Principles of Gas Nitriding 2

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Principles of Gas Nitriding (Part 2) by Daniel H. Herring May 3, 2011

Editors note: This is the third of a four-part presentation, including an online-exclusive article in April. In an effortto establish a logical order, we will label figures consecutively as theyappear, including the online content. Consequently, the numbering ofsome figures may appear to be missing, depending on your starting

point.

Nitriding is a case-hardening process in which nitrogen is introducedinto the surface of a ferrous alloy such as steel by holding the metalat a temperature below that at which the crystal structure begins totransform to austenite on heating (Ac 1 ) as defined by the Iron-CarbonPhase Diagram.

Fig. 14. Dissociation of ammonia and nitrogenpickup in steel during gas nitriding[4]

Gas Nitriding Reactions

Gas nitriding is typically done using ammonia with


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or without dilution of the atmosphere with dissociated ammonia ornitrogen (or nitrogen/hydrogen) in the temperature range of 925-1050F (500-565C). Ammonia (NH 3 ) is allowed to flow over theparts to be hardened.

Fig. 15. Nitriding surface and subsurface reactions[5]

Due to the temperature and the catalytic effect of the steel surface,the ammonia dissociates into atomic nitrogen and hydrogen inaccordance with equation 1:

2NH 3 2N + 6 H (1)

This is immediately followed by atomic nitrogen combining to formmolecular nitrogen per equation 2:

2N + 6 H N 2 + 3 H 2 (2)

During the period in which this nitrogen passes through the atomicstate, it is capable of being absorbed into the steel (Fig. 14).

So, the entire reaction equation 3, Figure 15 becomes:


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2NH 3 N 2 + 3 H 2 (3)

Gas Nitriding Activity

In accordance with the laws governing diffusion, the degree ofnitrogen penetration is governed by the temperature and the amountof nitrogen that can penetrate and diffuse into and away from theouter layer of the steel.

Fig. 16. Schematic representation of ammonia dissociation andnitrogen absorption[5]

In gas nitriding, the nitrogen activity is controlled by the degree ofdissociation and the flow rate of the gas (Fig. 16). The nitrogen issupplied by the dissociation of ammonia at the steel surface inaccordance with equation 4, a modified form of equation 1.

NH 3 [N] + 3/2 H (4)

By comparison with gas carburizing, the nitriding atmosphere is notin equilibrium since the flow rate of ammonia is too high to allowequilibrium to be achieved.

The amount of ammonia present in the outlet gas is a measure of the

degree of dissociation. The higher the flow rates of ammonia, thehigher the ammonia percentage in the exiting gas stream and the


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lower the degree of dissociation. However, a greater percentage ofammonia is present at the surface.

Equation 5 provides an explanation of the nitrogen activity in whichthe activity constant (a N ) is directly proportional to the degree ofammonia dissociation and the flow rate.

a N a v (5)

where a N is the activity of atomic nitrogena is the degree of dissociationv is the ammonia flow rate

Consequently, the nitrogen activity is a function of the number ofammonia molecules dissociated at the steel surface per unit of time.At constant pressure and temperature, the degree of dissociation isreduced as the flow rate increases, but the product (a v) increasesand so does aN.

Thus, the nitrogen potential (KN) derived from equation 4 can be

expressed as:

K N = pNH 3 / (pH 2 )3/2

(6)

The amount of white layer can be controlled by minimizing thenitriding potential. AMS 2759/10 (Automated Gaseous NitridingControlled by Nitriding Potential) indicates nitriding potential values(Table 5) for the various classes of white layer.

Nitrogen potential is also referred to as thenitriding parameter. At a constant temperature,the nitrogen activity, and consequently the nitrogen content, at thesurface of the nitrided surface layer are determined by the nitridingpotential. The various phases formed are expressed in the LehrerDiagram (Fig. 17).


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Fig. 17. Lehrer relationship between nitriding potential and the phaseformed within the compound layer[4]

It is also important to guarantee that there is an adequate amount ofnitrogen available during the process to harden the parts to

specification. If there is not enough nitrogen available, theconsequence will be low case depths and hardness, with relatedreductions in physical characteristics.

On the other hand, too much nitrogen at the part surface will result information of a brittle and excessively thick white layer, resulting inembrittlement of the nitrided case.

One of the keys to successful nitriding is controlling the percentage ofammonia available per square area of (work) surface that will supplyatomic (nascent) nitrogen at the surface. It is important to realizethat nitriding is due only to the dissociation of ammonia at the partsurface, not due to the presence of molecular nitrogen (N 2 ) ordissociated ammonia (N 2 + 3 H 2 ).

The nitriding reaction (Eq. 1, 2) will ultimately go to completion, butthis is a very slow reaction. Empirical work has resulted in a rule ofthumb that says if the furnace atmosphere is changed four times


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every hour, the amount of ammonia that is dissociated is 2510%.An approximate relationship between ammonia flow rate andpercentage dissociation exists (Fig. 18). The general shape of thecurve will vary as a function of the furnace style, workload size andsurface area.

Fig. 18. Percentage dissociation as a function of furnace volume[6]

Hence, the best control method for the process is one that measuresand controls the percentage of ammonia. When we talk about a 30%dissociation rate, we normally refer to a concentration of 70%ammonia and 30% dissociated ammonia in the exhaust gas. Inreality, due to the volume change involved, only 82.3% is ammoniawhile 17.7% is dissociated ammonia.

To nitride successfully, an adequate supply of atomic nitrogen mustbe available at the part surface. Thus, in gas nitriding, it becomesvery important to circulate the ammonia in such a way as toconstantly resupply the active nitrogen on all areas to be hardened.

Gas Nitriding Cycles and Case-Depth Determination


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Two types of nitriding processes are used: thesingle-stage process and two- stage or Floe (pronounced flow)process named after its inventor, Dr. Carl Floe.

Case-depth and case-hardness properties vary not only with theduration and type of nitriding being performed but also with steelcomposition, prior structure and core hardness. Case depths aretypically 0.008-0.025 inches (0.20-0.65 mm) and take 10-80 hoursto produce.

Single-Stage Nitriding Process In the single-stage process, a temperature range of 925-975F (500-

525C) is typical. The dissociation rate of ammonia into nitrogen andhydrogen ranges from 15-30%. The process produces a brittle,nitrogen- rich layer known as the white layer (compound zone) atthe surface and is comprised of various iron nitrides (FeN, Fe 4 N,Fe 16 N2 ).

Two-Stage Floe Process (U.S. Patent No. 2,437,249)The two-stage process (Table 7 and 8) was developed to reduce theamount of white layer formed by single-stage nitriding. The firststage is, except for time, the same as that of the single-stageprocess. In the second stage, however, the addition of a dilutant gas(dissociated ammonia or nitrogen) increases the percent dissociationto around 65-85%. The temperature is typically raised to 1025-1075F (550-575C), and the result is the reduction of the depth ofthe white layer, producing a deeper case of slightly lower hardness. Ifthe two-stage method is used, it is frequently possible to meet

dimensional tolerances without any final grinding operation.


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Dissociated ammonia is generally required for high second-stagedissociation (otherwise erratic control may result), and it is commonlyused as a dilutant (to change the percentage per square area thatNH 3 molecules are exposed to). In some cases, nitrogen is used.However, white-layer control and porosity can be affected. Loadingarrangement and the use of a furnace circulating fan are veryimportant so that a high dissociation level may be achieved. Thenitrogen potential varies with the composition of the gas mixture thatis being sent into the furnace.

Crystal (Lattice) Structure

Ferrite, or alpha ( ) iron, which is a body-centeredcubic (bcc) in crystal structure (Fig. 19), dissolves 0.001% nitrogenat room temperature and 0.115% nitrogen at 1095F (590C).Gamma prime ( ), or Fe4N, has a face-centered cubic (fcc) crystalstructure (Fig. 20) and dissolves 5.7-6.1% nitrogen. Fe 2 N and Fe 3 Nare called epsilon ( ), which has a hexagonal closed packed (hcp)crystal structure and dissolves between 8.0% and 11.0% nitrogen.

Fig. 19. Body-centered cubic (bcc) crystal structure; Fig. 20. Face-centered cubic (fcc) crystal structure

Control of the Nitriding Process


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There are several methods of controlling thenitriding process based on analysis of thepercentage of dissociation.

One method involves the use of an ammonia analyzer (Fig. 21),which is tied into ammonia and dissociated ammonia (or nitrogen)flowmeters (for use during the second stage of nitriding). Based onthe output from the ammonia analyzer, the process can be accuratelycontrolled.

Fig. 21. Ammonia control system (courtesy of Super Systems Inc.)

Another method used to measure the degree of dissociation is an

analysis of the amount of hydrogen in the exhaust gas (Fig. 22).From equation 4 we see:


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1 volume NH 3 1/2 volumeN4 + 3/2 volume H 4 (7)

Fig. 22. Anatomy of a hydrogen sensor[5]

For example, if the measured volume percentage of hydrogen is30%, the volume percentage nitrogen is 10% (30/3), and theremaining ammonia volume is 60% (100% 30% 10%). Given theoriginal volume of ammonia supplied ( ) into the furnace chamber,

equation 8 allows us to calculate the degree of dissociation ( ) in theexhaust gas.

1 /100 = (1 /100) [(1 /100 + 2( /100)] (8)Instruments for in-situ measurement of the nitriding potential via thehydrogen content (and other methods) are commercially availableand under development. These types of continuous-measurementdevices are especially important for the short cycles up to 20 hours.


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Alternately, a manual method for the control of the nitridingatmosphere involves the use of a dissociation pipette or burette (Fig.23).

Fig. 23. Manual measurement of percentage dissociation[8]

Ammonia is completely soluble in water. When water is introducedinto the dissociation pipette, any ammonia present dissolves,instantly forming ammonia hydroxide (NH 4 OH). Water continues toenter until it occupies a volume equivalent to that previously occupied

by the ammonia. The remainder of the exhaust gas, being insolublein water, collects at the top of the pipette. The height of the waterlevel is read directly from the scale of graduations, and this readingindicates the percentage of non-water-soluble hydrogen-nitrogen gasin the sample.

This reading, although not completely accurate, is the degree ofdissociation. It should be noted that the dissociation of ammonia

involves a twofold increase in volume as shown in equation 3. IH


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References available in Aprils online exclusive

For more information: Dan Herring is president of THE HERRING GROUPInc., P.O. Box 884 Elmhurst, IL 60126; tel: 630-834-3017; fax: 630-834-3117; e-mail: [email protected]; web:www.heat-treat- doctor.com. Dans Heat Treat Doctor columns appe armonthly in Industrial Heating , and he is also a research associateprofessor at the Illinois Institute of Technology/Thermal Processing

Technology Center.

Daniel H. Herring

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Principles of Gas Nitriding 2