Case Depth Measurement

Vacuum Hardening | Metal Technology Engineering

Case Depth Measurement

Case hardening is a thermochemical process that results in hard, wear-resistant part surfaces while maintaining soft, ductile cores. This is usually achieved by Carburizing or Nitriding. In flame and induction hardening, the case is developed by rapid heating of the surface layers followed by immediate quenching resulting in the surface layers converting to hard Martensite.

Measuring the depth of the case after treatment helps determine whether the part will perform as designed and conforms to the specification provided. We usually refer to two types of case depth: Total and effective.

Total case depth vs effective case depth

Total case depth is defined as the total distance carbon, nitrogen or both have diffused inward from the surface of the part. It can be observed under a microscope as the distance from the surface to where you cannot clearly distinguish between the part’s outer case and its inner core. Measuring total case depth is typically specified for parts requiring thinner cases and is usually a visual test.

Effective case depth is the distance measured from a part’s surface to a specific hardness called the cut-off. It’s meant to measure whether an intended hardness has penetrated to the correct depth as determined by a part’s design engineer. Effective case depth is what is generally specified for most engineering applications.

For effective case depth, the specified hardness is most often equivalent to 50HRc (513Hv) or 52 HRc (550Hv). The cut off is usually specified by the customer. The European definition for effective case depth uses 550Hv as the cut-off while the US definition is deeper with a cut-off at 513Hv. The measurement is usually carried out with a Vickers microhardness indenter and can be done with a Knoop or Vickers indenter and the resulting hardness values can then be converted to Rockwell C using a conversion chart.

Process control during case hardening is critical, especially for parts with thin sections or irregular shapes. Such parts are at risk of “through casing,” meaning the part becomes all case and no core. This results in a total loss of ductility, and results in parts that are very hard but very brittle and break easily. Case hardening treatments should be carefully specified so a part can perform in its intended application without risk of failure.

Total and effective case depth measurement techniques

Total case depth is usually measured visually by examining a part’s cross section under handheld microscopes. Applying a suitable etchant to the cross section prior to measurement creates the visual contrast needed to be able to distinguish the part’s case from its core.

At MTE, we use microscopes to aid our visual examination of treated parts.
Microhardness testing methods are used to determine effective case depth. A series of indentations are made on a part surface until a specified hardness is reported. Then, the distance from the surface of the part inward to that hardness level is measured.

Indentations from hardness testing increase in size from right to left, showing deeper penetration as the hardness tests are moved inward toward the softer core of this part.

It’s important to pay attention to how a sample part is prepared prior to total or effective case depth measurement. Invalid measurements can result if:

  • Sample cuttings or hardness tests are not performed perpendicular to the part surface. Non-perpendicular measurements will result in falsely high depth readings.
  • Parts must not be allowed to get too warm if they’re being cut prior to measurement. Increased heat softens parts, and the effective case depth will read falsely lower.
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Indentations from Vickers hardness testing are visible on the tooth of a gear. Notice how the indentations get larger as they move away from the surface.

Specification considerations for total and effective case depth

For case hardened parts, the part’s end use dictates which heat treatment process and case depth measurement is specified.

Typically, effective case depth is the preferred specified case depth for high-performance metals. These higher-alloy parts are already more expensive, so ensuring quality and limiting the risk of failure requires the more precise and informative case depth measurement.

In addition, effective case depth is preferred on any safety-critical parts such as those commonly found in automotive or aerospace applications.

Total case depth is commonly specified for non-safety critical parts where a hard surface and ductile core are still desired, but the design requirements aren’t as rigorous.