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What is Gas Nitriding?

Gas nitriding is a type of case-hardening process whereby nitrogen is introduced into the surface of a solid ferrous alloy (steel) by holding the metal at a suitable temperature in contact with a nitrogenous gas, usually ammonia. The nitriding temperature for steels is between 495 and 565°C (925 and 1050°F).

During nitriding, nitrogen atoms are absorbed into the surface to form hard nitrides. The maximum limit on case depth is about 0.040 inch (1.0 mm) maximum (in certain materials such as Nitralloy 135); typically 0.5 mm or less is achieved in AISI 4000 series steels.

Advantages of Gas Nitriding

Main advantages of gas nitriding are: very low distortion compared to carburizing or conventional hardening. Reason: no quenching is involved, and the nitriding temperatures are lower than other hardening processes.

Nitriding of Gears

SANDMARK uses nitriding extensively for many different gears that we manufacture for our customers. Nitriding is especially suitable for precision gears because gears have symmetric geometry (which is critical for nitriding), and precision and high-speed gears cannot tolerate even minor distortion and deformation, and require high levels of wear and pitting resistance.

Materials

Typical steels that are condusive to nitriding are chromoly steels and Nitralloys. Nitralloys are especially suited for nitriding.

Process

Typical sequence of processes employed by SANDMARK for nitriding resulting in low distortion and high repeatability: (1) core harden – quench and temper (2) rough machine (3) stress relieve (4) finish machine/grind (5) Nitride (6) optional: remove white layer by lapping

Pre Processing

Material must be hardened and tempered before being nitrided. The tempering temperature must be high enough to guarantee structural stability at the nitriding temperature.

In certain alloys, the hardness that can be achieved during nitriding is dependent on the core hardness of the part coming in. Hence, in order to obtain maximum case hardness, these steels should be provided with maximum core hardness by tempering them at the minimum allowable tempering temperature.

Single Stage Nitriding

In the single-stage process, a temperature in the range of about 495 to 525°C (925 to 975°F) is used, and the dissociation rate ranges from 15 to 30%. Single stage nitriding results in the creation of an iron nitride ‘skin’, about 0.001″ to 0.002″ thick. The white layer is not as hard as the case underneath it, and is very brittle. Frequently, this layer is asked to be removed. Lapping is an effective way to remove the white layer. The white layer can also be minimzed by employing dual-stage nitriding.

Dual Stage Nitriding

Dual stage nitriding, also known as Floe process, is simply two single stage processes back to back (the times are different however). Dual stage process is used primarily to reduce the thickness of the white layer (dual stage nitriding minizes the white layer to 0.0006″ or less). It can also increase the case depth especially if a higher temperature is used in the second stage.

Minimizing Distortion, Deformation

• Residual stresses from prior operations such as welding, hardening, machining, etc. can get relieved during nitriding, causing distortion of the geometry. Careful pre-processing which includes tempering will minimize this issue.
• Poor Process Control: Stresses are introduced during nitriding due to inadequate support in the furnace, or too rapid or nonuniform heating or cooling. Precise process control minimizes this issue.
• Uneven case depth is caused by inadequate surface preparation, which will cause distortion. This can be minimized by vapor degreasing, or by careful surface preparation.
• Geometry Factors: When the nitrided case develops, it expands. After the part cools to room temperature, the case region will be in compressive stress and the core area will be in tensile stress. If the part and the case are not symmetrical, the stresses are not balanced and distortion occurs. Because of this, nitriding is appropriate only for symmetrical parts such as shafts and gears. This distortion is dependent on the material being used and the geometry of the part.
• Grinding/Finishing Before Nitriding: All grinding (and finish machining) operations should be carried out to the final dimensions pior to nitriding. In nitrided parts, there is a balance between compressive stresses in the case and tensile stresses in the core (see ‘Geometry Factors’ above). If this balance is upset by grinding off a part of the case, it is possible to see dimensional instability over time. If the white layer needs to be removed, lapping should be used. Parts not ground after nitriding have excellent dimensional stability.