Ferritic nitrocarburizing

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Ferritic nitrocarburizing or FNC, also known by the proprietary names "Tenifer", "Tufftride", Melonite, and "Arcor", [Note 1] [1] 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. [2] There are four main classes of ferritic nitrocarburizing: gaseous, salt bath, ion or plasma, and fluidized-bed. [3]

Contents

The process improves three main surface integrity aspects: scuffing resistance, fatigue properties, and corrosion resistance. It has the advantage of inducing little shape distortion during the hardening process. This is because of the low processing temperature, which reduces thermal shocks and avoids phase transitions in steel. [4]

History

The first ferritic nitrocarburizing methods were done at low temperatures, around 550 °C (1,022 °F), in a liquid salt bath. The first company to successfully commercialize the process was the Imperial Chemical Industries in Great Britain. ICI called their process "the cassel" due to the plant where it was developed [5] [6] or "Sulfinuz" treatment because it had sulfur in the salt bath. While the process was very successful with high-speed spindles and cutting tools, there were issues with cleaning the solution off because it was not very water-soluble. [7]

Because of the cleaning issues, Lucas Industries began experimenting with gaseous forms of ferritic nitrocarburizing in the late 1950s. The company applied for a patent in 1961. It produced a similar surface finish as the Sulfinuz process, except for the formation of sulfides. The atmosphere consists of ammonia, hydrocarbon gases, and a small amount of other carbon-containing gases. [8]

This innovation spurred the development of a more environmentally friendly salt bath process by the German company Degussa after acquiring ICI patents. [9] Their process is widely known as the Tufftride or Tenifer process. Following this, the ion nitriding process was invented in the early 1980s. This process had faster cycle times, required less cleaning and preparation, formed deeper cases, and allowed for better control of the process. [10]

Processes

Despite the naming, the process is a modified form of nitriding and not carburizing. The shared attribute of this class of this process is the introduction of nitrogen and carbon in the ferritic state of the material. The processes are divided into four main classes: gaseous, salt bath, ion or plasma, or fluidized-bed. The trade name and patented processes may vary slightly from the general description, but they are all a form of ferritic nitrocarburizing. [11]

Salt bath ferritic nitrocarburizing

Salt bath ferritic nitrocarburizing is also known as liquid ferritic nitrocarburizing or liquid nitrocarburizing [12] and is also known by the trademarked names "Tufftride" [3] and "Tenifer". [13]

The simplest form of this process is encompassed by the trademarked "Melonite" process, also known as "Meli 1". It is most commonly used on steels, sintered irons, and cast irons to lower friction and improve wear and corrosion resistance. [14] [15]

The process uses a salt bath of alkali cyanate. This is contained in a steel pot that has an aeration system. The cyanate thermally reacts with the surface of the workpiece to form an alkali carbonate. The bath is then treated to convert the carbonate back to a cyanate. The surface formed from the reaction has a compound layer and a diffusion layer. The compound layer consists of iron, nitrogen, and oxygen is abrasion resistant and is stable at elevated temperatures. The diffusion layer contains nitrides and carbides. The surface hardness ranges from 800 to 1500 HV depending on the steel grade. This also inversely affects the depth of the case; i.e., a high carbon steel will form a hard, but shallow case. [14]

A similar process is the trademarked "Nu-Tride" process, also known incorrectly as the "Kolene" process (which is the company's name), includes a preheat and an intermediate quench cycle. The intermediate quench is an oxidizing salt bath at 400 °C (752 °F). This quench is held for 5 to 20 minutes before the final quenching to room temperature. This is done to minimize distortion and to destroy any lingering cyanates or cyanides left on the workpiece. [16]

Other trademarked processes are "Sursulf" and "Tenoplus". Sursulf has a sulfur compound in the salt bath to create surface sulfides creating porosity in the workpiece surface. This porosity is used to contain lubrication. Tenoplus is a two-stage high-temperature process. The first stage occurs at 625 °C (1,157 °F), while the second stage occurs at 580 °C (1,076 °F). [17]

Gaseous ferritic nitrocarburizing

Gaseous ferritic nitrocarburizing is also known as controlled nitrocarburizing, soft nitriding, and vacuum nitrocarburizing or by the tradenames "UltraOx", [18] "Nitrotec", "Nitemper", "Deganit", "Triniding", "Corr-I-Dur", "Nitroc", "Nitreg-C", [19] "Nitrowear", and "Nitroneg". [3] [20] The process works to achieve the same result as the salt bath process, except gaseous mixtures are used to diffuse the nitrogen and carbon into the workpiece. [21]

The parts are first cleaned, usually with a vapor degreasing process, and then nitrocarburized around 570 °C (1,058 °F), with a processing time that ranges from one to four hours. The actual gas mixtures are proprietary, but they usually contain ammonia and an endothermic gas. [21]

In comparison to a standard nitriding process, ferritic nitrocarburizing or FNC in a vacuum furnace takes less time to achieve case depth requirements - mainly in part due to the addition of carbon to achieve faster diffusion. [22]

Plasma-assisted ferritic nitrocarburizing

Plasma-assisted ferritic nitrocarburizing is also known as ion nitriding, plasma ion nitriding, or glow-discharge nitriding. The process works to achieve the same result as the salt bath and gaseous process, except the reactivity of the media is not due to the temperature but to the gas ionized state. [23] [24] [25] [26] In this technique intense electric fields are used to generate ionized molecules of the gas around the surface to diffuse the nitrogen and carbon into the workpiece. 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 gaseous ferritic nitrocarburizing with ammonia). Due to the relatively low-temperature range (420 °C (788 °F) to 580 °C (1,076 °F)) generally applied during plasma-assisted ferritic nitrocarburizing and gentle cooling in the furnace, the distortion of workpieces can be minimized. Stainless steel workpieces can be processed at moderate temperatures (like 420 °C (788 °F)) without the formation of chromium nitride precipitates and hence maintaining their corrosion resistance properties. [27]

Post-oxidation black oxide

An additional step can be added to the nitrocarburizing process called post-oxidation. When properly performed, post-oxidation creates a layer of black oxide (Fe3O4), that greatly increases the corrosion resistance of the treated substrate while leaving an aesthetically attractive black color. [28] Since the introduction of the Glock pistol in 1982, this type of nitrocarburizing with post-oxidation finish has become popular as a factory finish for military-style handguns.

This combination of nitrocarburizing and oxidizing is sometimes called "nitrox", but this word also has another meaning. [29]

Uses

These processes are most commonly used on low-carbon, low-alloy steels. However, they are also used on medium and high-carbon steels. Typical applications include spindles, cams, gears, dies, hydraulic piston rods, and powdered metal components. [30]

One of the initial applications of the hardening process for mass-produced automobile engines was by Kaiser-Jeep for the crankshaft in the 1962 Jeep Tornado engine. [31] This was one of many innovations in the OHC six-cylinder engine. The crankshaft was strengthened by Tufftriding in a unique salt bath for two hours at 1,025 °F (552 °C) that, according to Kaiser-Jeep, increased engine life by 50% and also made the journal surfaces hard enough to be compatible with heavy-duty tri-metal engine bearings. [32]

A first-generation Glock 17 adopted in 1985 by the Norwegian Armed Forces under the P80 designation Glock 17 (6825676904) bez fona.jpg
A first-generation Glock 17 adopted in 1985 by the Norwegian Armed Forces under the P80 designation

Glock Ges.m.b.H., an Austrian firearms manufacturer, utilized the Tenifer process until 2010, to protect the barrels and slides of the pistols they manufacture. The finish on a Glock pistol is the third and final hardening process. It is 0.05 mm (0.0020 in) thick and produces a 64 Rockwell C hardness rating via a 500 °C (932 °F) nitride bath. [33] The final matte, non-glare finish meets or exceeds stainless steel specifications, is 85% more corrosion resistant than a hard chrome finish, and is 99.9% salt-water corrosion resistant. [34] After the Tenifer process, a black Parkerized finish is applied and the slide is protected even if the finish were to wear off. In 2010, Glock switched to a gaseous ferritic nitrocarburizing process. [35] Besides Glock other pistol and other firearms manufacturers, including Smith & Wesson and HS Produkt, also use ferritic nitrocarburizing for finishing parts like barrels and slides, but they call it Melonite finish. Heckler & Koch use a nitrocarburizing process they refer to as Hostile Environment. Pistol manufacturer Caracal International, headquartered in the United Arab Emirates, uses ferritic nitrocarburizing for finishing parts such as barrels and slides with the plasma-based post-oxidation process (PlasOx). Grand Power, a Slovakian firearms producer, also uses a quench polish quench (QPQ) treatment to harden metal parts on its K100 pistols. [36]

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<span class="mw-page-title-main">Stainless steel</span> Steel alloy resistant to corrosion

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.

<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.

<span class="mw-page-title-main">Austenite</span> Metallic, non-magnetic allotrope of iron or a solid solution of iron, with an alloying element

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 1000 K (727 °C); other alloys of steel have different eutectoid temperatures. The austenite allotrope is named after Sir William Chandler Roberts-Austen (1843–1902). It exists at room temperature in some stainless steels due to the presence of nickel stabilizing the austenite at lower temperatures.

<span class="mw-page-title-main">Carbon steel</span> Steel in which the main interstitial alloying constituent is carbon

Carbon steel is a steel with carbon content from about 0.05 up to 2.1 percent by weight. The definition of carbon steel from the American Iron and Steel Institute (AISI) states:

<span class="mw-page-title-main">Quenching</span> Rapid cooling of a workpiece to obtain certain material properties

In materials science, quenching is the rapid cooling of a workpiece in water, gas, oil, polymer, air, or other fluids to obtain certain material properties. A type of heat treating, quenching prevents undesired low-temperature processes, such as phase transformations, from occurring. It does this by reducing the window of time during which these undesired reactions are both thermodynamically favorable and kinetically accessible; for instance, quenching can reduce the crystal grain size of both metallic and plastic materials, increasing their hardness.

<span class="mw-page-title-main">Carburizing</span> Heat treatment process in which a metal or alloy is infused with carbon to increase hardness

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.

<span class="mw-page-title-main">Gas tungsten arc welding</span> Welding process

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.

<span class="mw-page-title-main">Case-hardening</span> Process of hardening the surface of a metal object

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<span class="mw-page-title-main">Tempering (metallurgy)</span> Process of heat treating used to increase the toughness of iron-based alloys

Tempering is a process of heat treating, which is used to increase the toughness of iron-based alloys. Tempering is usually performed after hardening, to reduce some of the excess hardness, and is done by heating the metal to some temperature below the critical point for a certain period of time, then allowing it to cool in still air. The exact temperature determines the amount of hardness removed, and depends on both the specific composition of the alloy and on the desired properties in the finished product. For instance, very hard tools are often tempered at low temperatures, while springs are tempered at much higher temperatures.

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").

<span class="mw-page-title-main">Carbonitriding</span> Surface hardening process

Carbonitriding is a metallurgical surface modification technique that is used to increase the surface hardness of a metal, thereby reducing wear.

Bluing, sometimes spelled as blueing, is a passivation process in which steel is partially protected against rust using a black oxide coating. It is named after the blue-black appearance of the resulting protective finish. Bluing involves an electrochemical conversion coating resulting from an oxidizing chemical reaction with iron on the surface selectively forming magnetite, the black oxide of iron. In comparison, rust, the red oxide of iron, undergoes an extremely large volume change upon hydration; as a result, the oxide easily flakes off, causing the typical reddish rusting away of iron. Black oxide provides minimal protection against corrosion, unless also treated with a water-displacing oil to reduce wetting and galvanic action. In colloquial use, thin coatings of black oxide are often termed 'gun bluing', while heavier coatings are termed 'black oxide'. Both refer to the same chemical process for providing true gun bluing.

In metallurgy and materials science, annealing is a heat treatment that alters the physical and sometimes chemical properties of a material to increase its ductility and reduce its hardness, making it more workable. It involves heating a material above its recrystallization temperature, maintaining a suitable temperature for an appropriate amount of time and then cooling.

<span class="mw-page-title-main">Nitriding</span> Nitrogen diffusion case-hardening process

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.

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A cryogenic treatment is the process of treating workpieces to cryogenic temperatures in order to remove residual stresses and improve wear resistance in steels and other metal alloys, such as aluminum. In addition to seeking enhanced stress relief and stabilization, or wear resistance, cryogenic treatment is also sought for its ability to improve corrosion resistance by precipitating micro-fine eta carbides, which can be measured before and after in a part using a quantimet.

<span class="mw-page-title-main">Diffusion hardening</span> Process used in manufacturing

Diffusion hardening is a process used in manufacturing that increases the hardness of steels. In diffusion hardening, diffusion occurs between a steel with a low carbon content and a carbon-rich environment to increase the carbon content of the steel and ultimately harden the workpiece. Diffusion only happens through a small thickness of a piece of steel, so only the surface is hardened while the core maintains its original mechanical properties. Heat treating may be performed on a diffusion hardened part to increase the hardness of the core as desired, but in most cases in which diffusion hardening is performed, it is desirable to have parts with a hard outer shell and a more ductile inside. Heat treating and quenching is a more efficient process if hardness is desired throughout the whole part. In the case of manufacturing parts subject to large amounts of wear, such as gears, the non-uniform properties acquired through diffusion hardening are desired. Through this process, gears obtain a hard wear-resistant outer shell but maintain their softer and more impact-resistant core.

Martempering is also known as stepped quenching or interrupted quenching. In this process, steel is heated above the upper critical point and then quenched in a hot-oil, molten-salt, or molten-lead bath kept at a temperature of 150-300 °C. The workpiece is held at this temperature above martensite start (Ms) point until the temperature becomes uniform throughout the cross-section of the workpiece. After that, it is cooled in air or oil to room temperature. The steel is then tempered. In this process, austenite is transformed to martensite by step quenching, at a rate fast enough to avoid the formation of ferrite, pearlite, or bainite.

<span class="mw-page-title-main">Austempering</span>

Austempering is heat treatment that is applied to ferrous metals, most notably steel and ductile iron. In steel it produces a bainite microstructure whereas in cast irons it produces a structure of acicular ferrite and high carbon, stabilized austenite known as ausferrite. It is primarily used to improve mechanical properties or reduce / eliminate distortion. Austempering is defined by both the process and the resultant microstructure. Typical austempering process parameters applied to an unsuitable material will not result in the formation of bainite or ausferrite and thus the final product will not be called austempered. Both microstructures may also be produced via other methods. For example, they may be produced as-cast or air cooled with the proper alloy content. These materials are also not referred to as austempered.

<span class="mw-page-title-main">Ferritic stainless steel</span> High chromium, low carbon stainless steel type

Ferritic stainless steel forms one of the five stainless steel families, the other four being austenitic, martensitic, duplex stainless steels, and precipitation hardened. For example, many of AISI 400-series of stainless steels are ferritic steels. By comparison with austenitic types, these are less hardenable by cold working, less weldable, and should not be used at cryogenic temperatures. Some types, like the 430, have excellent corrosion resistance and are very heat tolerant.

References

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  2. ↑ Pye 2003 , p. 193.
  3. 1 2 3 Pye 2003 , p. 202.
  4. ↑ Pye 2003 , pp. 193–194.
  5. ↑ Archived at Ghostarchive and the Wayback Machine : ICI End of Process – Castner Process at Cassel Works. Transfilm UK – via YouTube.
  6. ↑ The 'Cassel' 'Sulfinuz' Process. Imperial Chemical Industries. 1954.
  7. ↑ Pye 2003 , p. 195.
  8. ↑ Pye 2003 , pp. 195–196.
  9. ↑ Velstrop, Hans (22 February 2015). "To find the way in the nomenclature jungle of nitrogen diffusion".
  10. ↑ Pye 2003 , pp. 196–197.
  11. ↑ Pye 2003 , pp. 201–202.
  12. ↑ Easterday, James R. "Liquid Ferritic Nitrocarburizing" (PDF). domino-69.prominic.net. Archived from the original (PDF) on 14 August 2011. Retrieved 19 August 2024..
  13. ↑ History of the company, archived from the original on 2009-08-26, retrieved 2009-09-29.
  14. 1 2 Pye 2003 , p. 203.
  15. ↑ Melonite Processing , retrieved 2009-09-17.
  16. ↑ Pye 2003 , pp. 208–210.
  17. ↑ Pye 2003 , p. 217.
  18. ↑ "UltraOx". ahtcorp.com. Retrieved 16 January 2023.
  19. ↑ "nitrocarburizing Nitreg-C". nitrex.com/en/. Retrieved 19 August 2024.
  20. ↑ Pye 2003 , p. 220.
  21. 1 2 Pye 2003 , p. 219.
  22. ↑ "Ferritic Nitrocarburizing (FNC) - Overview". ecm-usa.com. Retrieved 19 August 2024.
  23. ↑ Pye 2003 , p. 71.
  24. ↑ "An Introduction to Nitriding p. 9" (PDF). Archived from the original (PDF) on 2011-12-15. Retrieved 2011-05-27.
  25. ↑ Pye, David (2007). Steel Heat Treatment Metallurgy and Technologies. CRC Press. p. 493. ISBN   978-0-8493-8452-3.
  26. ↑ Mueller, Thomas; Gebeshuber, Andreas; Kullmer, Roland; Lugmair, Christoph; Perlot, Stefan; Stoiber, Monika (2004). "Minimizing Wear Through Combined Thermo Chemical and Plasma Activated Diffusion and Coating Processes" (PDF). Materiali In Tehnologije. 38 (6): 353–357. Retrieved 16 January 2023.
  27. ↑ 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.
  28. ↑ Holm, Torsten. "Furnace Atmospheres 3: Nitrading and Nitrocarburizing" (PDF). ferronova.com. Archived from the original (PDF) on 23 December 2015. Retrieved 8 May 2017.
  29. ↑ For references, see in wikt:nitrox.
  30. ↑ Pye 2003 , p. 222.
  31. ↑ Allen, Jim (30 October 2018). "Classic Engine: Jeep's Tornado Straight-Six". cars.com. Retrieved 16 January 2023.
  32. ↑ Page, Ben (2006). "Tornado 230 CI Engine Information / History". The International Full Size Jeep Association. Retrieved 16 January 2023.
  33. ↑ Kasler, Peter Alan (1992). Glock: The New Wave in Combat Handguns. Boulder, CO: Paladin Press. pp. 136–137. ISBN   978-0-87364-649-9. OCLC   26280979.
  34. ↑ Kokalis, Peter (2001). Weapons Tests and Evaluations: The Best of Soldier of Fortune. Boulder, CO: Paladin Press. p. 321. ISBN   978-1-58160-122-0.
  35. ↑ "Firearms History, Technology & Development". 2010-08-07. Retrieved 25 December 2014.
  36. ↑ "Grandpower on Tenifer QPQ". Archived from the original on 2014-10-26. Retrieved 2011-01-06.
  1. ↑ Other trade names include Tuffride/ Tuffrider, QPQ, Sulfinuz, Sursulf, Meli 1, and Nitride, among others

Bibliography

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