Pulse Plasma Nitriding for Aerospace Applications
When aerospace manufacturers seek to improve the reliability and durability of common materials such as aluminum, stainless steel, and titanium, advanced surface treatment case-hardening options are significantly enhancing wear resistance, fatigue and material performance.
Lightweight aluminum and its alloys are still used in machinery and engine components due to its relative strength and corrosion-resistance. Stainless steel alloys are utilized in aircraft components that require great strength and can handle increased weight, such as actuators, fasteners, and landing gear components. Due to its superior strength-to-weight ratio, titanium is often used in machinery, engine components, hydraulic systems, and the aircraft structural frame.
Traditionally, however, surface treatment has involved one-of-three processes: carburizing, salt-bath nitrocarburizing and gas nitriding. Each process has advantages and disadvantages, but those seeking more precise control of the diffusion layer formation, depth of case hardening, and preservation of component dimensions are increasingly turning to advanced pulse plasma nitriding.
Although pulse plasma has been utilized for decades, superior controls for the DC pulsing signal, along with improved chamber design and construction, allow for more precise temperature control and uniform distribution of the heat zone throughout the hotwall chamber. The result is extremely consistent and uniform nitriding batch-to-batch, with less gas consumption.
“The benefits are more precise control of the diffusion layers, and its broader appeal to heat treat more diverse materials, beyond steel, that include titanium, stainless steel, and even aluminum.” says Thomas Palamides, Senior Product and Sales Manager at PVA TePla America.
In addition, commercial heat treat shops and high-volume part producers can now select from multiple system configurations that offer flexibility, efficiency, repeatability, and throughput optimization. As a result, global aerospace OEMs and component manufacturers are now leveraging these systems to run a cleaner, more efficient operation.
In the aerospace industry, the number and range of pulse plasma applications is rapidly growing. Material-wise, it includes the low-temperature pulse plasma nitriding and nitrocarburizing (350°C - 430°C) of tools and components made of rust- and acid-resistant stainless steel to increase wear resistance without sacrificing corrosion resistance. It also encompasses the high-temperature pulse plasma nitriding (600°C - 900°C) of components made of titanium or titanium alloys.
Applications range from low-temperature pulse plasma nitriding and plasma nitrocarburizing of turbine parts made of high-alloy special steels, to pulse plasma nitriding of aircraft undercarriage components, to even the pulse plasma nitriding of satellite components made of high-alloy steels.
Pulse Plasma Nitriding Advantages
For steel and steel alloys, case-hardening can be achieved by carburizing, nitriding, cyaniding, or carbonitriding. Although carburizing, and nitrocarburizing of steel is a traditional approach, the part has to be raised above the A1 temperature (727°C) on the Iron-Carbon diagram, usually in the temperature range of 900°C - 930°C. Since the solubility of carbon is higher in the austenitic state than the ferritic state, a fully austenitic state is required for carburizing.
Along with the high temperatures and time-at-temperature associated with carburizing, parts can be distorted. Therefore, a post-carburizing heat treatment is required, at a minimum, to reduce internal part stresses. Depending on the part, and its geometric tolerances, limited machining may also be required.
An alternative to carburizing is nitriding, a lower-temperature, time-dependent, thermo-chemical process used to diffuse nitrogen into the surface of metal. One method is salt bath nitriding. In this process, liquid immersion is required, and is typically conducted at 550°C to 570°C. The nitrogen donating medium is a nitrogen-containing salt, such as sodium cyanide, often greater than 50% in concentration. However, with post-salt bath nitriding there is a high washing effort required to remove the residual cyanide-based treatment. In addition, there are disposal costs for salt and washing lye, environmental handling costs, as well as safety and operational liabilities.
Gas nitriding (500°C) and gas nitrocarburizing (540°C - 580°C) are universally accepted procedures, and typically require a high concentration of ammonia (NH3), and a high amount of carrier gas flow (normal pressure process) compared with pulse plasma nitriding. The elemental nitrogen gas constituent diffuses into iron and forms hard nitrides. Because of the reduced temperature compared to carburizing, no quenching is necessary, and therefore the chance for distortion and cracking are lower. Disadvantages of gas nitriding are that it requires the use of flammable gasses like ammonia, high gas consumption, and it is not able to treat nitride rust-and acid-resistant steels.
With recent advancements in pulse plasma nitriding, however, a new level of precision and control is possible which results in uniform and consistent case hardening. Together with the advantages of using environmentally friendly gasses only – in contrast to the use of ammonia in gas nitriding – plasma-based nitriding has become a focal point for additional innovations and a requirement for those that seek a more environmentally and safe solution.
In pulse plasma nitriding, parts are loaded into a heated vacuum chamber. After evacuating the chamber to a working pressure of 50 to 400 Pa, on a supporting fixture, it is covered by a bell chamber. The chamber is evacuated to below 10 Pascals prior to heating and a pulsating DC voltage of several hundred volts is applied between the charge (cathode) and the chamber wall (anode). The process gas in the chamber is then ionized and becomes electrically conducting. For this type of process, nitrogen and hydrogen gas mixtures and gasses with carbon additions, like methane, are often utilized.
Depending on treatment time and temperature, nitrogen atoms diffuse into the outer zone of components and form a diffusion zone. This can be as atomic nitrogen, dissolved in the iron lattice, as well as in the form of included nitrate deposition.
Adding further precision, innovators in pulse plasma have discovered methods to optimize the process through better control of the pulses. In the PulsPlasma® process developed by PVA TePla AG Industrial Vacuum Systems, for example, a precision regulated gas mixture of nitrogen, hydrogen, and carbon-based methane is used. A pulsating DC voltage signal of several hundred volts is delivered in less than 10 microseconds per pulse to ionize the gas. This serves to maximize the time between pulses for superior temperature control throughout the chamber.
“If you have a temperature variance of plus-minus 10 degrees within a batch, you will get completely different treatment results,” says Dietmar Voigtländer, Sales Manager at PlaTeG – Product Group with PVA Industry Vacuum Systems (IVS), Wettenberg, Germany, the manufacturer of PulsPlasma nitriding systems. “However, by controlling the pulse current by means of an exact pulse on and off time management, the overall temperature can be precisely managed with a uniform distribution, from top to bottom, throughout the hot wall chamber.”
A unique feature with this approach is that the system can be switched on to a stable glow discharge at room temperature. Most systems cannot do this because the generators do not supply stable plasma. To compensate, those systems must first be heated to 300°C - 350°C before plasma can be applied, adding time to the process. With PulsPlasma, that time can instead be used to prepare the surface by giving it a fine cleaning.
Even the materials of construction used to manufacture the nitriding system’s furnace itself have been optimized. In all systems, PlaTeG uses insulative materials developed in the aerospace industry to create a furnace wall as thin as 40 millimeters, compared to the industry standard of 150 millimeters. With less wall mass, the furnace requires less energy and time to heat, while still protecting workers that may accidentally touch the outside of the chamber.
With better overall control, the PulsPlasma nitriding furnaces offer multiple heating and cooling zones with each controlled by its own thermocouple. “This will create a very uniform temperature distribution within plus or minus 5 degrees Celsius from the bottom to the top of the furnace,” said Voigtländer.
Uniformity of temperature within a chamber pays a dividend beyond the consistency of nitriding results. With an even temperature throughout the chamber, the entire space is available for loading components which effectively increases the chamber’s capacity.
Stainless Steel – a Softer Steel
One of the key advantages of pulse plasma nitriding is that it is more suited to heat treating of high alloy materials such as stainless steel. When working with steels that have a higher chromium content, liquid nitriding can react with the elemental content and ‘rob’ the metal of some of its corrosion resistance.
Stainless steel has a natural passivation layer of chromium oxide, which inhibits corrosion. To bring nitrogen into the material, the chromium oxide layer must first be removed. With gas nitriding, removal of the passivation layer requires the application of a special chemistry. Stainless steels can also be nitrided in salt baths, but only with a sacrifice of some level of corrosion resistance.
With PulsPlasma nitriding, the treatment is applied directly through controlled ionic bombardment of the surface. By choosing a nitriding temperature below 450°C, and with exact control of the gas mixture, the material surface can be treated without reducing the corrosion resistance of the material.
Increased Production Throughput
Nitriding is a batch process. Innovation in furnace design, through an optimized mechanical operation, can increase efficiency and increase production capacity. While the actual time for nitriding does not change, efficient loading and unloading scenarios play an important part. The PlaTeG plant design can use any one of a mono, shuttle or tandem footprint, to manage throughput, resources, and operations costs.
As a batch process, nitriding typically requires waiting for the prior batch to be treated, cooled, and unloaded before a new batch can be started. Shuttle and tandem extensions are now available to streamline the batch process.
With a shuttle extension, an additional vacuum chamber bottom may be added to a furnace. During a running nitriding process, the unloading of an earlier batch and the loading/preparing of a subsequent batch on the second vacuum chamber is possible. The cycle time therefore for two consecutive batches is reduced because of the overlapping of the time for unloading/loading of a vacuum chamber with the treatment time of the running process.
With a tandem extension there are two complete vacuum chambers which are operated alternately by the vacuum pumps, the process gas supply, the plasma generator and the control unit of the system. In situations such as unmanned weekend operations, an automatic process can be started and controlled for both batches in succession. With this type of operational structure “it is possible to increase overall nitriding capacity by 30-60% annually,” according to Voigtländer.
Because plasma nitriding uses environmentally friendly nitrogen and hydrogen, the furnaces can be collocated with the machining of components without requiring a separate room. Moreover, the pulse plasma nitriding systems produce no polluting gases. This makes nitriding more efficient as part of an overall aerospace manufacturing process as an operator can locate the furnaces beside their drilling machines.
Pulse plasma offers significantly more precision in nitriding through the control of the mixture of gases, the controllability of glow discharge intervals, the design of the pulsed signal, and the use of a highly insulated hot wall nitride furnace. Together with innovations in the design of the furnaces to streamline batch management in nitriding operations, aerospace manufacturers who depend on nitriding components can benefit from greater uniformity of results, better-protected materials, and increased throughput.
This article was written by Jeff Elliott, Technical Writer, for PVA Industrial Vacuum Systems (Wettenberg, Germany). For more information, visit here .
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