Case depth is the thickness of the hardened surface layer produced by a surface-hardening process such as carburizing, carbonitriding, nitriding or induction hardening. It is measured with a Vickers microhardness traverse on a prepared cross-section, reporting the perpendicular distance from the surface to the point where hardness drops to a defined limit value. Correct case depth is critical for the wear resistance and fatigue life of gears, shafts, camshafts and bearings. For the wider method context, see the Hardness Testing Academy.

What is case depth?

Surface-hardening treatments create a hard outer case over a tougher core. Case depth quantifies how deep that hardened layer extends. Because hardness decreases gradually from the surface to the core, the depth is always defined relative to a limit hardness specified by the relevant standard, not to the point where hardness simply ends.

How is case hardening depth measured?

The standard method is a Vickers microhardness traverse. A metallographic cross-section is cut, mounted, ground and polished, then a row of Vickers indentations (typically HV1, sometimes HV0.5 or HV0.3) is made at increasing distances from the surface. Hardness is plotted against depth, and the case depth is read where the curve crosses the limit value. A calibrated microhardness tester and certified test blocks are essential for reliable results. See the case depth section of the Vickers guide.

Step-by-step: how to perform a case depth test

This practical procedure describes a CHD measurement on a carburized or carbonitrided part, from sectioning the piece to reading the final report. It follows ISO 2639 (limit 550 HV1) with a Vickers microhardness traverse per ISO 6507.

1. Section the part. Cut a cross-section perpendicular to the hardened surface with an abrasive cut-off machine and plenty of coolant. Heat from cutting can temper or alter the case, so never dry-cut.
2. Mount the sample. Embed the section in resin (hot compression mounting or cold mounting) using an edge-retention compound, so the surface stays flat and the edge is not rounded during preparation.
3. Grind. Grind progressively with finer silicon carbide papers, keeping the face flat and exactly perpendicular to the surface, until the deformation left by cutting is removed.
4. Polish. Polish to a mirror finish (diamond suspensions down to 1 micron) so the indentations have sharp, clearly measurable corners. Avoid relief and edge rounding.
5. Etch only if needed. For CHD measurement the polished surface is enough; a light etch (for example Nital) is needed only if you also want to reveal the microstructure or the visible case boundary.
6. Verify the tester. Before measuring, verify the microhardness tester at the chosen load with a certified test block and confirm calibration traceability.
7. Set the parameters. Select the test force (typically HV1 for CHD), set the dwell time, and define a measurement line perpendicular to the surface starting close to the edge.
8. Run the indentation traverse. Place a row of Vickers indentations at increasing distances from the surface into the core, respecting the minimum spacing between indentations and from the edge required by ISO 6507. See the Vickers hardness testing guide for the spacing rules.
9. Measure each indentation. Measure both diagonals of every indentation and record the hardness value against its depth from the surface.
10. Plot the hardness profile. Plot hardness (HV) versus depth to obtain the case hardness profile curve.
11. Determine the CHD. Find the depth at which the curve crosses the limit hardness (550 HV1 for CHD per ISO 2639), interpolating between the two nearest points.
12. Issue the report. Record the method and standard, the test force, the limit hardness, the resulting CHD value, the tester identification and calibration traceability, the operator and the date, and include the hardness versus depth curve.

CHD, SHD and Nht: the three case-depth types

Which standards define case depth?

The main references are ISO 2639 (case hardening depth after carburizing), ISO 18203 (measurement of the depth of surface-hardened and nitrided layers), ASTM E1077 and SAE J423. Hardness measurement itself follows ISO 6507 and ASTM E384 (Vickers and microhardness).

What are common errors in case depth measurement?

Typical sources of error include poor sample preparation (edge rounding, deformation), indentations placed too close together, the wrong test force, an uncalibrated tester and measuring the perpendicular distance incorrectly on an angled section. Verify the tester before each measurement campaign and follow the spacing rules in ISO 6507.

Frequently asked questions