless steels can be nitrided without the formation of chromium nitride precipitates and hence maintaining their corrosion resistance properties.^[5]

In the plasma nitriding processes, nitrogen gas (N₂) is usually the nitrogen carrying gas. Other gasses like hydrogen or argon are also used. Indeed, argon and hydrogen 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 hydrogen 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^[6]).

Materials for nitriding

Examples of easily nitridable steels include the SAE 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, molybdenum and titanium.

In 2015, nitriding was used to generate a unique duplex microstructure in an iron-manganese alloy (martensite-austenite, austenite-ferrite), known to be associated with strongly enhanced mechanical properties.^[7]

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.

Related Research Articles

Stainless steel, also known as inox, corrosion-resistant steel (CRES), and rustless steel, is an alloy of iron that is resistant to rusting and corrosion. It contains iron with chromium and other elements such as molybdenum, carbon, nickel and nitrogen depending on its specific use and cost. Stainless steel's resistance to corrosion results from the 10.5%, or more, chromium content which forms a passive film that can protect the material and self-heal in the presence of oxygen.

Corrosion is a natural process that converts a refined metal into a more chemically stable oxide. It is the gradual deterioration of materials by chemical or electrochemical reaction with their environment. Corrosion engineering is the field dedicated to controlling and preventing corrosion.

In physical chemistry and engineering, passivation is coating a material so that it becomes "passive", that is, less readily affected or corroded by the environment. Passivation involves creation of an outer layer of shield material that is applied as a microcoating, created by chemical reaction with the base material, or allowed to build by spontaneous oxidation in the air. As a technique, passivation is the use of a light coat of a protective material, such as metal oxide, to create a shield against corrosion. Passivation of silicon is used during fabrication of microelectronic devices. Undesired passivation of electrodes, called "fouling", increases the circuit resistance so it interferes with some electrochemical applications such as electrocoagulation for wastewater treatment, amperometric chemical sensing, and electrochemical synthesis.

<span class="mw-page-title-main">Heat treating</span> Process of heating something to alter it

Heat treating 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 also used in the manufacture of many other materials, such as glass. Heat treatment involves the use of heating or chilling, normally to extreme temperatures, to achieve the desired result such as hardening or softening of a material. Heat treatment techniques include annealing, case hardening, precipitation strengthening, tempering, carburizing, normalizing and quenching. Although the term heat treatment applies only to processes where the heating and cooling are done for the specific purpose of altering properties intentionally, heating and cooling often occur incidentally during other manufacturing processes such as hot forming or welding.

Brazing is a metal-joining process in which two or more metal items are joined by melting and flowing a filler metal into the joint, with the filler metal having a lower melting point than the adjoining metal.

Carburizing, or carburising, 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. The intent is to make the metal harder and more wear resistant. Depending on the amount of time and temperature, the affected area can vary in carbon content. Longer carburizing times and higher temperatures typically increase the depth of carbon diffusion. When the iron or steel is cooled rapidly by quenching, the higher carbon content on the outer surface becomes hard due to the transformation from austenite to martensite, while the core remains soft and tough as a ferritic and/or pearlite microstructure.

Gas tungsten arc welding is an arc welding process that uses a non-consumable tungsten electrode to produce the weld. The weld area and electrode are protected from oxidation or other atmospheric contamination by an inert shielding gas. A filler metal is normally used, though some welds, known as 'autogenous welds', or 'fusion welds' do not require it. A constant-current welding power supply produces electrical energy, which is conducted across the arc through a column of highly ionized gas and metal vapors known as a plasma.

Plasma arc welding (PAW) is an arc welding process similar to gas tungsten arc welding (GTAW). The electric arc is formed between an electrode and the workpiece. The key difference from GTAW is that in PAW, the electrode is positioned within the body of the torch, so the plasma arc is separated from the shielding gas envelope. The plasma is then forced through a fine-bore copper nozzle which constricts the arc and the plasma exits the orifice at high velocities and a temperature approaching 28,000 °C (50,000 °F) or higher.

Case-hardening or carburization is the process of introducing carbon to the surface of a low carbon iron or much more commonly low carbon steel object in order to enable the surface to be hardened.

Quench polish quench (QPQ) is a specialized type of nitrocarburizing case hardening that increases corrosion resistance. It is sometimes known by the brand name of Tufftride, Tenifer or Melonite. Three steps are involved: nitrocarburize ("quench"), polish, and post-oxidize ("quench").

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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, and corrosion and oxidation resistance.

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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 polymers. PVD is characterized by a process in which the material transitions from a condensed phase to a vapor phase and then back to a thin film condensed phase. The most common PVD processes are sputtering and evaporation. PVD is used in the manufacturing of items which require thin films for optical, mechanical, electrical, acoustic or chemical functions. Examples include semiconductor devices such as thin-film solar cells, microelectromechanical devices such as thin film bulk acoustic resonator, aluminized PET film for food packaging and balloons, and titanium nitride coated cutting tools for metalworking. Besides PVD tools for fabrication, special smaller tools used mainly for scientific purposes have been developed.

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<span class="mw-page-title-main">Sputter deposition</span> Method of thin film application

Sputter deposition is a physical vapor deposition (PVD) method of thin film deposition by the phenomenon of sputtering. This involves ejecting material from a "target" that is a source onto a "substrate" such as a silicon wafer. Resputtering is re-emission of the deposited material during the deposition process by ion or atom bombardment. Sputtered atoms ejected from the target have a wide energy distribution, typically up to tens of eV. The sputtered ions can ballistically fly from the target in straight lines and impact energetically on the substrates or vacuum chamber. Alternatively, at higher gas pressures, the ions collide with the gas atoms that act as a moderator and move diffusively, reaching the substrates or vacuum chamber wall and condensing after undergoing a random walk. The entire range from high-energy ballistic impact to low-energy thermalized motion is accessible by changing the background gas pressure. The sputtering gas is often an inert gas such as argon. For efficient momentum transfer, the atomic weight of the sputtering gas should be close to the atomic weight of the target, so for sputtering light elements neon is preferable, while for heavy elements krypton or xenon are used. Reactive gases can also be used to sputter compounds. The compound can be formed on the target surface, in-flight or on the substrate depending on the process parameters. The availability of many parameters that control sputter deposition make it a complex process, but also allow experts a large degree of control over the growth and microstructure of the film.

Black oxide or blackening is a conversion coating for ferrous materials, stainless steel, copper and copper based alloys, zinc, powdered metals, and silver solder. It is used to add mild corrosion resistance, for appearance, and to minimize light reflection. To achieve maximal corrosion resistance the black oxide must be impregnated with oil or wax. One of its advantages over other coatings is its minimal buildup.

Ferritic nitrocarburizing or FNC, also known by the proprietary names "Tenifer", "Tufftride", Melonite, and "Arcor", is a range of proprietary case hardening processes that diffuse nitrogen and carbon into ferrous metals at sub-critical temperatures during a salt bath. Other methods of ferritic nitrocarburizing include gaseous processes such as Nitrotec and ion (plasma) ones. The processing temperature ranges from 525 °C (977 °F) to 625 °C (1,157 °F), but usually occurs at 565 °C (1,049 °F). Steel and other ferrous alloys remain in the ferritic phase region at this temperature. This allows for better control of the dimensional stability that would not be present in case hardening processes that occur when the alloy is transitioned into the austenitic phase. There are four main classes of ferritic nitrocarburizing: gaseous, salt bath, ion or plasma, and fluidized-bed.

References

↑ Kunst, Helmut; Haase, Brigitte; Malloy, James C.; Wittel, Klaus; Nestler, Montia C. "Metals, Surface Treatment". Ullmann's Encyclopedia of Industrial Chemistry . Weinheim: Wiley-VCH. ISBN 978-3527306732.
↑ Ion Nitriding and Nitrocarburizing of Sintered PM Parts, October 7, 2004
↑ Menthe, E; Bulak, A; Olfe, J; Zimmermann, A; Rie, KT (2000). "Improvement of the mechanical properties of austenitic stainless steel after plasma nitriding". Surface and Coatings Technology. 133 (1): 259. doi:10.1016/S0257-8972(00)00930-0.
1 2 Zagonel, L; Figueroa, C; Droppajr, R; Alvarez, F (2006). "Influence of the process temperature on the steel microstructure and hardening in pulsed plasma nitriding". Surface and Coatings Technology. 201 (1–2): 452. doi:10.1016/j.surfcoat.2005.11.137.
↑ Larisch, B; Brusky, U; Spies, HJ (1999). "Plasma nitriding of stainless steels at low temperatures". Surface and Coatings Technology. 116: 205–211. doi:10.1016/S0257-8972(99)00084-5.
↑ Zagonel, L; Figueroa, C; Alvarez, F (2005). "In situ photoemission electron spectroscopy study of nitrogen ion implanted AISI-H13 steel". Surface and Coatings Technology. 200 (7): 2566. arXiv: 1712.01483 . doi:10.1016/j.surfcoat.2004.10.126. S2CID 119102526.
↑ Meka, S.R.; Chauhan, A.; Steiner, T.; Bischoff, E.; Ghosh, P.K.; Mittemeijer, E.J. (2015). "Generating duplex microstructures by nitriding; nitriding of iron based Fe–Mn alloy". Materials Science and Technology. 32 (9): 883–889. doi: 10.1179/1743284715Y.0000000098 .

External links

"MIL-S-6090A, Military Specification: Process for Steels Used In Aircraft Carburizing and Nitriding". United States Department of Defense. 7 Jun 1971. Archived from the original on 29 August 2019. Retrieved 20 June 2012.
An Introduction to Nitriding Archived 2011-12-15 at the Wayback Machine
ECM USA, Vacuum Furnace Manufacturer: Vacuum Purged Gas Nitriding. https://www.ecm-usa.com/applications/nitriding
ECM USA, Vacuum Furnace Manufacturer: FNC Ferritic Nitrocarburizing. https://www.ecm-usa.com/applications/ferritic-nitrocarburizing-fnc

↑ Pye, David. "The Heat Treatment Library". pye-d.com. Archived from the original on 2017-01-11. Retrieved 2017-01-10.

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[1] Kunst, Helmut; Haase, Brigitte; Malloy, James C.; Wittel, Klaus; Nestler, Montia C. "Metals, Surface Treatment". Ullmann's Encyclopedia of Industrial Chemistry . Weinheim: Wiley-VCH. ISBN 978-3527306732.

[2] Ion Nitriding and Nitrocarburizing of Sintered PM Parts, October 7, 2004

[MecProp-3] Menthe, E; Bulak, A; Olfe, J; Zimmermann, A; Rie, KT (2000). "Improvement of the mechanical properties of austenitic stainless steel after plasma nitriding". Surface and Coatings Technology. 133 (1): 259. doi:10.1016/S0257-8972(00)00930-0.

[LowTemp-4] 1 2 Zagonel, L; Figueroa, C; Droppajr, R; Alvarez, F (2006). "Influence of the process temperature on the steel microstructure and hardening in pulsed plasma nitriding". Surface and Coatings Technology. 201 (1–2): 452. doi:10.1016/j.surfcoat.2005.11.137.

[Stainless-5] Larisch, B; Brusky, U; Spies, HJ (1999). "Plasma nitriding of stainless steels at low temperatures". Surface and Coatings Technology. 116: 205–211. doi:10.1016/S0257-8972(99)00084-5.

[H2-6] Zagonel, L; Figueroa, C; Alvarez, F (2005). "In situ photoemission electron spectroscopy study of nitrogen ion implanted AISI-H13 steel". Surface and Coatings Technology. 200 (7): 2566. arXiv: 1712.01483 . doi:10.1016/j.surfcoat.2004.10.126. S2CID 119102526.

[Duplex-7] Meka, S.R.; Chauhan, A.; Steiner, T.; Bischoff, E.; Ghosh, P.K.; Mittemeijer, E.J. (2015). "Generating duplex microstructures by nitriding; nitriding of iron based Fe–Mn alloy". Materials Science and Technology. 32 (9): 883–889. doi: 10.1179/1743284715Y.0000000098 .

[8] Pye, David. "The Heat Treatment Library". pye-d.com. Archived from the original on 2017-01-11. Retrieved 2017-01-10.

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