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Nitriding is a process that involves diffusing nitrogen into a ferrous part’s surface. The result is a part with increased surface hardness, wear and corrosion resistance, anti-galling properties and improved fatigue strength.
|Fig. 1. Pit furnace line with removable retort|
Both batch and continuous equipment can be used for gas nitriding. High-volume components are often run in continuous systems with lower per-unit cost. One type of continuous furnace used for nitriding is a pusher-style. Pusher systems could include a washer, oxidation furnace, purge chamber, nitriding furnace, cooling zone, post-purge chamber and load/unload area. Continuous systems tend to be expensive from a capital cost and installation standpoint and are less flexible than batch equipment. The focus of this article will be on batch equipment and the features essential to successful gas nitriding.
Let’s start with a brief explanation of how the process works. In gas nitriding, nitrogen forms via a catalytic decomposition of ammonia (NH3) on the metal surface of a part according to equation 1.
While in this state, nitrogen can diffuse into the surface, and the remaining nitrogen molecules combine with other nitrogen molecules according to equation 2.
The nitrogen activity is controlled by the degree of ammonia dissociation and the ammonia flow rate. Gas nitriding normally consists of one or two stages. During single-stage nitriding, a load of parts is maintained at 925-975°F in an atmosphere controlled at an ammonia dissociation rate of 15-30%. A compound, or white, layer that consists of gamma prime and epsilon nitrides forms on a part’s surface during single-stage nitriding. Because the compound layer can be brittle and could spall in service, it is desirable to minimize white-layer thickness. This led to the development of the two-stage, or Floe, process named after its inventor, Dr. Carl Floe (U.S. Patent No. 2,437,249), which reduces the amount of white layer. This process is described here:
1. Other than time, the first stage is very similar to the single-stage nitriding.
2. During the second stage, the temperature of the parts is either maintained or increased to 1025-1075°F and the ammonia dissociation rate is increased to 65-85%.
|Fig. 2. Tip-up furnace|
Nitriding and nitrocarburizing can be accomplished in a wide range of batch-style furnaces, including pit, bell, box, integral-quench and tip-up. All of these batch furnaces include some of the same essential features and are described below.
Sealed Work Chamber
An atmosphere-tight work chamber is required since ammonia flows through the furnace. In most cases, a retort is located within a furnace. Retorts are normally constructed using Inconel 600 and in some instances Inconel 601 or other stainless steels. It is important to avoid having the retort interact with the ammonia and absorb atomic nitrogen intended for the parts.
A seal is particularly important. A leak not only affects the nitride case but leads to potential safety concerns for the operators. The presence of large amounts of raw ammonia (over about 50 ppm) can lead to irritation of the eyes, shortness of breath and other physiological symptoms. In extreme cases or under the wrong circumstances violent explosions can also occur. To protect the seal, heat is normally removed from the area by water tracing.
Since gas nitriding is accomplished in the convection temperature range, fans are required to circulate not only the nitriding processing gases but also to uniformly transfer the energy from the heat source to the workload. The result is uniform case depth throughout a workload. The placement (above load, below load or in separate heating chamber) is dependent on the type of furnace and load configuration.
Nitriding furnaces are available with gas-fired and electric heating systems. With either method, the heat source (gas burners or electric elements) must be isolated from exposure from the processing gas. In addition, direct radiation is often undesirable due to the risk of overheating. The heating source can be located outside the retort if the furnace includes a retort. Otherwise, it can be located in radiant tubes.
As indicated previously, ammonia provides the source of nitrogen in gas nitriding. Control of the dissociation rate or nitriding potential can be accomplished using either dissociated ammonia (75% hydrogen, 25% nitrogen) or 100% nitrogen. Better white-layer control has been reported when dissociated ammonia is used during the second and subsequent stages of nitriding. Ammonia, like endothermic gas, is extremely flammable. By comparison, carburizing is typically performed at 1600-1750°F in an endothermic processing gas. Since this temperature range is well above the auto-ignition temperature of endothermic gas, it is possible to safely use the burn-in technique when introducing the atmosphere.
In contrast, air must be purged out of the furnace prior to admission of ammonia into a nitriding furnace. The most common method is flowing nitrogen through the furnace. National Fire Protection Agency (NFPA) 86 “Standard for Ovens and Furnaces,” requires that the oxygen be less than 1% prior to admission of ammonia. Typically, between 5 and 10 volume changes of the nitrogen is required to purge the air out of the furnace and can be determined when initially starting up a new furnace.
The exhaust gas contains nitrogen, ammonia and more than 4% hydrogen. To help control the internal furnace pressure and render the exhaust gas harmless, a bubbler is commonly found in the exhaust gas stream followed by a burn-off system to ignite the mixture as it exits the furnace. In many cases, it is also necessary to decompose the exhaust gases in an afterburner, fume scrubber or both.
NFPA 86 also requires that combustible gas be purged out of a furnace at the end of a cycle. The atmosphere must contain less than 50% of the LEL (lower explosion limit) before the furnace can be opened.
Several types of cooling systems are available for nitriding furnaces. The first type includes a water-cooled heat exchanger and blower. In this case, the blower draws the atmosphere out of the furnace, through the heat exchanger and then forces the atmosphere back into the furnace. Another type of cooling system available for retort furnaces includes a blower that forces air through the cavity outside the retort. In some instances, the retort is pulled, and an external cooling stand and blower arrangement are employed.
In the past, the control system for a gas nitriding furnace simply included temperature-control instrumentation. Atmosphere measurement was accomplished using a burette and required manual adjustments of the process-gas flow rates to maintain a specific ammonia dissociation rate. In some instances, this can require 20 or more temperature and ammonia adjustments during a cycle.
Today, metallurgical properties are often achieved by controlling the nitriding potential (KN factor). For example, AMS 2759/10, “Automated Gaseous Nitriding Control by Nitriding Potential,” set forth three classifications, which clearly define the maximum white layer allowed. To obtain repeatable metallurgical results, it is necessary to control the nitriding potential (KN), which is defined by equation 3.
AMS 2759/10 defines nitriding potential range for various materials (4140, 4340, H11, etc.) based upon nitriding process classification. Programmable logic controllers (PLCs) use the process-value data measured by electronic process-gas flowmeters, a pressure transducer and hydrogen analyzer to calculate the nitriding potential. The PLC then adjusts the electronic flowmeters to maintain a specific nitriding potential. The integration of these tools has made it possible to automate the process (time, temperature and nitriding potential).
|Fig. 3. Manipulator for tip-up furnace|
Batch Gas Nitriding Equipment Types
Batch equipment is available in many configurations, including pit, bell, box and tip-up furnaces. Selecting the optimum solution starts with gathering the following information:
1. Number of parts required per unit time as well as the number of production hours available
2. Part size and weight, including critical tolerances
3. Part configuration: Long parts would typically be processed vertically while small parts could be loaded into baskets for processing in either horizontal or vertical equipment
4. Case depth and required metallurgical properties
5. Available floorspace, preferred handling methods (e.g., powered load-transfer cart, overhead crane, forklift truck) and the degree of flexibility and automation needed
6. Process flexibility requirements: One specific part type with one specific recipe (time, temperature, nitriding potential) or multiple parts and/or multiple recipes
7. System configuration: Single stand-alone furnace or part of system
8. Space limitations
9. Degree of integration: Stand-alone or integrated with existing equipment and/or an existing facility
This information is used to help determine the load size, work-chamber size, equipment type and number of pieces of equipment.
This type of furnace includes a cover that provides access for transferring workloads in baskets or loaded fixtures in and out of the unit using an overhead crane (Fig. 1). To increase throughput, a system sometimes includes two removable retorts and two cooling stands. This configuration enables unloading and loading one chamber while the other retort is in the pit furnace. Pit furnaces are especially convenient for processing long parts hung vertically, heavy loads and long-cycle work. Ceiling height can be an issue with these types of systems.
Circulating fans are located in the cover or bottom of the furnace. Positioning the fan in the cover provides easier access for maintenance. The addition of circulation baffles and diffusers provides a defined flow throughout the work area.
Typically, this type of furnace includes a stationary base, heating bell, cooling bell and retort. To increase throughput, the furnace could include two bases and retorts. The advantage of this type of system is that it does not normally require a pit. However, it does require quite a bit more ceiling height than a pit furnace.
This type of furnace is designed for horizontal loading/unloading and uses either a vertical lift or side-swing door at its entrance. They can be part of a line or stand-alone. If part of a line including a powered load-transfer cart, the workload supports consist of roller rails and chain guides similar to internal-quench furnace lines. If it stands alone, the furnace could include alloy skid rails to allow transferring loads in and out via forklift truck.
Either in/out or straight-through types can be used for controlled nitriding and nitrocarburizing (ferritic or austenitic). Gas flow must be carefully controlled to avoid air introduction into the system during load transfer.
This type of furnace consists of a stationary base and tilting top (Fig. 2). A forklift truck can be used to transfer loads in and out of the furnace. As an alternative, manipulators like the one pictured in Figure 3 can be used.
To be energy efficient, this type of furnace requires large workloads. At the same time, it provides flexibility to run different recipes for each workload. In contrast to the other types of batch nitriding furnaces, this furnace does not include a retort. Thus, it requires a gas-tight frame and radiant tubes. Fans can be positioned above or below the workload area. Positioning the fans below the workload area more evenly distributes the heat and atmosphere throughout dense workloads. However, a pit is required to position the motors and provide a means for technicians to service the furnace (Fig. 4).
|Fig. 6. Load of the 1008 steel automotive transmission hubs after gas nitrocarburizing|
Several types of gas nitriding equipment are available today, including pit, bell, box and tip-up furnaces. Selection of the right type of equipment is based upon part size, configuration and weight; cycle times, production rates and load sizes; process requirements; loading method; and system orientation, space limitations and orientation of existing equipment. No matter which type of equipment is selected, it is also important that a forced-circulation system is included to obtain uniform results. Other items to consider are the type of heating and cooling systems, manual or automatic dissociation, or nitriding-potential measurement and/or control. IH
For more information: Contact Patrick Weymer, product manager, BeaverMatic, Inc. Rockford, IL. 61104; tel: 815-963-0005; fax: 815-963-5673; e-mail: patrick.weymer@ beavermatic.com; web: www.beavermatic.com