|Fig. 21. Ammonia control system (courtesy of Super Systems Inc.)|
|Fig. 14. Dissociation of ammonia and nitrogen pickup in steel during gas nitriding|
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.
|Fig. 15. Nitriding surface and subsurface reactions|
Gas Nitriding Reactions
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)
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)
|Fig. 16. Schematic representation of ammonia dissociation and nitrogen absorption|
Gas Nitriding Activity
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 (aN) is directly proportional to the degree of ammonia dissociation and the flow rate.
aN a a · v (5)
where aN is the activity of atomic nitrogen
a is the degree of dissociation
v is the ammonia flow rate
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 aN.
Thus, the nitrogen potential (KN) derived from equation 4 can be expressed as:
KN = 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|
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.
|Fig. 18. Percentage dissociation as a function of furnace volume|
Gas Nitriding Cycles and Case-Depth Determination
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.
|Fig. 19. Body-centered cubic (bcc) crystal structure; Fig. 20. Face-centered cubic (fcc) crystal structure|
Crystal (Lattice) 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.
|Fig. 22. Anatomy of a hydrogen sensor|
Control of the Nitriding Process
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:
1 volume NH3 → 1/2 volume
N4 + 3/2 volume H4 (7)
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|
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. IH
References available in April’s online exclusive. For more information: Dan Herring is president of THE HERRING GROUP Inc., P.O. Box 884 Elmhurst, IL 60126; tel: 630-834-3017; fax: 630-834-3117; e-mail: email@example.com; web: www.heat-treat-doctor.com. Dan’s Heat Treat Doctor columns appear monthly in Industrial Heating, and he is also a research associate professor at the Illinois Institute of Technology/Thermal Processing Technology Center.