Composition of the Formed Case
The surface properties of the compound layer formed at the surface will generally display a dual phase of gamma prime and epsilon.
The dual phase is generally at 50% for each phase and primarily dependent on the components’ carbon content. The general rule of thumb is 0.40% carbon. Below that carbon content, the surface compound layer tends to be predominantly gamma prime. Above 0.40% carbon, the phase of the compound layer tends to become dominantly epsilon.
The compound layer with dominant gamma prime will tend to produce a surface compound layer that will produce good impact strength without the risk of surface fracture and a surface hardness of approximately 720 HV up to approximately 820 HV. Generally, one would consider utilizing the Floe two-stage process with a temperature of 900°F.
The compound layer with dominant epsilon is strongly influenced by the carbon content of the steel being processed. This type of surface compound zone will have excellent wear properties, but the impact strength is reduced significantly. Process temperatures are generally 900-935°F.
The diffusion layer is created by nitrogen interstitially diffusing into the steel body to interact with the (if present) nitride-forming elements (including iron). The diffusion layer is, in reality, the main body of the nitriding process result.
Harris of The Massachussets Institute of Technology pioneered a table of diffusion rates based on temperature and time in relation to accomplished total case depth.
Plain-Carbon Steel Formed Case Depths Based on the Harris Formula
(Please note: The following is based on plain-carbon steels up to 0.40% carbon at different process temperatures) and calculated on the formula for case-depth calculation:
Case depth = √t x factor
The Harris Formula Factors Based on a Simple Nitriding Steel (Temperature-Temperature Factor)
- 865°F - 0.00221
- 875°F - 0.00233
- 885°F - 0.00259
- 900°F - 0.00289
- 930°F - 0.0030
- 950°F - 0.0033
- 960°F - 0.0035
- 975°F - 0.0037
- 1000°F - 0.0038
These values are suggested factors only. It must not be construed that these factors will suffice for all types of steel. On the alloying of the steel with suitable nitride-forming alloys, the rate of diffusion becomes more difficult, and these values then become retarded.