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Principles of Gas Nitriding - Part 3-4

By Dan Herring

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

This is the third and four part of a four-part article on the principles of Gas Nitriding (PART 1-2). Nitriding is a case-hardening process in which nitrogen is introduced into the surface of a ferrous alloy such as steel by holding the metal at a temperature below that at which the crystal structure begins to transform to austenite on heating (Ac₁) as defined by the Iron-Carbon Phase Diagram.

Gas Nitriding Reactions

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

Gas nitriding is typically done using ammonia with or without dilution of the atmosphere with dissociated ammonia or nitrogen (or nitrogen/hydrogen) in the temperature range of 925-1050°F (500-565°C). Ammonia (NH₃) is allowed to flow over the parts to be hardened.

Due to the temperature and the catalytic effect of the steel surface, the ammonia dissociates into atomic nitrogen and hydrogen in accordance with equation 1:
2NH₃ > 2N + 6 H (1)

This is immediately followed by atomic nitrogen combining to form molecular nitrogen per equation 2:
2N + 6 H > N₂ + 3 H₂ (2)

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

During the period in which this nitrogen passes through the atomic state, it is capable of being absorbed into the steel (Fig. 14).
So, the entire reaction – equation 3, Figure 15 – becomes:

2NH₃ > N2 + 3 H2 (3)

Gas Nitriding Activity

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

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

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

NH₃ > [N] + 3/2 H (4)

By comparison with gas carburizing, the nitriding atmosphere is not in equilibrium since the flow rate of ammonia is too high to allow equilibrium 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, the higher the ammonia percentage in the exiting gas stream and the lower the degree of dissociation. However, a greater percentage of ammonia is present at the surface.

Equation 5 provides an explanation of the nitrogen activity in which the activity constant (a_N) is directly proportional to the degree of ammonia dissociation and the flow rate.

a_N a a · v (5)

where a_N is the activity of atomic nitrogen

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

Thus, the nitrogen potential (K_N) derived from equation 4 can be expressed as:

K_N = pNH₃ / (pH₂)^3/2 (6)

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

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

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

It is also important to guarantee that there is an adequate amount of nitrogen available during the process to harden the parts to specification. If there is not enough nitrogen available, the consequence will be low case depths and hardness, with related reductions in physical characteristics.

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

One of the keys to successful nitriding is controlling the percentage of ammonia available per square area of (work) surface that will supply atomic (nascent) nitrogen at the surface. It is important to realize that nitriding is due only to the dissociation of ammonia at the part surface, not due to the presence of molecular nitrogen (N2) or dissociated ammonia (N₂ + 3 H₂).

The nitriding reaction (Eq. 1, 2) will ultimately go to completion, but this is a very slow reaction. Empirical work has resulted in a rule of thumb that says if the furnace atmosphere is changed four times every hour, the amount of ammonia that is dissociated is 25±10%. An approximate relationship between ammonia flow rate and percentage dissociation exists (Fig. 18). The general shape of the curve will vary as a function of the furnace style, workload size and surface area.

Hence, the best control method for the process is one that measures and 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. In reality, due to the volume change involved, only 82.3% is ammonia while 17.7% is dissociated ammonia.

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

Gas Nitriding Cycles and Case-Depth Determination

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

Two types of nitriding processes are used: the single-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 the duration and type of nitriding being performed but also with steel composition, prior structure and core hardness. Case depths are typically 0.008-0.025 inches (0.20-0.65 mm) and take 10-80 hours to produce.

Single-Stage Nitriding Process

In the single-stage process, a temperature range of 925-975°F (500-525°C) is typical. The dissociation rate of ammonia into nitrogen and hydrogen ranges from 15-30%. The process produces a brittle, nitrogen-rich layer known as the “white layer” (compound zone) at the surface and is comprised of various iron nitrides (FeN, Fe₄N, Fe₁₆N₂).

Two-Stage Floe Process (U.S. Patent No. 2,437,249)

The two-stage process (Tables 4, 5) was developed to reduce the amount of white layer formed by single-stage nitriding. The first stage is, except for time, the same as that of the single-stage process. In the second stage, however, the addition of a dilutant gas (dissociated ammonia or nitrogen) increases the percent dissociation to around 65-85%. The temperature is typically raised to 1025-1075°F (550-575°C), and the result is the reduction of the depth of the white layer, producing a deeper case of slightly lower hardness. If the two-stage method is used, it is frequently possible to meet dimensional tolerances without any final grinding operation.

Dissociated ammonia is generally required for high second-stage dissociation (otherwise erratic control may result), and it is commonly used as a dilutant (to change the percentage per square area that NH₃ molecules are exposed to). In some cases, nitrogen is used. However, white-layer control and porosity can be affected. Loading arrangement and the use of a furnace circulating fan are very important so that a high dissociation level may be achieved. The nitrogen potential varies with the composition of the gas mixture that is being sent into the furnace.

Crystal (Lattice) Structure

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

Ferrite, or alpha (a) iron, which is a body-centered cubic (bcc) in crystal structure (Fig. 19), dissolves 0.001% nitrogen at room temperature and 0.115% nitrogen at 1095°F (590°C). Gamma prime (g’), or Fe₄N, has a face-centered cubic (fcc) crystal structure (Fig. 20) and dissolves 5.7-6.1% nitrogen. Fe₂N and Fe₃N are called epsilon (e), which has a hexagonal closed packed (hcp) crystal structure and dissolves between 8.0% and 11.0% nitrogen.

Control of the Nitriding Process

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

There are several methods of controlling the nitriding process based on analysis of the percentage 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 on the output from the ammonia analyzer, the process can be accurately controlled.

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:

For example, if the measured volume percentage of hydrogen is 30%, the volume percentage nitrogen is 10% (30/3), and the remaining ammonia volume is 60% (100% – 30% – 10%). Given the original volume of ammonia supplied (a) into the furnace chamber, equation 8 allows us to calculate the degree of dissociation (b) in the exhaust gas.

1 – b/100 = (1 – a/100) ÷
[(1 – a/100 + 2(a/100)] (8)

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

Instruments for in-situ measurement of the nitriding potential via the hydrogen content (and other methods) are commercially available and under development. These types of continuous-measurement devices are especially important for the short cycles – up to 20 hours.

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

Ammonia is completely soluble in water. When water is introduced into the dissociation pipette, any ammonia present dissolves, instantly forming ammonia hydroxide (NH₄OH). Water continues to enter until it occupies a volume equivalent to that previously occupied by the ammonia. The remainder of the exhaust gas, being insoluble in water, collects at the top of the pipette. The height of the water level is read directly from the scale of graduations, and this reading indicates the percentage of non-water-soluble hydrogen-nitrogen gas in the sample.

This reading, although not completely accurate, is the degree of dissociation. It should be noted that the dissociation of ammonia involves a twofold increase in volume as shown in equation 3.

Earlier in this article we have addressed the basics of the process and the details of the process chemistry, now let's talk about selecting the proper backfill gas and backfill system, whether it be a partial-pressure atmosphere, a process gas, a quench gas or a gas used to equalize pressure between chambers for load transfer or removal.

Nitridable Steels

Fig. 24.  Effect of individual alloying elements on nitriding

A variety of steels can be nitrided. Steels with alloying elements such as aluminum, chromium, vanadium and molybdenum are more easily nitrided because they form nitrides that are stable at nitriding temperature. In general, the higher the percentage of alloying elements, the lower the nitriding temperature required and the higher the hardness achieved.

Effects of Alloying Elements

Individual alloying elements (Fig. 24) have different responses to nitriding. These include:

Properties of Nitrided Steels

Mechanical properties (Table 11) can be enhanced by gas nitriding.

Fig. 25.  Influence of nitriding temperature on surface hardness and depth of nitriding (Nitralloy 135M, 60 hour cycle)[4]

Factors Affecting Case Depth

Users of the nitriding process are interested in the variables that determine the depth and properties (particularly the hardness) of the case produced. The key factors affecting the case depth are: the nature of the nitriding medium; nitriding time and temperature; the amount and nature of nitride-forming elements in solid solution in the steel; and the type, amount and nature of other elements in the steel (C, Ni, Si). In particular:

• The hardness of the case decreases with temperature. Lower temperatures decrease case depth for a given treatment time.
• The amount and nature of the nitride-forming elements in the steel affect the case depth to the extent that the penetration of nitrogen is inversely proportional to the amount of nitrogen that is precipitated (as the alloy nitrides) for a given cycle. The lower the alloy content, the deeper the case for a given time cycle.
• The major function of carbon is to combine with other alloying elements to form carbides, thus removing them from possible reaction with nitrogen.
• It is also known that nickel and silicon together reduce case depth slightly.

Fig. 26.  Influence of nitriding time on surface hardness (Nitralloy 135M nitrided at 950°F (510°C)[4]

A change in nitriding temperature (Fig. 25) from 900° to 1110°F (480° to 600°C) influences the case depth and relative surface hardness. Similarly, a change in nitriding time at constant temperature can result in a variation in surface hardness (Fig. 26).   

The Iron-Nitrogen phase diagram (Fig. 27) provides a “road map” to help determine the type of structures that will be produced in the nitriding process. This figure tells us what happens when nitrogen diffuses into the surface of pure iron.

Fig. 27.  Iron- nitrogen equilibrium diagram[10]

At nitriding temperatures above 840°F (450°C), nitrogen will dissolve (interstitially) in ferrite (alpha-iron) but only up to a concentration of 0.1% at 1095°F (590°C). When nitrogen content exceeds this value, Fe₄N or gamma prime (y') forms up to a nitrogen content of 5.7-6.1%. This nitride will precipitate at the grain boundaries and preferentially along certain crystallographic planes. As nitriding continues, these nitrides increase in size (as well as quantity) until the entire microstructure has been transformed into a layer of y’. This is the so-called “compound or white layer.”

Fig. 28.  Initiation of nitriding[9]

As the concentration of nitrogen continues to increase, the nitrogen content in the layer also increases. When the nitrogen concentration exceeds 6.1%, y’ nitride starts to change to Fe₃N and Fe₂N, both referred to as epsilon (e) nitride. This transformation starts at the surface (where the nitrogen concentration is greatest), and the y’ layer gradually transforms into epsilon nitride. Meanwhile, the y’ layer is driven deeper (Fig. 28).

Fig. 29.  Composition of nitrided layer[9] (a) Layer representation; (b) Actual layers (compound and diffusion zones)

Despite the high iron content of the epsilon layer (7.5-9%), the white (compound) layer is nonmetallic in nature. Simultaneously with the increase in thickness of the white layer, the nitrogen diffuses deeper into the steel and new nitrides are precipitated. The zone below the compound layer is called the “diffusion zone” (Fig. 29).   

Since most steels also contain carbon, carbonitrides are formed as well. The nitrides and carbonitrides, in combination with the distortion of the lattice from interstitial atom addition, are quite hard, typically exhibiting ≥ 67 HRC (900-1100 HV) equivalent.

Daniel H. Herring / Tel: (630) 834-3017) /E-mail: dherring (at) heat-treat-doctor.com

Dan Herring is president of THE HERRING GROUP Inc., which specializes in consulting services (heat treatment and metallurgy) and technical services (industrial education/training and process/equipment assistance. He is also a research associate professor at the Illinois Institute of Technology/Thermal Processing Technology Center.

References

1. Herring, D. H., "What Happens to Steel During Heat Treatment? Part One: Phase Transformations," Industrial Heating, April 2007.
2. Tarfa, Tahar Nabil, "Controlled Gas Nitriding for Powertrain Components," Global Powertrain Conference, 2004, Nitrex Metals (www.nitrex.com)
3. Diggs, Thomas G., Rosenberg, Samuel J., and Geil, Glenn W., "Heat Treatment and Properties of Iron and Steel," National Bureau of Standards, Monograph 88, 1966.
4. Principles of Nitriding and Nitrocarburizing, Atmosphere Heat Treating, Basic Seminar, Ipsen International, 1998.
5. Winter, Karl-Michael, "Nitriding Sensors & Controls," Nitrex Metals Nitriding Symposium, 2006 and private correspondence.
6. Lotze, Thomas, Gas Nitriding, Application Bulletin A4601, Super Systems, Inc.
7. Mr. James Oakes, Super Systems, Inc. (www.supersystems.com), private correspondence.
8. ASM Handbook, Volume 4, Heat Treating, 1991.
9. Pye, David, "Practical Nitriding and Ferritic Nitrocarburizing," ASM International, 2003.
10. Source Book on Nitriding, ASM International, 1977.
11. Braziunas, V. Prokka, L, and Herring, D. H, "Automated Measuring System for Gas Nitriding/Nitrocarburizing," Industrial Heating, January 2005.
12. Metals Handbook, Volume 2: Heat Treating, Cleaning and Finishing, ASM International, 1964, pg. 153.
13. Herring, D. H., “Cleaning of Parts and Fixtures,” Industrial Heating, Online Exclusive, Part 13, 2010.
14. Vanderwoort, George F., Metallography Principles and Practices, McGraw-Hill Book Company, 1984.