Nitrogen is a case-hardening process in which nitrogen is introduced into the surface of a ferrous alloy such as steel by holding the metal at a temperature below that at which the crystal structure begins to transform to austenite on heating as defined by the Iron-Carbon Phase Diagram.

Techniques for Inspection and Quality Control

What is the White Layer?
The “white layer” or compound zone is a very hard, brittle layer that does not diffuse into the steel but remains on the immediate surface. The process variables that control the depth (and make up) of the white layer are time, temperature and gas composition. It typically comprises two intermixed phases, gamma prime ( γ’) and epsilon (ε).

Thickness of the White Layer
The amount of carbon has a small effect on thickness, but it has a pronounced effect on the composition of the white layer (that is, the percentages of gamma prime and epsilon in the layer).

AMS (Aerospace Material Specification) 2759/10 (2006) identifies three classes of processes:
  • Class 0: No white layer
  • Class 1: White layer permitted, 0.00005 inch (maximum)
  • Class 2: White layer permitted, 0.001 inch (maximum)


Measuring the White Layer
Hardness (Rockwell superficial) and microhardness (Knoop or Vickers) tests are commonly used to measure the case depth. Hardness of the white layer is done using microhardness techniques only (metallographic technique and proper selection of indentation load are critical to accurate readings).

The effective case depth in gas nitriding is defined as that point on the curve where the hardness is equal to the core hardness plus 4 HRC points (50 HV). For example, when the core hardness equals 30 HRC (300 HV), the effective case depth is defined as that point where the hardness is 34 HRC (350 HV).

Metallographic Evaluation
Case-depth determination (surface to a point of contrast between the case and core) can be done by metallographic techniques. Suitable etchants for macroscopic (Table 1) and microscopic (Table 2) examination are shown.

Typical Results

White layer (iron nitride) can be 0.0002-0.0020 inch (0.005-0.050 mm) depending on length of cycle and whether single- or dual-stage nitriding is performed. Thickness is measured by metallographic methods: 5% nital or 10% ammonium persulfate. (Note: Etchants darken the case but not white layer.)
  • The two-stage (Floe) process produces a shallower, softer and more ductile white layer than does the single-stage process. The white layer in the Floe process can be held to a maximum 0.0005-0.00075 inch (0.013-0.02 mm), which still may be excessive.


Elimination of White Layer

Engineering drawing specifications may call for the complete removal of the white layer, i.e. “no white layer” (0.0000 inch white layer depth). The ways to achieve this include:

1. The use of several patented processes:
  • U.S. Patent 2,960,421 achieves the removal of the iron-nitride white layer by a diffusion process. Parts are copper plated all over and then heated to and held at 975ºF for periods up to 40 hours depending on the thickness of the white layer to be removed.
  • U.S. Patent 3,069,296 achieves the removal of the iron-nitride white layer by use of a simple alkaline solution that decomposes the iron nitride, making it friable and removed by post nitriding blast cleaning. 200-mesh aluminum-oxide grit is normally recommended. Depending on the surface-finish requirements, either liquid-abrasive or peening with glass bead may be substituted for grit blasting. The procedure does not harm the surface finish and has the added advantage of removing copper plate (during immersion in the alkaline solution) from parts plated for selected nitriding. Tests indicate no decrease in hardness, fatigue strength or impact strength, and etching or pitting of the surface does not occur if done properly.
2. The use of chemical techniques:
  • Citric Acid – Parts are pre-cleaned then immersed in a citric-acid solution heated to 154-170ºF (68-77ºC) for several minutes. After rinsing, the parts are immersed in a neutralizing solution then rinsed again. Glass-bead blasting (400 mesh, 40-80 psig) is then performed, followed again by a cleaning operation. A stress relief operation at 335-365ºF (168-185ºC) for several hours (and within four hours of the acid immersion) completes the process.
  • Sulfuric Acid – Parts are pre-cleaned then immersed in a sulfuric acid solution heated to 165-195ºF (75-90ºC) for several seconds. After rinsing, the parts are immersed in a neutralizing solution, then rinsed again. Alumina oxide blasting (180 mesh, 40-60 psig) is then performed followed again by a cleaning operation. A stress relief operation at 335-365ºF (168-185ºC) for several hours (and within four hours of the acid immersion) completes the process.
3. The use of ion nitriding techniques: post-nitriding machining and pos-nitriding glass-bead blasting

Re-Nitriding

Re-nitriding is not recommended. A nitrided part on which close dimensional tolerances are required must be correctly nitrided the first time. There is no satisfactory way of "de-nitriding" and then re-nitriding (only a brittle case is produced). The nitrided surface could be removed by grinding or sandblasting and the piece then re-nitrided if the resultant change in dimensions could be tolerated (it usually cannot).

Troubleshooting

The most common nitriding problems are detailed in Table 4.

Tips for Nitriding

A wide variety of steels, some stainless steels and certain tool steels can be nitrided using gas, ion (plasma) or salt bath methods. Three of the most common nitrided steels are:
  • SAE 4140. This low-alloy steel is commonly used for nitriding applications. The combination of carbon and alloying elements allows core hardness in the range of 28-32 HRC developed by quenching and tempering at temperatures exceeding approximately 50°F above the nitriding process temperature.
  • SAE 4340. A higher-alloy steel used when a higher core hardness is required, up to 39 HRC, or heavier section sizes require a higher hardenability steel.
  • Nitralloy. This family of steels was specifically designed for nitriding. Typical quenched-and-tempered core hardness is 25-35 HRC. The advantage of the Nitralloy steels is their excellent response to nitriding and the resulting (very) high surface hardness, typically in the 62-65 HRC equivalent range.
Regardless of the steel used for nitriding, two heat-treat methods are commonly used:
  • Method 1. For minimal distortion: Quench-and-temper stock to specified core hardness – rough machine – stress relieve – finish machine – nitride – lap or lightly grind as necessary.
  • Method 2. For maximum machinability: Rough machine – quench and temper to specified core hardness – finish machine – nitride – lap or lightly grind as necessary.
Typical nitrided case depths for steel are 0.010-0.020 inches (0.25-0.50 mm). Shallower or deeper case depths are possible. Significantly longer cycles are required for case depths about 0.020 inch due to the slow diffusion rate of nitrogen into steel. Stainless and tool steel case depths are typically limited to 0.001-0.003 inch (0.025-0.075 mm).

Case depth is generally specified as total case determined by etching a mounted sample, or it can be defined as the depth at which a certain hardness is obtained. The case depth hardness should be specified in terms of the actual core hardness (e.g., case depth at 110% of “core” or core hardness plus 3 HRC points), as the hardness gradient in a nitrided part depends heavily on the prior hardness.

A typical by-product of nitriding is the white layer, a thin layer of extremely hard iron nitride. This layer may or may not be objectionable, but in most cases it must be kept thin.

Fig. 13. Nitriding cost comparison[2]

Economic Considerations

Nitriding is often more expensive than other case-hardening processes (due primarily to the length of cycle), but the increased cost is often offset by the savings resulting from the small amount of distortion. Frequently, the increased cost of the nitriding process (over other methods of surface hardening) is justified by: simplification in manufacturing procedures; low-temperature process with associated low furnace maintenance; low labor costs; and cleanliness of the work. Nitriding is less expensive than most coating or other surface treatments (Fig. 13).

Nitriding should be considered for any application where wear resistance (particularly metal-to-metal wear), fatigue resistance and freedom from distortion are particularly important. Nitriding is not successful for applications involving erosion or low-stress scratching abrasion.

Typical Nitriding Applications

Basically, all iron and steel can be nitrided. Low- and non-alloyed steels can be nitrided in the annealed condition. Medium- and high-alloyed steels should be austenitized, quenched and tempered to develop optimum core properties prior to nitriding. Typical applications include:
  • Cylinder barrels and liners
  • Bushings
  • Gears
  • Piston pins
  • Rotors
  • Shafts
  • Clutch hubs
  • Shackle bolts
  • Thread guides and spindles
  • Cams
  • Crankshafts
  • Gauges
  • Rubber and paper mill rolls
  • Boring bars
  • Camshafts
  • Die-cast dies
  • Clutch Plates