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Ferritic nitrocarburizing is a range of case hardening processes that diffuse nitrogen and carbon into ferrous metals at sub-critical temperatures. The processing temperature ranges from 525 °C (977 °F) to 625 °C (1,157 °F), but usually occurs at 565 °C (1,049 °F). At this temperature steels and other ferrous alloys are still in a ferritic phase, which is advantageous compared to other case hardening processes that occur in the austenitic phase. There are four main classes of ferritic nitrocarburizing: gaseous, salt bath, ion or plasma, and fluidized-bed.

The process is used to improve three main surface integrity aspects:

• • scuffing resistance
• • fatigue properties
• • corrosion resistance

It has the added 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.

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 was the Imperial Chemical Industries in England. They called their process a "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.Because of the cleaning issues the Joseph Lucas Limited company began experimenting with gaseous forms of ferritic nitrocarburizing in the late 1950s. The company applied for a patent by 1961. It produced a similar surface finish as the Sulfinuz process with the exception of the formation of sulfides. The atmosphere consisted of ammonia, hydrocarbon gases, and a small amount of other carbon-containing gases.This spurred the development of a more environmentally friendly salt bath process by the German company Degussa. Their process is the widely known Tufftride 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.

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, creating barriers to slip, 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.[1] Surface hardness of carbonitrided parts ranges from 55 to 62 HRC. Certain pre-industrial case hardening processes include not only carbon-rich materials such as charcoal, but nitrogen-rich materials such as urea, which implies that traditional surface hardening techniques were a form of carbonitriding.

The Carbonitriding process is similar to gas carburization with the addition of ammonia to the carburizing atmosphere, which provides a source of nitrogen. Nitrogen is adsorbed 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 (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. The lower temperature allows oil quenching, or even gas quenching with a protective atmosphere.

Carburizing is a heat treatment process in which iron or steel absorbs carbon liberated when the metal is heated in the presence of a carbon bearing material, such as charcoal or carbon monoxide, with the intent of making the metal harder. 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 via the transformation from austenite to martensite, while the core remains soft and tough as a ferritic and/or pearlite microstructure.[1] This manufacturing process can be characterized by the following key points: It is applied to low-carbon workpieces; workpieces are in contact with a high-carbon gas, liquid or solid; it produces a hard workpiece surface; workpiece cores largely retain their toughness and ductility; and it produces case hardness depths of up to 0.25 inches (6.4 mm). In some cases it serves as a remedy for undesired decarburization that happened earlier in a manufacturing process.

Method

Carburization of steel involves a heat treatment of the metallic surface using a source of carbon. Carburization can be used to increase the surface hardness of low carbon steel.

Early carburization used a direct application of charcoal packed onto the metal (initially referred to as case hardening), but modern techniques apply carbon-bearing gases or plasmas (such as carbon dioxide or methane). The process depends primarily upon ambient gas composition and furnace temperature, which must be carefully controlled, as the heat may also impact the microstructure of the rest of the material. For applications where great control over gas composition is desired, carburization may take place under very low pressures in a vacuum chamber.

References

[1] Oberg, E., Jones, F., and Ryffel, H. (1989) Machinery's Handbook 23rd Edition. New York: Industrial Press Inc.

Further reading

• Geoffrey Parrish, Carburizing: Microstructures and Properties. ASM International. 1999. pg 11 External links
• "MIL-S-6090A, Military Specification: Process for Steels Used In Aircraft Carburizing and Nitriding" (http:/ / www. everyspec. com/ MIL-SPECS/ MIL-SPECS-MIL-S/ MIL-S-6090A_8810/ ) (PDF). United States Department of Defense. 07 JUN 1971.

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Definition

Nitrocarburizing is a thermo-chemical diffusion process in which nitrogen, carbon and oxygen atoms diffuse into the surface of a steel part to form a compound layer on the part's surface. It is a variation of the nitriding process, which is done mainly to provide anti-wear resistance to a metal's surface and to improve its fatigue resistance.

Nitrocarburizing is carried out in gas nitriding furnaces or pit-type gas nitriding furnaces, which operate at temperatures up to 1202°F (650°C) and are used for surface hardening of components by nitrogen impregnation. These furnaces are fitted with an atmosphere circulation system and a provision for purging ammonia (because nitrogen is inert at that temperature, ammonia is used).

The benefits of nitrocarburizing

• • Improved wear
• • Resistance to corrosion
• • Better fatigue strength

Applications for nitrocarburizing

• • Gears and pinion shafts\Plastic injection molding
• • Piston rings
• • Aluminum extrusion dies
• • Forging and forming dies

References

[1] Oberg, E., Jones, F., and Ryffel, H. (1989) Machinery's Handbook 23rd Edition. New York: Industrial Press Inc.

Further reading

• Geoffrey Parrish, Carburizing: Microstructures and Properties. ASM International. 1999. pg 11 External links
• "MIL-S-6090A, Military Specification: Process for Steels Used In Aircraft Carburizing and Nitriding" (http:/ / www. everyspec. com/ MIL-SPECS/ MIL-SPECS-MIL-S/ MIL-S-6090A_8810/ ) (PDF). United States Department of Defense. 07 JUN 1971.

Quench Tempering/Neutral Hardening

This is a process that produces a martensitic microstructure usually performed on machined parts. Also known as “martensitic hardening”, “quench hardening”, "quench tempering" and "neutral hardening".

This process is used, like other processs, to enable the steel to achieve a higher level of hardness and strength. Other benefits include toughness, temperature resistance and high strength for heavy-loaded parts, allowing both lighter and stiffer characteristics. Parts will have the necessary high wear and heat resistance but will still remain tough.

Parts that require grinding to low roughness can get the necessary machinability. The martensitic stainless steel parts will be corrosion-resistant after this type of heat treatment. Tool steels will also be more resistant to wear and heat, and they will achieve a higher level of hardness and machinability.

The “neutral” in neutral hardening simply means that the hardening process is not meant to change the chemical composition of the steel. The neutral hardening process should help the core harden but should not change the surface, so the heated parts of the surfaces are not decarburized or enriched. The parts are heated in a neutral atmosphere and then quenched into agitation-controlled, temperature-controlled, furnace-integral oil baths. This helps to harden the parts. The parts are then tempered to relieve the stresses from the heat treating process reduceing the hardness to the desired specific range.

A lot of terms for similar processes

These processes all share a single idea of returning or limiting a parts distortion or warping during the heat treatment. MPC can perform heat tempering or press tempering to restore heat treated components back to required dimensional tolerances. Most hardening processes involve heating metal work pieces to high temperatures and rapid quenching to generate a martensitic transformation. The heating and rapid quenching along with phase transformations that occur all cause distortion of the work pieces. Flat work pieces such as transmission plates will bend and not be flat after heat treatment. MPC has specific equipment and extensive experience working with and returning automotive parts such as transmission plates to a flat condition by tempering them after hardening while they are clamped or pressed flat. It is this experience which helps MPC deliver parts within the most demanding tolerances and reduce our clients scrap and waste.

Vibratory finishing / Post Cleaning

Vibratory finishing is a type of mass finishing manufacturing process used to deburr, radius, descale, burnish, clean, and brighten a large number of relatively small parts after heat treating. In this batch-type operation, specially shaped pellets of media and the parts are placed into the tub of a vibratory tumbler. The tub of the vibratory tumbler and all of its contents are then vibrated. The vibratory action causes the media to rub against the metal parts which yield the desired result. Depending on the application this can be either a dry or wet process. Unlike tumbling this process can finish internal features, such as holes. It is also quicker and quieter. The process is performed in an open tub so the operator can easily observe if the required finish has been obtained. MPC has several processes with tubs up to 10 feet in diameter.

Vibratory tumblers have an action that is similar to filing. An eccentric, rotating weight shakes the tub in a circular path, during which the entire load is lifted up at an angle and then dropped. As the load is falling (but not actually airborne) the tub returns to an upward position, applying an upward and angular force that causes a shearing action where the parts and media rub against each other. Vibratory finishing systems tend to produce a smooth finish because the media essentially laps the parts. Since the load is moving as a unit, very fragile parts are quite safe in the vibrator. There is no tearing action or unequal forces that tend to bend and distort parts. The larger the parts or media are, the faster the cutting action.

The frequency and amplitude of the machine controls the finish of the parts. The frequencies can vary from 900 to 3600 cycles per minute (CPM) and the amplitude can vary from 0 to 3⁄16 in (4.76 mm). High frequencies, 1800 CPM or greater, and small amplitudes are used for fine finishes or delicate parts, whereas large amplitudes are used for heavier cutting. High frequencies and amplitudes can roll burrs and peen edges. The circulation of parts is best at higher frequencies, therefore, heavy pieces are run at these high frequencies with moderate amplitudes of 3⁄32 to 1⁄8 in (2.38 to 3.18 mm).

Degarmo, E. Paul; Black, J T.; Kohser, Ronald A. (2003), Materials and Processes in Manufacturing (9th ed.), Wiley, ISBN 0-471-65653-4
Wikipedia, The Free Encyclopedia. Retrieved 17:18, August 29, 2014, from http://en.wikipedia.org/w/index.php?title=Vibratory_finishing&oldid=598390776