Industrial Heating Experts Speak Blog

Nitride Surface Metallurgy (part 2)

September 24, 2012
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David Pye

When specifying the hardness required on an engineering drawing for a nitrided component, the engineer producing the drawing should state:
  • The case depth required
  • Hardness value required
  • The method of hardness testing – micro or macro (Vickers, Knoop, Rockwell superficial)
  • The hardness-test load value
  • Cross transverse traverse or direct indenter load onto the immediate surface
The most accurate method of case-depth measurement is by a transverse cross traverse of a cut sample and at right angles to the sample surface.

Surface Metallurgy
When the case is being formed by the diffusion of nitrogen into the steel surface from a conventional process gas such as ammonia, there are generally three distinct areas of the formed case:
  • The immediate surface is known as the compound layer simply because there are two metallurgical phases – gamma prime and epison – within that layer. It is more commonly known as the white layer because it will etch out as a white area when polished and etched with nital etch.
  • The second part of the formed metallurgy is located immediately below the compound layer and is known as the diffusion zone. This is the area that the diffused nitrogen begins to react with the nitride-forming elements in the steel to form stable nitrides.
  • The third area is located immediately below the diffusion zone, which is known as the core material. The transition of the diffusion zone into the core is not a definitive line of transition but a gradual reduction of the formed stable nitrides into the core. The hardness of the core is what will give support to the formed nitrided case. So, it is most important that the process temperature does not exceed the pre-tempering temperature selection. It is advisable to stay approximately 50

    °F (35°C) below the pre-tempering temperature.

Control of the Compound Layer Thickness
The control of the compound layer thickness will be by the process temperature selection, the time at that temperature and the process-gas decomposition ratios. The writer estimates that (on a plain-alloy, nitriding-type steel) the thickness of the compound layer is generally around 10% of the total formed case (from the surface hardness down to a depth of 500HV using a 200-gram micro-hardness load). Thus, the compound-layer actual thickness will be controlled by time and temperature.

Makeup of the Phases of the Compound Layer
The composition of the two phases – gamma prime and epsilon – is generally 50% of each of the two phases, based on a gas ratio of 1 part nitrogen and 2 parts hydrogen (ammonia) at a process temperature of 925°F (500°C).

However, if the gas ratios are changed (such as can be accomplished with the process of plasma nitriding, which uses molecular nitrogen and molecular hydrogen), then the phase ratio percentages can be changed as can the thickness of the compound layer.

There will be a continuation in part 3 of this presentation.
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