Active screen plasma nitriding technology (ASPN) is a new industrial solution that enjoys all the advantages of traditional plasma nitriding but does not have its inconveniences. Different-size parts can be treated in the same batch. It also offers the possibility of oxynitriding and nitriding of stainless steels.





 

Nitriding, which is a nitrogen diffusion technique, has the great advantage of using relatively low temperatures (450-590°C/850-1100°F), allowing it to be utilized on most mechanically finished parts.



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Fig. 1. Active screen surrounding the parts

Development of Nitriding Technologies

Several techniques have been developed to industrially achieve the nitriding process over the last century.  

Gaseous Nitriding (1910-1920)
This process using ammonia (NH3) is still the most commonly used technology for nitriding today. Though the equipment is not expensive, it comes with very high running cost due to high energy and gas consumption. In addition, it does not allow a good control of the structure.

Salt-Bath Nitriding (1940-1950)
Its use today is decreasing, and in many countries it is not even allowed anymore due to the environmental impact of the chemicals and the emissions. Its high running costs stem from the energy consumption and expenses to clean the parts.  

Plasma Nitriding (1965-1975)
First developed using cold-wall furnaces, plasma nitriding has significant advantages: very low running costs (reduced consumption of energy and gases); controllable; optimized structure and layers; and nitriding of stainless steels. Plasma nitriding is totally safe and has no poisonous gas emissions and no negative environmental impact. However, conventional plasma nitriding has a number of well-known difficulties, including the direct application of plasma on the parts to be treated, the risk of arcing, hollow cathodes, white layers, non-homogenous batch temperature and the impossibility to mix parts of different geometries in the chamber. Thus, it requires well-trained, highly skilled operators.

In the early 1980s, plasma nitriding became much easier thanks to the development of pulsed power supplies and automatic gas-flow control. All these innovations using hot-wall furnaces were first presented by co-author Pierre Collignon in 1982. Within 10 years, this new type of furnace and process became the industrial standard. However, some inconveniences exist to this day with hot-wall furnaces (e.g., temperature homogeneity, limited control of metallurgical structure, expensive equipment and maintenance).



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Fig. 2. Commercial ASPN furnace (Courtesy of Plasma Metal/PD2i)

Active Screen Plasma Nitriding

ASPN is a technology that has been developed and commercially used in recent years. There is finally a technology that resolves the difficulties of conventional plasma nitriding.  

ASPN Technology: How it Works
With the active screen, the plasma is no longer applied to the workload but rather to a metallic screen that surrounds the parts (Fig. 1). The parts to be nitrided are either placed on a floating potential or a light bias is applied. Plasma forms on the screen and not on the parts under these conditions. The screen heats up quickly, which heats the workload by radiation. In this way, the whole workload heats up to the correct nitriding temperature. The screen also supplies the active species, which quickly diffuses into the materials to form the expected nitrided layers. The temperature control is achieved by regulating the plasma power on the active screen. Reactive gases (N2, H2, CxHy) are pumped from the middle of the chamber and flow through the active screen. This assures a gentle flow of reactive species over the entire workload.



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Fig. 3. Mixed batches can be treated

 

Treatment of the Parts
Because the parts are no longer directly exposed to the glow, the problems related to conventional plasma nitriding are eliminated (arcing, hollow cathodes, temperature heterogeneity, etc.). Degreasing with chloride solvents or ultrasonic-assisted cleaning for complicated parts is no longer necessary. Simply degreasing by washing (steam cleaning) is sufficient. Homogenous batch loading is no longer required. Therefore, parts with different shapes, geometries and steel types can be treated in the same batch (Fig. 3). The space between parts can be reduced to increase the load density. The entire vacuum chamber can be used due to the excellent temperature homogeneity. In contrast, only 50% of the available chamber volume can be used most of the time with DC plasma-nitriding equipment.  

Metallurgical Structures
Similar to the classic processes, the diffusion layer thickness as well as the hardness depend primarily on the steel used. The hardness obtained typically varies from 600 to 1,200 Vickers, and layer depths vary from 0.01-0.3mm. In addition, ASPN delivers essentially the same structures as conventional DC plasma nitriding.



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Fig. 4.  Hardness and case depth for H13 (left) and D2 (right) tool steels treated at 500°C (932°F)with ASPN

 

ASPN of Tool Steel
Tool steels are nitrided to improve their surface hardness, scratch resistance and wear resistance. To investigate the response of tool steel toward ASPN, H13 (0.35C, 1.5Mo, 5Cr, 1V) and D2 (1.5C, 1.0Mo, 12Cr) steels were nitrided at 500°C for 5-40 hours. Figure 4 shows that both tool steels were hardened considerably after ASPN treatment. The maximum hardness in the nitrided case was more than 1,000 HV. The case depth varied from 100-300 µm, depending on treatment time. Containing a higher amount of chromium, D2 steel shows higher case hardness but shallower case depth than H13.



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Fig. 5.  Optical (a) and SEM (b) microstructures of ASPN 316 steel

 

ASPN of Stainless Steels
It is possible to make low-temperature treatments (350-450°C) on stainless steels due to good temperature homogeneity achieved with ASPN. The complete absence of precipitates, nitrides and carbides allows the steel to remain stainless while increasing its wear resistance by a factor of 100 (Fig. 5). ASPN is applied to parts in nuclear, medical, aerospace, food and chemical applications.  

Nitriding + Post Oxidation
With the active screen, the nitriding can be immediately followed by a plasma-assisted oxidizing process of approximately 30 minutes. This increases the corrosion resistance to the same level as obtained with conventional salt-bath processes. The potential applications are numerous (e.g., tool holders or automotive components like steering knuckles).

ASPN of Polymeric Materials
The ASPN technique has been used to modify the surface of several polymeric materials, including ultrahigh molecular-weight polyethylene (UHMWPE), polypropylene and polyoxymethylene (POM). It has been shown that ASPN can effectively modify the surface chemical composition and bond structure. The ASPN-treated polymers have considerably improved hardness, elastic modulus, creep resistance, scratch resistance and wear resistance.  

ASPN - An Economical Trump
Significant productivity gains are achieved with ASPN due to simplified load buildup, less complex degreasing procedures and higher load density. Furthermore, ASPN enables exceptional savings on consumables and energy (see Sidebar 1). For example, savings on electricity were 15% and 95% on gases for a 1-ton load of forging dies of X40CrMoV5 steel.



Conclusion

ASPN is a recently developed technology that provides solutions to many wear-resistance problems. It resolves the various inconveniences of conventional DC plasma nitriding while maintaining its numerous advantages. Metallurgically, the obtained structures are optimal and may be adapted to any need. They are fully reproducible since every parameter is piloted by automatic controls. The treatments are mostly done on steel but can be extended to other materials (Ti, polymers and powders). High-density loads, reduced energy consumption and reduced gas consumption make ASPN an economically competitive process. It does not produce any dangerous or toxic products or wastes, and it allows working in fully secure conditions with complete respect to the environment. IH  

For more information:  Contact Dr. Christian Kunz, president, PD2i North America, 12816 Stahl Drive, Knoxville, TN 37934; tel: 865-321-3932; e-mail: christian.kunz@pd2i.com; web: www.pd2i.com  



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SIDEBAR 1

ASPN – Lowest Running-Cost Technology for Nitriding

  • Very low gas consumption
  • Low energy consumption due to efficient heating
  • Short cycle times
  • Very low spare-parts consumption

  ASPN – An Environmentally Friendly Technology

  • No poisonous emissions
  • No environmentally hazardous materials
  • Safe for operators


SIDEBAR 2

Typical Applications

  • Components (crankshafts, camshafts, gears, pistons, valves, cylinders, valve springs, shafts, spindle, sliding rails, pump cylinders)
  • All types of hot-forging dies
  • All types of extrusion dies
  • Sheet-metal forming punches and dies
  • Rolling dies
  • Aluminum die-casting molds
  • Plastic extruder screws, molds for injection molding
  • Tool holders and taps