Modern designing of mechanical components and tools should take under consideration their intrinsic stresses and surface condition. The ability of the manufacturing technologies as well as the heat-treating processes to affect such properties must be assessed early in the design process if improving performance and durability of the component are needed.[1-3]

Mechanical surface technologies such as rolling and shot peening play important roles in inducing the surface compressive stress needed for increasing bending fatigue of components such as powdered-metal gears.[4] However, those processes are not always sufficient or well-suited for increasing surface properties to the required level.

Tribological as well as corrosion properties of the product depend not only on the material used for the design, its mechanical technologies used or heat treatment but also on a possible modification of its surface chemistry.[5]

 

Thermochemical Treatment

Thermochemical treatment has been the most effective way of increasing hardness and producing compressive stresses. Those treatments are needed for increasing the life of the components by enhancing their tribological properties and reducing friction and wear. Of various thermochemical treatments, nitriding is the most effective way of generating such stresses. As can be seen from Fig. 1, the presence of compressive stress at the surface minimizes maximum surface stress generated during rotational bending.[4] As a result, a greater bending stress can be applied at the surface before the level of stress at the case-core interface is high enough to initiate failure.[4]

Nitriding and ferritic nitrocarburizing (FNC), occasionally combined with post-oxidizing, are carried out below critical temperatures of the steel and can therefore be easily applied to the majority of fully finished mechanical components as well as to metal-forming tools. Both processes can be carried out in gas using ammonia and/or in plasma using a mixture of nitrogen and hydrogen doped with hydrocarbons, if needed. The plasma ion-nitriding process is shown in Fig. 2.

 

Selecting the Proper Nitriding Method


When metal components or tools require surface enhancements, the designers should decide the treatment method early in the process to mitigate possible future problems and achieve optimal product performance. The most-common methods of nitriding and nitrocarburizing are plasma/ion and gas processes. Both can be applied to finished components with typically minimal effect on their dimensions and surface finish. Any dimensional change can be predicted, and proper tolerances can be written in advance for the machining process.

The sputtering present in plasma/ion nitriding allows treatment of any ferrous or titanium alloys without a need for additional preparation or activation of the surface. This is especially important when the treated objects are made of stainless steel or any self-passivating alloys. On the contrary, gas nitriding with ammonia can only be applied to stainless steels if a proper activation step, usually with corrosive gases, is applied.

The ion nitriding process is called a low-nitriding-potential process since the compound layers (white layers) formed are not very rich in nitrogen. Therefore, they are not brittle. When the components are made of low-alloy or plain-carbon steels, which lack nitride-forming elements, it might be necessary to produce a very thick white layer (WL). This can be achieved using the gas or plasma ferritic nitrocarburizing (FNC) process, which allows for a WL production of 0.025-0.050 mm (0.001-0.002 inch) thick.

   The ion/plasma nitriding process can be selective, if desired, by applying mechanical masking, which eliminates the contact of the glow discharge with the treated surface. This is especially important when fine threads, sharp edges, etc. must not be nitrided.

A simple plug/bolt or a steel cylinder covering the threaded hole or shaft assures complete protection from the treatment. Selective hardening in gas nitriding is more difficult to achieve since it requires a galvanic coating (e.g., copper plating) if 100% effectiveness is desired. On the other hand, gas nitriding can be used to treat various components all-over by ensuring the ammonia has unrestricted contact with the treated surface.

Low-density powder-processed components cannot be nitrided using the gas method since ammonia may penetrate interconnected porosities, resulting in through-hardening and brittleness of the products. Therefore, they should be nitrided using the plasma method.

 

Nitrided surfaces are very hard (>65 HRc equivalent for Nit135M steel) and under compressive stress. As a result of the superior hardness obtained from nitriding, frictional properties and the wear resistance of the component are significantly increased.[4,6] It should be noted that the nitrided layer can resist wear by friction under a high-contact Hertzian stress up to 250 MPa. An example of the tribological behavior of nitrided Nit135 M samples is shown in Fig. 3.

 

Applications of Thermochemical Treatments

Applications of nitriding are very broad. Common examples and benefits resulting from using specific nitriding processes are shown in the figures.

Machinery components requiring partial treatment are good candidates for plasma/ion nitriding (e.g., crankshafts). Bearing journals subjected to wear by friction and bending fatigue are hardened by plasma, but the internal treads are protected from the treatment by applying mechanical masks (Fig. 4).

Large and expensive parts such as engraved rolls and long shafts are very often treated by plasma because of the accessibility of the large equipment and very good control of the layer structure, as well as process uniformity (Fig. 5).

Metal-forming tools such as a large stamping dies for automotive applications, especially those made of cast iron, should only be plasma/ion nitrided. Gas nitriding may result in a significant roughening of the surface (Fig. 6).[3,7]

Various small mechanical parts usually need all-over nitriding treatment, especially when good wear and corrosion resistance are needed. The latter can be increased when the nitrided layer has a sufficiently thick compound zone (white layer). In those situations, the gas nitriding process with ammonia is used (Fig. 7).

Corrosion resistance of the nitrided parts can also be enhanced by formation of a thin magnetite layer on the surface through post-oxidation. This layer has a uniform black or blue color, which increases the product’s attractiveness (Fig. 8).

It should be noted that thermochemical processes also include carburizing, boronizing and other treatments,[2] but those are carried out at a high temperature. Therefore, unlike nitriding, they cannot be applied to finished components.

For more information: Contact Edward Rolinski, DrEng, Dr-habil, VP technology; Advanced Heat Treat Corp., 1625 Rose St., Monroe, Mich., 48162; tel: 319-291-3396; fax: 734-243-4066; e-mail: doctorglow@ion-nitriding.com; web: www.ahtcorp.com.


References

  1. T. Burakowski and T. Wierzchon´, Surface Engineering of Metals, Principles, Equipment, Technologies, CRC Press LLC, 1999
  2. Thermochemical Surface Engineering of Steels, Ed. E. J. Mittemeijer and M. A. J. Somers, Pub. Woodhead Publishing, 2014
  3. E. Rolin´ski, “Plasma Assisted Nitriding and Nitrocarburizing of Steel and other Ferrous Alloys,” Chapter 11 in Thermochemical Surface Engineering of Steels Ed. E. J. Mittemeijer and M. A. J. Somers, Pub. Woodhead Publishing, 2014, pp. 413-449
  4. T. Frech, Ph. Scholzen, Ch. Löpenenhaus and F. Klozke, “Influence of Different Manufacturing Processes on Properties of Surface-Densified PM Gears,” Gear Technology, Sept./Oct. 2018, pp. 66-77
  5. T. Bell and N.L. Loh, Journal of Heat Treating, American Ed. American Society for Metals, Vol. 2 No 3, June 1982, pp. 232-237
  6. J. Senatorski, J. Tacikowski, E. Rolinski and S. Lampman, “Tribology of Nitrided and Nitrocarburized Steels,” ASM Handbook Vol 18, Friction, Lubrication and Wear Technology, ed. G. Totten ASM International, 2017, pp.638-652
  7. E. Rolinski, A. Konieczny, G. Sharp, “Nature of Surface Changes in Stamping Tools of Gray and Ductile cast Iron During Gas and Plasma Nitrocarburizing,” Journal of Materials Engineering and Performance, 2009, Vol. 18 No 8, pp. 1052-1059