Gas Nitriding

Gas nitriding is a case-hardening process whereby nitrogen is introduced into the surface of a solid ferrous alloy by holding the metal at a suitable temperature in contact with a nitrogenous gas, usually ammonia. The nitriding temperature for all steels is between 495 and 565°C (925 and 1050°F).

Principal reasons for nitriding are:

• To obtain high surface hardness
• To increase wear resistance and antigalling properties
• To improve fatigue life
• To improve corrosion resistance
• To obtain a surface that is resistant to the softening effect of heat at temperatures up to the nitriding temperature.

Because of the absence of a quenching requirement, with attendant volume changes, and the comparatively low temperatures employed in this process, nitriding of steels produces less distortion and deformation than either carburizing or conventional hardening.

Nitridable Steels

Of the alloying elements commonly used in commercial steels, aluminum, chromium, vanadium, tungsten, and molybdenum are beneficial in nitriding because they form nitrides that are stable at nitriding temperatures. Molybdenum, in addition to its contribution as a nitride former, also reduces the risk of embrittlement at nitriding temperatures. Other alloying elements, such as nickel, copper, silicon, and manganese, have little, if any, effect on minding characteristics.

Although at suitable temperatures all steels are capable of forming iron nitrides in the presence of nascent nitrogen, the nitriding results are more favorable in those steels that contain one or more of the major nitride-forming alloying elements. Because aluminum is the strongest nitride former of the common alloying elements, aluminum-containing steels (0.85 to 1.50% Al) yield the best nitriding results in terms of total alloy content. Chromium-containing steels can approximate these results if their chromium content is high enough. Unalloyed carbon steels are not well suited to gas nitriding because they form an extremely brittle case that spalls readily, and the hardness increase in the diffusion zone is small.

The following steels can be gas nitrided for specific applications:

• Aluminum-containing low-alloy steels 7140 (Nitralloy G, 135M, N, EZ)
• Medium-carbon, chromium-containing low-alloy steels of the 4100, 4300, 5100, 6100, 8600, 8700, and 9800 series
• Hot-work die steels containing 5% chromium such as H11, H12, and H13
• Low-carbon, chromium-containing low-alloy steels of the 3300, 8600 and 9300 series
• Air-hardening tool steels such as A-2, A-6, D-2, D-3 and S-7
• High-speed tool steels such as M-2 and M-4
• Nitronic stainless steels such as 30, 40,50 and 60
• Ferritic and martensitic stainless steels of the 400 and 500 series
• Austenitic stainless steels of the 200 and 300 series
• Precipitation-hardening stainless steels such as 13-8 PH, 15-5 PH, 17-4 PH, 17-7 PH, A-286, AM350 and AM355.

Aluminum-containing steels produce a nitrided case of very high hardness and excellent wear resistance. However, the nitrided case also has low ductility, and this limitation should be carefully considered in the selection of aluminum-containing steels. In contrast, low-alloy chromium-containing steels provide a nitrided case with considerably more ductility but with lower hardness. Tool steels, such as H11 and D2, yield consistently high case hardness with exceptionally high core strength.

Nitriding Process

Prior Heat Treatment. All hardenable steels must be hardened and tempered before being nitrided. The tempering temperature must be high enough to guarantee structural stability at the nitriding temperature: the minimum tempering temperature is usually at least 30°C (50°F) higher than the maximum temperature to be used in nitriding.

In certain alloys, such as series 4100 and 4300 steels, hardness of the nitrided case is modified appreciable by core hardness: that is, a decrease in core hardness results in a decrease in case hardness. Consequently, in order to obtain maximum case hardness, these steels are usually provided with maximum core hardness by being tempered at the minimum allowable tempering temperature.

Single-Stage and Double-Stage Nitriding. Either a single- or a double-stage process may be employed when nitriding with anhydrous ammonia. In the single-stage process, a temperature in the range of about 495 to 525°C (925 to 975°F) is used, and the dissociation rate ranges from 15 to 30%. This process produces a brittle, nitrogen-rich layer known as the white nitride layer at the surface of the nitrided case.

The first stage of the double-stage process is, except for time, a duplication of the single-stage process. The second stage may proceed at the nitriding temperature employed for the first; stage, or the temperature may be increased to from 550 to 565°C (1025 to 1050°F): however, at either temperature, the rate of dissociation in the second stage is increased to 65 to 80% (preferably, 75 to 80%). Generally, an external ammonia dissociator is necessary for obtaining the required higher second-stage dissociation.

To summarize, the use of a higher temperature during the second stage:

• Lowers the case hardness
• Increases the case depth
• May lower the core hardness depending on the prior tempering temperature and the total nitriding cycle time
• May lower the apparent effective case depth because of the loss of core hardness, depending on how effective case depth is defined.

Operating Procedures

After hardening and tempering, and before nitriding, parts should be thoroughly cleaned. Most pans can be successfully nitrided immediately after vapor degreasing. However, some machine-finishing processes such as buffing, finish grinding, lapping, and burnishing may produce surfaces that retard nitriding and result in uneven case depth and distortion. There are several methods by which the surfaces of parts finished by such methods may be successfully conditioned before nitriding.

One method consists of vapor degreasing pans and then abrasive cleaning them with aluminum oxide grit or other abrasives such as garnet, or silicon carbide, immediately prior to nitriding. Any residual grit must be brushed off before pans are loaded into the furnace. Pans should be handled with clean gloves.

A second method consists of preoxidizing the pans in an air atmosphere at approximately 330°C (625°F). This may be done as a separate operation, or it may be incorporated as part of the healing portion of the nitriding cycle if suitable precautions are taken.

Furnace Purging. After loading and sealing the furnace at the start of the nitriding cycle, it is necessary to purge the air from the retort before the furnace is heated to a temperature above 150°C (300°F). This prevents oxidation of parts and furnace components, and, when ammonia is used as the purging atmosphere, avoids production of a potentially explosive mixture. Nitrogen is preferred in place of ammonia for purging, but the same precautions should be taken to avoid oxidation of parts, except when preoxidation is intentionally included as part of the cycle.

A typical purging cycle using anhydrous ammonia follows:

• Close furnace and start flow of anhydrous ammonia gas at as fast a flow rate as is practical with first step.
• Set furnace temperature control at 150°C (300°F) simultaneously. Heat furnace to this temperature but do not exceed.
• When the furnace has been purged to the degree that 10% or less air and 90% or more ammonia are present in the retort, the furnace may be heated to the nitriding temperature.

It is not feasible to incorporate preoxidation as part of the cycle unless nitrogen is available as a purging medium at the end of the 320°C (625°F) oxidizing stage. Under no circumstances should ammonia be introduced into a furnace containing air at 330°C (625°F) because of the explosion hazard.

Purging is employed also at the conclusion of the nitriding cycle when the furnace is cooled from the nitriding temperature. It is common practice to remove the ammonia remaining in the retort with nitrogen to reduce the amount of ammonia that would otherwise be released into the immediate area when the load is removed. Dilution of the ammonia lessens the discomfort to employees working near the furnace. The introduction of nitrogen into the retort can be delayed until the nitrided parts have cooled to below 150°C (300°F).

Nitrogen versus Ammonia for Purging. Advantages of nitrogen as a purging gas include its safety, ease of handling, and ease of control. The use of nitrogen, however, requires additional equipment, including piping.

Ammonia requires no additional equipment and is relatively safe when properly handled; mixtures of 15 to 25% ammonia in air, however, are explosive if ignited by a spark.

Dissociation Rates. The nitriding process is based on the affinity of nascent nitrogen for iron and certain other .metallic elements. Nascent nitrogen is produced by the dissociation of gaseous ammonia when it contacts hot steel parts.

Although various rates of dissociation can be used successfully in nitriding, it is important that the nitriding cycle begin with a dissociation rate of about 15 to 35% and that this rate be maintained for 4 to 10 h. Depending on the duration of the total cycle, temperature should be maintained at about 525°C (975°F).

Typically ammonia is supplied at a flow rate to achieve a minimum of four (4) atmosphere changes in the retort per hour. This initial cycle develops a shallow white layer from which diffusion of nitrogen into the main case structure proceeds.

When nitriding with dissociation rate of 15 to 35%, it is normal to control this rate entirely by the flow rate of ammonia. At a dissociation rate of 75 to 80%, however, it is necessary to introduce completely dissociated ammonia.

Distortion and Dimensional Changes. Distortion in nitriding may result from:

• Relief of residual stresses from prior operations such as welding, hardening, machining, and so forth
• Stress introduced during nitriding due to inadequate support in the furnace, or too rapid or nonuniform heating or cooling.
• Stress is introduced by the increase in volume that occurs in the case. This change causes a stretching of the core, which results in tensile stresses that are balanced by compressive stresses in the case after the parts have cooled to room temperature. The magnitude of the permanent set in the core and case is affected by yield strength of the material, thickness of the case, and by the amount and nature of the nitrides formed.

Stabilizing Treatment. In nitrided pans, there is a balance between compressive stresses in the case and tensile stresses in the core. If this balance is upset by grinding off a part of the case, slow dimensional changes may occur as the stresses approach equilibrium. To prevent these changes, nitrided pans are first ground almost to the final dimensions, then heated to 565°C (1050°F) for 1 h. and finally finish ground or lapped. Parts nitrided and not ground after nitriding have excellent dimensional stability.

Finishing Costs. The amount of distortion resulting from nitriding is small compared to that resulting from other case-hardening processes, which involve quenching to form martensite. Consequently, the increase; cost of the nitriding operation and of steel suitable for nitriding often can be offset by the savings resulting from finishing to size prior to nitriding.