An Overview of Nitriding – Technology and Tribological Benefits
This article will provide an overview of nitriding, which is a gaseous diffusion process in which nitrogen is absorbed into the surface of the steel.
The nitriding process (Fig. 1) begins at the surface of the part with the formation of a compound (aka white) layer, which depends to a large extent on the composition of the steel (in particular, the carbon content). The phases formed within this surface layer are epsilon (e) and gamma prime (g¢). Each has attributes that enhance the mechanical properties of the case and can be varied in percentage as a function of temperature and gas chemistry. The thickness of the compound layer is a function of material (plain-carbon steel forming a thicker compound layer than alloy steels), temperature, time and the process-gas composition.
Three methods of nitriding are commonly used in the industry today: gas nitriding (Fig. 2), ion (plasma) nitriding (Fig. 3) and salt-bath nitriding (Fig. 4). Each method is unique and has both advantages and limitations (Table 1). Salt-bath (aka liquid) nitriding and gas nitriding have similar uses, although gas nitriding is often preferred for environmental reasons and in applications where heavier case depths or stop-off paints are required. In general, nitriding provides better control of case chemistry as well as good case uniformity and has many other advantages (such as lower distortion) over other higher-temperature diffusion processes (e.g., carburizing).
Single-stage and two-stage gas nitriding, as originally developed, is still in use today. By the 1990s, however, Professor Leszek Małdziński (Poland), after analysis of the results of experiments also involving thin iron foils, modified the traditional Lehrer gas nitriding diagram to incorporate iso-concentration curves (Fig. 5). In simplest terms, these curves represent constant nitrogen concentration in iron (both on the surface and at the interphase boundary) as a function of temperature and nitriding potential. This overcame a shortcoming in the Lehrer diagram, which predicts the phase structure of the nitrided layer but does not provide information about the concentration of nitrogen (in a, g¢ and e phases) as a function of temperature and nitriding potential. This is the basis for understanding the kinetics of nitrided layer growth.
Not only can we determine the nitriding potential at any given temperature, but we can also predict the microstructural phases that will be produced, paving the way for practical process simulators. Hence, modern gas nitriding was born, and all new equipment, processes and controls in use today are based on this diagram.
Underneath the compound layer is the diffusion layer, where the diffusion of nitrogen away from the surface results in the formation of nitrides and, in some instances, carbonitrides. In general, the compound zone will have high hardness, good wear resistance and improved corrosion and other tribological properties, while the diffusion zone improves fatigue resistance and other mechanical properties.
The parameter that defines the rate of dissociation of ammonia is the nitriding potential (KN). In practice, the nitriding potential is selected so as to produce the required case depth and microstructure. The higher the nitriding potential, the higher the nitrogen concentration will be, both at the surface and in a particular phase (a, g¢ or e). Thus, in order to maintain a given nitriding potential, one must increase or decrease the flow of ammonia. Clearly, the higher the nitriding potential that is needed, the greater the flowrate of ammonia. The same is true of temperature; as it increases so too does the ammonia flowrate and the speed of nitriding.
Today, advanced nitriding processes control the ammonia flowrate into the furnace retort at a given temperature. As a result, they are capable of producing single (a), dual (g¢+a) or three-phase (e + g¢ + a) compound layers (Fig. 6).
Gas nitriding equipment in use today can be either horizontal or vertical in orientation (Fig. 7 a-b). The furnaces are typically a sealed-retort style with options for vacuum purging to save on nitrogen gases, and they also offer better process control. In older equipment, an ammonia smell was always present in the heat-treat shop. Today’s gas nitriding furnaces have a very important feature: in-situ nitriding potential measurement. The most common method of measuring nitriding potential (aka KN) is to measure the hydrogen content.
Ion (plasma) nitriding equipment is often supplied in either horizontal or vertical (Fig. 8) configurations. Cold-wall designs, which rely on plasma generation for heating, must be very carefully controlled to avoid significant issues with case uniformity. In addition, ion nitriding produces an “edge effect,” where fillets and corners do not see the same electric field configuration/density and can produce a non-uniform case in these areas. Also, non-uniform heating in narrow spaces may cause local overheating and result in more part distortion. Finally, ion nitriding is an inherently low-nitriding-potential process, one that results in low-nitrogen compound-layer formation. Therefore, special process controls are needed similar to the gas nitriding process for applications requiring heavier white-layer formation.
Newer units incorporate a “bipolar pulsed plasma” with DC overcurrent protection, ultra-fast arc detection systems, and frequency and duty-cycle flexibility.
Gas and ion nitriding are growth markets for surface thermochemical treatments due in part to tribological benefits (related to fatigue, lubricity and wear). In addition, nitriding allows for low-temperature, low-distortion heat treatment. Nitriding also offers enhanced corrosion resistance.
For example, the nitride layer can be engineered to achieve specific properties (e.g., wear) according to the needs of the design engineer (Fig. 9).
Several examples of enhanced component design by use of nitriding technology (and related processes) can be summarized as follows.[9-15]
- Nitriding can increase abrasion/wear resistance and improve bending and/or contact-fatigue properties. For example, nitriding increases the bending-fatigue strength of a 3% Cr-Mo steel from 480 to 840 MPa – a 75% improvement. Rolling-contact fatigue time-to-failure of M50 steel bearing components (see #4 in this list) increases by a factor of 10. Nitriding also decreases a part’s coefficient of friction.
- Nitriding leads to improved corrosion resistance over coatings such as hard-chrome and nickel plating (although corrosion resistance is not considered a tribological property, per se). The ability of the nitrided layer to withstand thermal stresses improves part stability, which extends the surface life of tools and other components exposed to heat. Most importantly, the nitrogen picked up by the diffusion layer increases the rotating-bending fatigue strength in parts.
- Medium-carbon steels such as 1045 in the pre-hardened state (quench + temper) can be run using complex nitrocarburizing processes. The microstructures, in particular the epsilon (e) phase formed on the surface, has a significant effect on reducing the sliding friction coefficient and improving wear resistance. It was also noted that the oxidation after nitrocarburizing rarely changes the friction coefficient but increases the wear resistance. With oxidation plus polishing and secondary oxidation treatments, the wear resistance of nitrocarburized specimens is improved even further.
- For high-performance (M50, M50 NiL) tool steels, ferritic nitrocarburizing is known to improve rolling-contact-fatigue (RCF) life, especially in high-stress, marginally lubricated environments (e.g., bearings). The (iron-carbonitride) compound layer is detrimental, while the diffusion zone below the surface is considered beneficial. FNC has little effect on sliding wear performance of M50, which is superior to ferritic nitrocarburized M50 NiL or untreated M50 NiL. The hardness of the diffusion zone and the generation of large compressive residual stresses are the principle contributors to improved mechanical properties.
5.Wear resistance of nitrided engine camshafts and other engine components (e.g., bolts) is enhanced in all cases. This includes resistance to elevated temperatures, which is of particular interest to the racing industry.
- Dry sliding resistance of the Ti-6Al-4V alloys is improved by nitriding or ferritic nitrocarburizing. The nitriding temperature must be selected based on understanding the main wear mechanism for a given application (i.e., specific load and sliding speed conditions). When wear is determined by the resistance of the compound layer (low loads and low sliding speeds), the nitriding treatment has to be carried out at 800˚C (1470˚F). In this case, the compound layer has optimal properties with respect to resistance to adhesion and fragmentation. When the material is exposed to delamination (high loads and high sliding speeds), the strength of the diffusion layer has to be maximized. In this case, the nitriding temperature should be in the range of 900˚C (1650˚F) in order to enhance the hardening of the diffusion layer.
The Future of Nitriding Technology
A more advanced form of gas nitriding known as ZeroFlow® technology offers much tighter nitriding-potential control. In addition, it uses less process ammonia and is equipped with a vacuum purge to minimize nitrogen usage. For example, this technology can extend life in aluminum extrusion dies made of H10, H11 and H13 tool steel that experience severe wear in service due in part to elevated operating temperatures. The bottom line was that a higher volume of aluminum could be extruded into a die that was nitrided only once as opposed to conventional (single-stage) gas nitriding that required reworking (re-nitriding) the die a total of seven times (Fig. 10 – online only).
The die failure mechanisms were found to be due to wear, heat and edge cracking (fracturing). The fracturing was due to the formation of nitriding networks at the corners from over-nitriding (Fig. 11). Resolution of the problem required stringent nitriding-potential control (e.g., going from a KN value of 1.0 to 0.4).
Nitriding is a case-hardening process that offers the design engineer control over important tribological properties as well as minimal component distortion. Optimizing diffusion-related processes and the use of modeling and simulation to control heat treatment are now a practical reality.
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