Gears, shafts, pins and bearing parts are often surface hardened by carburising, carbonitriding, nitriding or induction hardening. The goal is a hard, wear-resistant surface over a tougher core. Whether the treatment worked depends not only on surface hardness but on how deep the hard layer goes. That depth is measured by a hardness traverse, most often with a Vickers hardness tester.
What effective case depth means
Hardness falls gradually from the surface into the core, so there is no sharp boundary. Standards therefore define an effective case depth: the distance from the surface to the point where hardness falls to a specified limit value. For carburised and hardened steels, ISO 2639 defines effective case depth as the depth at which hardness reaches 550 HV, measured with a specified test force, unless another limit is agreed. Other limits are used for induction-hardened or nitrided parts, defined in their own standards or on the drawing.
Other terms you may see include total case depth (to where hardness reaches core level) and, for nitrided parts, nitriding hardness depth. Always use the definition given on the drawing.
Why Vickers
A Vickers indenter is a square-based diamond pyramid. With low test forces it makes small indentations that can be placed close together, so many readings can be taken over a depth of a millimetre or less. The diagonal of each indentation is measured optically, and the hardness is calculated from the force and the diagonal length. The Vickers method is specified in IS 1501 and ISO 6507.
Sample preparation
- Section the part perpendicular to the surface at the location specified, using cooling to avoid heating the sample.
- Mount the section if needed, so the edge is supported and stays flat.
- Grind and polish to a finish good enough to see the indentation corners clearly. Edge rounding must be avoided, since the first readings are near the surface.
- Light etching may help locate the case but is not always required for the traverse.
Running the traverse
- Place indentations along a line perpendicular to the surface, starting close to the edge.
- Keep the spacing between indentations at least the minimum required by the standard relative to the indentation size, so that one indentation does not affect the next. A staggered or zig-zag pattern lets you take readings at small depth increments while keeping that spacing.
- Continue until the hardness is clearly below the limit value, ideally to core hardness.
- Measure the depth of each indentation from the surface accurately.
Determining the depth
Plot hardness against distance from the surface. The effective case depth is where the curve crosses the limit value, found by interpolating between the two readings on either side. Report the depth in millimetres together with the limit hardness and the test force, for example "CHD 550 HV1 = 0.8 mm" in the style used on many drawings.
| Treatment | Typical depth definition |
|---|---|
| Carburising | Depth to 550 HV (ISO 2639) unless otherwise agreed |
| Induction or flame hardening | Depth to a limit related to the specified surface hardness |
| Nitriding | Depth to a limit above core hardness |
Common mistakes
- Edge rounding from poor polishing, which lowers near-surface readings.
- Indentations placed too close together.
- Measuring the depth from a rounded or chipped edge instead of the true surface.
- Using a different test force from the one specified, which changes the result.
- Sectioning at the wrong location, such as the tip of a gear tooth instead of the specified pitch circle.
Equipment
Case depth work benefits from a tester with low test forces, a precise motorised stage for positioning, and image analysis to measure diagonals consistently. Akuracy offers a computerized Vickers hardness tester and a Vickers hardness tester; browse the Vickers hardness tester collection for options, and share your test forces and part types when you ask for a quote.
Surface hardness and core hardness
Case depth is usually reported together with surface hardness and core hardness. Surface hardness may be measured on the finished surface with a suitable Vickers or superficial Rockwell test, while core hardness is measured at the centre of the section or at a specified depth. Together the three values describe the hardness profile: a good surface value with a shallow case suggests insufficient carburising time, while a deep case with low surface hardness may point to decarburisation or incorrect quenching.
Test pieces and process control
For production control, heat treaters often process a test coupon of the same steel alongside the parts, then section the coupon instead of a finished part. The coupon must be representative in size and steel grade, since section size affects cooling rate and therefore hardness. Recording the furnace batch, cycle, coupon ID and the full hardness traverse makes it easy to correlate case depth with process settings and to investigate any non-conforming batch.
Keeping the polished section after testing is also good practice, so the traverse can be re-examined or repeated if a result is challenged.
Frequently asked questions
Can I measure case depth with a Rockwell tester?
Rockwell indentations are too large to resolve shallow cases. Superficial Rockwell can give a surface value, but depth traverses are normally done with Vickers or Knoop.
What test force should I use?
Use the force specified on the drawing or in the standard. ISO 2639 specifies a test force for carburised case depth; other forces need agreement.
Why stagger the indentations?
Staggering keeps the required spacing between indentations while still taking readings at small steps in depth.
Request a quote
Akuracy (Ratnakar Enterprises) can advise on a configuration for your materials, standards and test volume. Share your specimen details and the standard you test to, and our team will reply with a suitable option. Request a quote online or message us on WhatsApp at +91 94038 91579.