Engineering the Surface
The importance of early-stage design decisions for enhancing surface properties of steel and cast-iron components with nitriding and nitrocarburizing cannot be overstated.
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. 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.
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. 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.
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.
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, 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: firstname.lastname@example.org; web: www.ahtcorp.com.
- T. Burakowski and T. Wierzchon´, Surface Engineering of Metals, Principles, Equipment, Technologies, CRC Press LLC, 1999
- Thermochemical Surface Engineering of Steels, Ed. E. J. Mittemeijer and M. A. J. Somers, Pub. Woodhead Publishing, 2014
- 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
- 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
- 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
- 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
- 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