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On Friday, April 4, 2025
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Thermochemical surface treatments such as nitriding (or carburizing) diffuse nitrogen (or carbon) into the surface of steel alloys. The diffusion layers can increase the near-surface hardness of the material, providing enhanced tribological properties (wear resistance, coefficient of friction, Hertzian contact stresses, etc.). Rightfully so, these heat treatments are often named “case hardening” type processes, since there is a hardened layer on the outside of the material leaving the internal material softer.
Two Methods to Evaluate the Diffusion Layer (Case Depth)
Effective Case Depth (ECD)
Effective Case Depth (ECD) is defined as the depth to a particular hardness value. On a metallographic mount, microhardness testing (Vickers- HV or Knoop- HK) is performed at standard intervals (this is called a “microhardness traverse”) on the cross-section. The resultant hardness values are plotted against the depth for each indentation. The depth to the desired hardness value is then interpolated.
Total Case Depth (TCD)
Total Case Depth (TCD) is defined as the full or complete depth of atomic (nitrides/nitrogen or carbon) diffusion. For some steel alloys, TCD can be evaluated visually after etching with an appropriate acid to reveal the diffusion layer. For steel alloys that DO NOT reveal the diffusion layer, a microhardness traverse is performed. The resultant hardness values are plotted against the depth for each indentation. The full microhardness traverse is used to calculate the TCD, based on the extent of hardness drop from surface hardness to core hardness.
Note: TCD is typically defined as the depth to 50 HV or 50 HK above the core hardness average. Alternatively, TCD can be defined as the depth to 110% of the core hardness average.
Case Depth Common Uses
Carburizing
For carburizing treatments, case depths are usually defined as “ECD to 50 HRC” as this usually coincides with total carbon diffusion depth and the martensitic transformation (quenching) of the applied carbon diffusion. For common carburizing steel alloys (e.g. 8620, 9310, 5120, or 1018), “ECD” is most used because of the universal resultant case hardness for this process type.
Nitriding
For nitriding treatments, case depths are usually defined as “TCD”. This is mainly because the resultant case hardness varies greatly from low alloy steels to high alloys steels.
- For nitrided LOW alloy steels (such as 1018), case hardness may peak at approximately 300 HV (~30 HRC eqv).
- For nitrided HIGH alloy steels (such as 17-4 PH), case hardness values can easily exceed 1000 HV (above the HRC eqv chart).
Because of this non-uniform resultant hardness of nitrided steels, it disallows the use of ECD to be used to compare case depths of different alloys.
What Affects Case Depth During Case Hardening?
For optimum diffusion to occur with either process type - nitriding or carburizing - the surface of the material should be clean and free of surface oxidation and heat treatment scale.
For carburizing applications, the combination of carbon diffusion and sufficient case depth are necessary to produce the effective case depth. If the parts are not quenched or insufficiently quenched (e.g. slack quench), carbon diffusion would remain, but the case properties would remain soft since the quenching operation has not yet produced the case hardening.
For nitriding applications, core hardness can play a crucial role in determining the ECD. Core hardness serves as the foundation upon which the case is built – nitriding improves the already-established core hardness. Considering two specimens of identical alloy (chemical composition), but varying core hardness, when nitrided under the same conditions the one with a higher core hardness will exhibit a deeper effective case depth.
AHT Quality: Focusing on Your ECD Specification
Prior to nitriding, AHT often tests the incoming hardness of parts to identify any potential issues, ensuring that any concerns are addressed before processing.
When sending parts with an ECD requirement, it is helpful to also provide a sample piece of material that has undergone the same heat treatments as the parts in the order. While AHT includes sample materials in every load, discrepancies between the condition of our sample materials and the submitted parts can lead to skewed metallurgical results.
Want to Speak with an AHT Representative about your Case Depth Requirements?
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