CHAPTER 18 What Is Meant by Ferritic Nitrocarburizing? FERRITIC NITROCARBURIZING accomplishes surface treatment ofa part in the ferrite region of the iron-carbon equilibrium diagram (Fig. 1). As the process takes place in the ferrite region, both nitrogen and carbon diffuse into the steel surface. The process is categorized as a thermochemi- cal treatment and is carried out at temperatures between 525 and 650 °C (975 and 1200 °F); the typical process temperature is approximately 565 °C (1050 °F). The purpose of the process is to diffuse nitrogen and carbon atoms into a solid solution of iron, thus entrapping the diffused atoms in the interstitial lattice spaces in the steel structure (Ref 1). As with the nitriding procedure, there are many methods and deriva- tives of ferritic nitrocarburizing. These are discussed in the chapters that follow. Process Benefits Ferritic nitrocarburizing improves the surface characteristics of plain carbon steels, low-alloy steels, cast irons, and sintered ferrous alloys. As described in later sections of this chapter, resistance to wear, fatigue, and corrosion are improved with the introduction of nitrogen and carbon. Scuffing resistance means the resistance to wear on the metal sur- face. This is accomplished by changing the nature of the surface com- pound layer, which is also known as the white layer. The completed compound layer will form with both epsilon ( ε) and gamma prime ( γ ′) phases. The dominant ε-phase resists abrasive wear. Fatigue properties of steel are greatly improved by altering the com- position of the compound layer. This means that treated steel has greater resistance to fatigue failure than an untreated steel (Ref 1). Corrosion Resistance. After ferritic nitrocarburizing, steel parts can withstand many hours in a salt spray environment, whereas an untreated plain carbon steel will fail the corrosion test very rapidly.
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Ferritic nitrocarburizing has been a proven process for many years and
is now gaining much acceptance by engineers. This increased interest in
the process, the author believes, is due to engineers gaining a better under-standing of materials selection and metallurgists gaining a greater under-
standing of process capabilities and restrictions. In addition, many furnace
manufacturers want to serve their clients by developing new and more
efficient process methods and equipment.
The early methods of ferritic nitrocarburizing were accomplished in
low-temperature (550 °C, or 1020 °F) salt baths working on the principle
of the decomposition of cyanide to cyanate (in the ferrite region). Imper-
ial Chemical Industries in England pioneered the salt bath process,
which was called the “Sulfinuz” treatment (Ref 2). The salt also con-
tained a sulfur compound in its chemistry. The process was based on theformation of:
• Nitrides: The nitrides were formed as a result of the nitrogen compo-
nent contained in the cyanide salt. The nitrogen diffused into the steel
to form iron nitrides in low-alloy steels and stable nitrides in higher
alloyed steels.
• Carbon: The carbon was supplied from the salt in limited quantities
and formed carbides, interspersed with the formed nitrides.
• Sulfides: The sulfur addition to the salt formed sulfides in the case,
providing a self-lubricating property.
The action of the molten salt at the process temperature also caused
slight surface porosity on the treated steel. This allowed the surface pores
to become minute reservoirs, retaining lubricant on the immediate surface.
The net result was that the treated component resisted scuffing and
exhibited excellent resistance to frictional wear problems. The process
was a great success with high-speed spindles and high-speed cutting tools.
It did, however, require careful salt bath analysis on a daily basis (Ref 1).
Another challenge of the process was that the salt was not very water solu-ble. The treated component required extensive hot water cleaning after
treatment. Cleaning became a major issue.
Problems associated with salt bath processing led to experimentation
with gaseous methods of ferritic nitrocarburizing. Experiments were con-
ducted in the late 1950s with gaseous methods by Cyril Dawes of Joseph
Lucas Ltd. in England. The company successfully applied for a patent on
the process in 1961 (Ref 3).
The gaseous procedure produced a porous layer very similar to the layer
produced with the Sulfinuz process (with the exception of forming surface
sulfides), which claimed to provide good antifrictional properties. Theprocess patent stated that the gaseous atmosphere consisted of ammonia,
Chapter 18: What Is Meant by Ferritic Nitrocarburizing? /
With the advent of pulsed plasma technology in the early 1980s for ion
nitriding, it did not take long to realize that another method of ferritic nitro-
carburizing had been discovered. This procedure was soon commercial-
ized. Advantages include faster process cycle times, less surface cleaning
and preparation, deeper case formation, and better control of surface metal-lurgy formation. Equipment is now being built that is capable of performing
Chapter 18: What Is Meant by Ferritic Nitrocarburizing? /
Work-holding fixtures and wiring techniques used in liquid nitrocar-
burizing. (a) Typical holding basket for small parts, equipped with afunnel for loading parts into the basket without splashing. Funnel, which is madeof sheet metal, also insures that parts are coated with salt before nesting together.Basket may be made of carbon or alloy steel rod and steel wire mesh. Work mustbe free from oil, or the parts will stick together. Parts must be dry. (b) Inconel bas-ket of simple design. Upper loop of the handle is for lifting; lower loop accom-modates a rod which supports the basket over the furnace. (c) Simple basket withtrays, intended for small parts. Trays provide a maximum of loading space with-out adversely affecting circulation. Entire fixture is made of Inconel. (d) Nettedfixture, of Inconel, for holding small parts with a head or shoulder. (e) Methods of wiring small parts. Black annealed steel wire is used for parts weighing less than10 lb; annealed stainless wire is used for heavier parts. (f ) Hooks, made of nickelalloy rod, for holding circular parts. (g) Method for holding large parts in whichtapped handling holes are available or can be provided. Nickel alloys are used
for such fixtures because of the need for high-temperature strength. Resistance tooxidation is not a factor, as liquid carburizing salts are reducing. (h) Rack forholding six small crankshafts; exploded view shows a crankshaft in position.(i) Special rack for carburizing the outside diameters of bearing races. Holdingplates are made of mild steel; rods, of Inconel.
In Japan Neutrino Inc.Takahashi Bldg., 1-44-3 Fuda, Chofu-Shi, Tokyo 182 Japan
Telephone: 81 (0) 424 84 5550
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