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Nitriding

Nitriding

Image courtesy of S zillayali (own work) on Wikipedia Commons, published under CC BY 3.0

Image courtesy of S zillayali (own work) on Wikipedia Commons, published under CC BY 3.0

Manufacturers today often seek to harden the exterior of steel and iron parts in order to enhance strength and fracture-resistance properties. A component which has undergone this form of "case hardening" will retain a softer, more pliable core while displaying greater abrasion-resistance. 

Parts producers have developed a variety of methods to achieve surface hardening. Popular techniques include induction hardening, carburizing, and nitriding. The latter process involves using high temperatures to help diffuse nitrogen atoms across the exterior layers of a work piece. The deposition of nitrogen on the surface of steel provides a cost-effective way to strengthen a metal part utilizing easily accessed, inexpensive resources.

 

Nitriding Process

During the nitriding process, nitrogen atoms diffuse across the surface of a low carbon steel part. This deposited material helps form an outer nitride layer which, at sufficiently high temperatures, contributes to case hardening. 

Nitrogen dissolves into the surface of molten iron and steel alloys within very specific temperature ranges. Nitrides also form on the exterior and assist in the formation of a case hardened outer layer. This transformation will cause the surface of the metal to grow more brittle; the hardened metal part resists chipping, abrasions, corrosion and wear more effectively.

 

Nitriding Methods

Manufacturers today rely primarily on three different methods for achieving nitriding. The selection of a particular methodology largely depends upon the manufacturer, the steel alloy and the metal part's intended purpose.

 

Gas Nitriding

Also called "ammonia nitriding", manufacturers have relied upon this method of case hardening steel since the 1920s.

Process Details

A manufacturer will expose the surface of a metal part at high temperatures to gasses containing nitrogen. Production facilities usually utilize ammonia, an inexpensive nitrogen-rich gas, for this purpose.

Benefits

By carefully controlling environmental factors, such as the flow rates of the gases involved and the temperature of the furnace, companies today can accurately determine the amount of available nitrogen and control the nitriding process with some precision. This method currently permits the nitriding of high volumes of parts very cost-effectively. However, manufacturers need to monitor the production process closely to prevent explosions. The cleanliness of the metal parts also impacts the effectiveness of gas nitriding; an oily or greasy surface impedes the diffusion of nitrogen into the metal.

Examples

Manufacturers during the 1930s in Germany developed a variety of different grades of "nitriding steels" using ammonia nitriding. This process has played a significant role in the construction, metals fabrication and transportation industries by providing hardened, corrosion-resistant metals.

 

Salt Bath Nitriding

Manufacturers can also soak metal in a liquid bath of cyanide salt, or other nitrogen-rich material. Due to the potentially toxic by-products resulting from this process, salt bath nitriding no longer enjoys as much popularity in this century as it did several decades ago in the USA. Manufacturing facilities which utilize this process must control environmental risks.

Process Details

When a part undergoes successive exposures to different salt baths, the surface hardening manufacturing process may occur in conjunction with carburizing in an automated production environment. Manufacturers can completely immerse a part in the liquid, ensuring exposure of the entire surface.

Benefits

This form of nitriding occurs faster than some forms of gas nitriding. It can occur within fairly "low tech" production facilities.

Examples

Manufacturers have used salt bath nitriding extensively within the oil and gas drilling industry to create hardened pipes and drills.

 

Plasma Nitriding/Ion Nitriding

Sometimes referred to as "glow discharge nitriding", his high tech manufacturing process relies on electrical fields to create nitrogen ions in a plasma state.

Process Details

The manufacturer can deposit nitrogen from nitrogen gas directly on the surface of a metal work piece via "hot plasmas" used in metal spraying, welding or cladding procedures. Recently, some companies have also developed very sophisticated "cold plasma" methods which take place within a vacuum in a specialized low pressure oxygen-free chamber.

Benefits

This technique for performing nitriding developed comparatively recently. It offers a number of benefits, including the ability to control the deposition of nitrides within fairly close tolerances. The process occurs rapidly since production facilities can utilize nitrogen gas. Parts which undergo plasma nitriding often require no further finishing. Since manufacturers can accurately control the case hardening deposition, plasma nitriding technology sometimes permits the production of very strong case hardened steel parts.

Examples

Many examples of this technology occur today within the aerospace, aviation and biomedical industries. Since plasma nitriding permits the controlled deposition of nitrides, it can produce exceptionally strong metal surfaces.

 

Applications

Modern manufacturing facilities perform nitriding on steel and iron parts used in a wide assortment of industries. Case hardening enables certain alloys and steels to perform significant roles in the transportation, aviation and aerospace industries, for instance. Firearms manufacturers also use this process extensively to harden steel components in weapons. 

Nitriding strengthens metal parts widely used in many industrial environments, such as cam shafts, gears, valves and screws. This low-cost process also provides important surface hardening of tools in some situations.

 

Key Advantages

The nitriding process offers a number of important advantages. First, nitriding strengthens industrial steel comparatively cost-effectively. Nitrogen occurs widely in the environment and remains relatively inexpensive. Second, nitriding sometimes enables companies to extend the effective lifespan of metal tools, components and fabrication materials by promoting greater corrosion-resistance. It case hardens low carbon steel alloys against abrasion, fractures and wear. Third, nitriding enhances steel's surface hardness without causing brittle interiors.


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