Nitriding of steels

In this column, we will discuss the nitriding of steels.

Nitriding introduces nitrogen into the surface of a steel at a lower temperature than carburizing. Temperatures of 500-550°C are often used. Since nitriding does not involve heating into the austenite region and subsequent quenching, nitrided parts offer minimum distortion and good dimensional control. In most commercial applications the surface layer is up to 200-300 µm thick and rarely exceeds 600 µm.

Nitrogen forms a solid solution with ferrite at nitrogen contents up to 6 percent. At a nitrogen concentration greater than 6 percent, γ’ (gamma prime) with a composition of Fe₄N, is formed. At nitrogen contents greater than 8 percent, the equilibrium product ε is formed, with the composition of Fe₃N.

In general, nitrided surface layers are stratified into layers, with the outer surface as γ’, and the inner layer as ε. The outer layer of γ’ is often referred to as the “white-layer.” This surface layer is undesirable as it is hard, but very brittle. Typical microstructure of a nitrided part is shown in Figure 1.

For gas nitriding, the primary reaction is the decomposition of ammonia to form elemental nitrogen which diffuses into the steel:

The depth of case and case properties are dependent on the amount of nitride-forming elements in the steel. As the alloying content is increased, so is the hardness of the case. However, the additions of alloying elements also slow down diffusion of nitrogen, thus increasing process time. Figure 2 shows typical process times to achieve a specific case depth for typical nitriding materials.

Nitriding steels are medium carbon steels that contain strong nitride-forming elements such as aluminum, chromium, or molybdenum. The highest hardening is achieved with 1 percent aluminum. These steels are usually quenched and tempered prior to nitriding.

Nitriding is accomplished by gas, salt, or ion nitriding. Process times for gas nitriding can be rather long, depending on the depth of effective case desired. Ion nitriding has faster nitriding times, due to the saturation of the surface with nitrogen resulting in faster diffusion.

The total case depth is often defined as the dark-etching zone on the component cross-section, determined metallographically. For alloys that do not exhibit a sharp transition between the base material and the diffusion zone, the total case depth is defined as the depth below the surface where the hardness is 10 percent higher than the core hardness. The effective case depth is defined as the case depth where the hardness exceeds a certain value. This value is defined by specification or drawing.

Processes

Gas Nitriding

Gas nitriding uses anhydrous ammonia in either a single-stage or two-stage process [2]. In the single-stage process, parts are processed at 495-525°C (925-975°F) for a desired time. This process produces a brittle white compound nitride layer at the surface. The Floe process, which is a two-stage process, reduces the amount of brittle white layer present. The first stage is performed similarly to the single-stage nitriding, while the second stage is performed at a higher temperature of 550-565°C (1,025-1,050°F). This second stage acts in an identical fashion as the diffuse stage in boost-diffuse carburizing. This second stage decreases surface hardness, reduces the thickness of the white layer, and increases the case depth of nitriding.

Molten Salt Nitriding

Molten salt bath nitriding is still done, but at a reduced level due to environmental and regulatory concerns. In this process, at temperatures like gas nitriding, a molten bath of nitrogen containing salt containing cyanide or cyanates is used. Critical dimensions can be held because of the uniform heat transfer of molten salt, and because of the low temperatures used. Molten salt nitriding is also quicker due to the higher nitrogen potential of the bath, compared to gas nitriding.

Plasma (Ion) Nitriding

In plasma (ion) nitriding, parts are processed in a vacuum. The parts are electrically charged with a high voltage, and nitrogen is introduced into the chamber. A plasma is formed due to voltage potential. This voltage potential causes the individual nitrogen atoms to accelerate and impact the part. This process heats up the part, cleans the surface, and nitrogen is absorbed into the part. It is possible to selectively mask parts for nitriding through control of the plasma. In gas or salt nitriding, various stop-off or other chemicals are used. A beneficial compression residual stress field at the surface of the part increases fatigue resistance. This process reduces even further the distortion that can occur in gas nitriding, and provides excellent control of case depth, chemistry, and uniformity. 

References

1. A. K. Rakhit, “Nitriding of Gears,” in Heat Treatment of Gears: A Practical Guide for Engineers, Materials Park, OH, ASM International, pp. 133-158.
2. J. R. Davis, “Process Selection Guide,” in Surface Hardening of Steels, Materials Park, OH, ASM International, 2002, pp. 1-16.
3. D. K. Dwivedi, Surface Engineering — Enhancing Life of Tribological Components, New Delhi, India: Springer (India) Pvt. Ltd., 2018.