Nitro- Carburizing

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CHAPTER

18What Is Meant by

Ferritic Nitrocarburizing?

FERRITIC NITROCARBURIZING accomplishes surface treatment of

a 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, andcorrosion 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 canwithstand many hours in a salt spray environment, whereas an untreated

l i b l ill f il h i idl



Low Distortion. Another major advantage of the ferritic nitrocarburiz-

ing process is that the procedure is carried out at a low temperature that

prevents phase changes in the steel (from ferrite to austenite), thus reduc-

ing the risk of distortion. Distortion is the result of the release of induced

stresses, the thermal shock of quenching, and the risk of incomplete trans-

formation to martensite. No phase change occurs during the ferritic nitro-carburizing treatment.

194 / Practical Nitriding and Ferritic Nitrocarburizing

The iron-carbon equilibrium diagram. The nitrocarburizing process is carried out in the ferrite region(alpha iron) of the diagram.

Fig. 1



Early History of Ferritic Nitrocarburizing

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 a hydrocarbon gas and other small amounts of carbon-containing

gases (Ref 3).

An important study that contributed greatly to the scientific understand-

ing of gas nitrocarburizing treatments and compound layer structure was

published by Prenosil in 1965 (Ref 4). As a result of the study, many com-panies developed variations of the original patented process and the pro-

cedure was accepted by engineers and metallurgists alike.

Advances in gaseous nitrocarburizing did not stop or hinder the process

technique of using salt baths for the ferritic nitrocarburizing process. If

anything, it spurred on the salt manufacturers to develop more environ-

mentally friendly salts and cleaner procedures. Degussa of Germany

developed the salt bath process of “Tufftride,” a two-component process

that formed both nitrides and carbides in the immediate surface of the

steel (Ref 1). The process will produce only very shallow case depths,

approximately 0.05 mm (0.002 in.) deep, but with high surface hardnessvalues, good fatigue properties, and excellent corrosion resistance. The

process cycle times are relatively short (in the region of 1.5 h), followed

by a quench (Fig. 2).

Once again, the process relies on the decomposition of cyanide to

cyanate, which is accelerated by the introduction of a titanium aeration

tube. The aeration tube passes air through the molten salt from the bottom

of the salt pot. The system requires good operational maintenance in terms

of regular bath desludging, salt analysis, and periodic regeneration. This

requires raising the bath temperature to 575 °C (1070 °F) and holding for

approximately 2 h, followed by another desludging operation. The pur-

pose is to precipitate out of the molten salt any free iron originating from

work support baskets and fastening wire used to wire the components in

place in the work basket (Fig. 3).


Typical time-temperature process cycle for a ferritic nitrocarburizingprocedure using salt baths

Fig. 2



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


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.

Fig. 3



both pulsed plasma ion nitriding and ferritic nitrocarburizing in the same

process chamber and with the same pulsed power pack. The procedure

offers a more controllable, repeatable surface metallurgy.

Why Ferritic Nitrocarburize?The physical benefits of ferritic nitrocarburizing have been listed. The

choice to nitrocarburize is an economic one, when compared to other

methods of achieving the same benefits. Figure 4 presents an approximate

cost comparison of various surface treatments (Ref 5). Besides the direct

cost of the equipment, the process selection procedure should consider the

total investment costs.

Cost of floor space involves direct purchase or rental of space.

Remember, floor space also includes storage area for fixtures and fittings

and workload preparation area.Installation costs are sometimes overlooked. The cost of installation

means the cost of unloading equipment from the delivery vehicle and

positioning the equipment in place. Will riggers need to be hired? It also

means the cost of a new facility if one is built to accommodate the new

equipment, including all plumbing, electrical wiring, gas delivery sys-

tems, water delivery system, and effluent exhaust system.


Cost

CVD TiC

PVD TiN

Cobalt-base

Vapor deposited:

Surface weld:

Iron-base

Ni-Cr-B + WC and fuse

Ni-Cr-B and fuse

13% Cr wireCombustion gun sprayed:

Al2O3

Electrochemical:

Thermochemical:

Electroless:

Plasma sprayed:

WC-Co

Nickel

Cobalt + Cr3C2

Chromium

Nitrocarburizing

Nitriding

Carburizing

Fig. 4 Approximate relative costs of various surface treatments. Source: Ref 5



Cost of Insurance and Freight. What type of crating or container will

be used? After loading is complete, a visual inspection should be carried

out; if possible, digital photographs should be taken of the load in its posi-

tion in the event of possible insurance claims when the contents are later

uncrated. Other related costs and concerns include:

• Loading and delivery from the manufacture site to the point of departure

• Type of shipping line (conference or nonconference)

• Paperwork delays. If the equipment is shipped internationally, incom-

plete or improper paperwork can cause serious delays at either the port

of departure or the port of arrival. Such delays can be very costly.

Check on daily demurrage rates and duties payable.

• Insurance. The equipment must leave the manufacturing facility fully

insured. Before installation, check the suitability of the intended site

with the insurance carrier. Does the room or building have the neces-sary fire protection? Would the existing fire protection system damage

the new equipment?

• Road transport from the port of arrival. Road transportation permits

may be necessary if the vehicle load is considered a wide load.

• Access. Be sure that before the equipment arrives, doors and wall

apertures are large enough to allow easy access of the equipment into

the facility. It can be embarrassing if the furnace will not fit through

the door. Preplanning models can sometimes be used to navigate large

equipment through plants.

Operating costs include materials, energy, disposal of spent chemicals,

labor (including training), rejected materials, and time. All these costs must

be evaluated on a per item or other basis before making a final decision.

Training

To ensure that the furnace goes together the first time (and hopefully

starts the first time), at least two primary discipline people—the operating

person and the maintenance person—should visit the manufacturing sitewhen the furnace is being assembled. They also should be present after the

hot trials to see how the furnace is dismantled. Photographically docu-

ment the critical assembly areas using a camera or video recorder.

Project training can then be broken down into:

• An understanding of both the process and its results: This means under-

standing the process principles, the method of nitrogen diffusion, and

the expected results in relation to the steel being treated.

• An understanding of the equipment performance: This means under-

standing the operation, functions, and capabilities of the equipment, aswell as reactions of the process in relation to part geometry.




While many more considerations may arise when preparing to ship a

furnace from one place to another, the previously mentioned ones will

serve to stimulate thinking between the team responsible for delivering

the equipment and the client. In order for a furnace project to be success-

ful for both the purchaser and the seller, there must be clear lines of com-munication regarding each party’s responsibilities. This must include

expectations of performance from both the furnace manufacturer and the

client.

REFERENCES

1. T. Bell, Ferritic Nitrocarburizing, Met. Eng. Q., May 1976, reprinted

in Source Book on Nitriding, P.M. Unterweiser and A.G. Gray, Ed.,

American Society for Metals, 1977, p 266–278

2. The Cassel Manual of Heat-Treatment and Case Hardening, 7th ed.,

Imperial Chemical Industries Ltd., United Kingdom, 19643. Joseph Lucas Ltd., United Kingdom, British Patent 1,011,580

4. B. Prenosil, Structures of Layers Produced by Bath Nitriding and by

Nitriding in Ammonia Atmospheres with Hydrocarbon Additions,

Härt.-Tech. Mitt., Vol 20 (No. 1), April 1965, p 41–49 (BISI transla-

tion 4720)

5. J.R. Davis, Ed., Surface Engineering for Corrosion and Wear Resis-

tance, ASM International, 2001, p 191




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