Evaluating Case Depth for Case Hardening Treatments

posted On Thursday, April 3, 2025 in Blog

Thermochemical surface treatments such as nitriding (or carburizing) diffuse nitrogen (or carbon) into the surface of steel alloys. The diffusion layers increase the surface and 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.

In Figure 1, the red horizontal line represents 50 HRC equivalent (513 HV). The intersection of the red line with the hardness curve (purple) determines the resultant ~0.016” effective case depth (at 50 HRC).

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 near-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 in other ways, such as the depth to 110% of the core hardness average.

In Figure 1, the green horizontal line represents the core hardness average of 334 HV. The blue horizontal line represents hardness value of 50 HV points above the core hardness average (50 HV + 334 HV= 384 HV). The intersection of the blue line with the hardness curve (purple) determines the resultant ~0.022” total case depth.

Microhardness Traverse Profile

Figure 1. Graph (microhardness vs depth) associated with Figure 2.

Figure 3. Nitrided medium alloy steel showing compound zone (white layer) at the surface and nitride

Figure 2. Nitrided medium alloy steel showing compound zone (white layer) at the surface and nitride diffusion zone. Hardness indentations shown are produced using Vickers indenter (HV).

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.

Figure 1. Carburized and Quenched 8620 alloy showing higher hardness and carbon content martensite n

Figure 3. Carburized and Quenched 8620 alloy showing higher hardness and carbon content martensite near the surface.

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 within the diffusion zone may peak at approximately 300 HV (~30 HRC eqv).
For nitrided HIGH alloy steels (such as 17-4 PH), case hardness within the diffusion can easily exceed 1000 HV (above the HRC scale).

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.

For some highly alloyed steels, such as stainless steels, the total case depth (usually observed visually after etching) is approximately the same as effective case depth due to the abrupt change from case hardness to core hardness. Reference Figure 4.

Figure 4. Nitrided stainless steel (17-4 PH) showing sharp transition from case to core. Hardness in

Figure 4. Nitrided stainless steel (17-4 PH) showing sharp transition from case to core. Hardness indentations shown are produced using Knoop indenter (HK).

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 or slow cooling), 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 specimen with the 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.