Microhardness Case Depth Determination: Principles, Standards, Methods, and Applications

Microhardness case depth determination is the reference method for quantifying the depth of hardened or modified surface layers in carburized, nitrided, induction-hardened, and coated metal components. By traversing micro-Vickers indentations across a polished cross-section, the technique produces a hardness-depth profile from which effective case depth, total case depth, and layer boundaries are precisely defined. This article explains the measurement principle, applicable ISO, ASTM, GB, and JIS standards, specimen preparation, test procedure, data evaluation, and typical industrial applications.

Technical Overview

Microhardness case depth determination, also referred to as microhardness layer depth measurement or hardness-depth profiling, is a quantitative metallographic technique used to establish how deeply a surface treatment has modified the hardness of a component. Hardened or engineered layers rarely end abruptly; instead, hardness decreases progressively from the surface toward the unaffected core. The measurement maps this gradient and locates the depth at which hardness falls to a defined limit value, which constitutes the effective depth of the layer.

The test is performed on a metallographically prepared cross-section using a micro-Vickers hardness tester. A diamond pyramidal indenter is pressed into the polished surface at controlled test forces, most commonly HV1 (9.807 N) and HV0.5 (4.903 N), with smaller forces such as HV0.1 (0.9807 N) reserved for thin layers and coatings. Indentations are placed in a line perpendicular to the treated surface, the diagonal lengths are measured optically, and each hardness value is plotted against its distance from the surface. The resulting curve is then evaluated against the limit hardness specified by the applicable product standard or the customer drawing.

Because hardness is a directly traceable mechanical property and indentation positioning can be controlled to within a few micrometres, the method achieves depth resolutions on the order of 0.01 to 0.05 mm. It is universally accepted for acceptance testing, heat-treatment process validation, and failure analysis, and is routinely performed by accredited third-party laboratories as well as in-house metallurgical departments.

Testing Samples

Essentially any metallic component with a hardness-modified surface layer can be evaluated, provided a representative cross-section can be extracted. Typical sample categories include the following.

Carburized and Carbonitrided Steel Components

Gears, pinions, shafts, camshafts, bearing rings, and constant-velocity joints made from low-carbon carburizing steels such as 20CrMnTi, 20CrMo, 16MnCr5, 5120, and 8620 after gas carburizing, vacuum carburizing, or carbonitriding followed by quenching and tempering. Effective case depths typically range from 0.3 to 2.0 mm.

Nitrided and Nitrocarburized Components

Gas-nitrided, plasma-nitrided, and salt-bath nitrocarburized (including QPQ-treated) parts manufactured from alloy steels such as 38CrMoAl, 42CrMo, and 40Cr, as well as tool steels and stainless steels. These layers are generally thinner, commonly 0.1 to 0.7 mm, and often include a hard compound layer at the surface.

Induction-Hardened and Flame-Hardened Parts

Crankshafts, axle shafts, wheel spindles, guideways, rails, and other medium- or high-carbon steel components hardened by induction or flame heating. The hardened zone may be a full peripheral case or a localized pattern, and depth is assessed against a percentage of the specified minimum surface hardness.

Coated and Surface-Engineered Parts

Hard chromium plating, thermal spray coatings, PVD and CVD films, hard anodized aluminum layers, and diffusion coatings. Microhardness traverses verify both the hardness of the deposit and the integrity and effective thickness of the engineered layer.

Welded, Cladded, and Heat-Affected Structures

Cross-sectional hardness traverses are used to map heat-affected zone softening, weld dilution, cladding depth, and hardness transitions in dissimilar metal joints, providing layer-depth data alongside conventional weld qualification.

Testing Items

Standard deliverables from a microhardness layer depth study include the following measurement items:

• Effective case hardening depth (CHD) defined at 550 HV1, per ISO 2639 and GB/T 9450
• Total hardened layer depth, taken where hardness merges with the unaffected core hardness
• Nitriding hardness depth (NHD), defined where hardness equals core hardness plus 50 HV, per ISO 15787 and GB/T 11354
• Surface hardening depth (SHD) after induction or flame hardening, defined at 80 percent of the specified minimum surface hardness, per ISO 3754 and GB/T 5617
• Shallow hardened layer depth (up to approximately 0.3 mm) per ISO 4970 and GB/T 9451
• Complete hardness-depth profile with tabulated indentation data and graphical curve
• Surface hardness, near-surface hardness, and core hardness values
• Depth at any customer-specified limit hardness value
• Coating microhardness (HV0.01 to HV0.1) and transition zone characterization

Testing Methods

Governing Standards

The measurement combines the general micro-Vickers hardness test method with layer-specific depth determination standards:

• ISO 6507-1, ASTM E384, GB/T 4340.1, and JIS Z 2244 for Vickers and micro-Vickers hardness testing
• ISO 2639 and GB/T 9450 for determination and verification of case hardening depth after carburizing and hardening
• ISO 4970 and GB/T 9451 for shallow hardened layers
• ISO 15787 and GB/T 11354 for nitriding hardness depth
• ISO 3754 and GB/T 5617 for surface hardening depth after induction or flame hardening
• ASTM E1077 for estimating the depth of carburization or decarburization
• ASTM E3 and GB/T 13298 for metallographic specimen preparation

Specimen Preparation

A full-thickness section is cut perpendicular to the treated surface using abrasive or precision cutting with abundant coolant to avoid altering the hardness gradient. The specimen is mounted in a hard resin that provides excellent edge retention, ground through successive silicon-carbide grades, and polished to a mirror finish, typically with 1 micrometre or finer diamond suspension. Edge rounding, smearing, and grinding-induced heat must be strictly avoided, since even minor edge preparation defects distort the hardness readings closest to the surface, where the depth measurement is most sensitive. A light etch, such as 2 to 3 percent nital for steels, may be applied after hardness testing to support complementary microstructural examination.

Measurement Procedure

The indentation sequence follows the requirements of the governing depth standard:

• Select the test force according to the layer type: HV1 for carburized cases, HV0.5 for nitrided cases, and HV0.1 or lower for thin coatings; where HV0.5 is used for carburized cases, the limit hardness is reduced to 525 HV0.5 in accordance with ISO 2639
• Position the first indentation close to the surface, typically within 0.05 mm, and place subsequent indentations at spacings not exceeding about 0.1 mm through the transition region
• Maintain a center-to-center spacing between adjacent indentations of at least 2.5 to 3 times the indentation diagonal to prevent interaction effects
• Apply a dwell time of 10 to 15 seconds and measure both diagonals of each indentation optically
• Continue the traverse until the hardness stabilizes at the core level, well beyond the transition zone
• Perform at least two traverses, either on the same section or on separate sections, and average the results as required by the standard

Data Evaluation and Depth Determination

Hardness values are plotted against distance from the surface to generate the hardness-depth profile. The depth at the limit hardness is obtained by linear interpolation between the two bracketing indentations, using the formula depth equals d1 plus (d2 minus d1) multiplied by the ratio of (HV1 minus limit hardness) to (HV1 minus HV2), where d1 and d2 are the distances of the inner and outer bracketing points and HV1 and HV2 are their corresponding hardness values. The result is rounded in accordance with the governing standard and compared with the specified minimum or range on the engineering drawing. Modern testers with automated stages and digital image analysis can execute the full traverse, construct the profile, and compute the interpolated depth automatically, reducing operator influence and improving repeatability.

Applications

Microhardness case depth determination supports quality control, process development, and failure investigation across virtually every engineered-metals industry:

• Automotive powertrain: acceptance testing of transmission and differential gears, camshafts, crankshafts, and drive shafts, where specified CHD values typically range from 0.5 to 1.5 mm
• Aerospace: verification of case-hardened pins, bushings, actuators, and bearing components against stringent drawing requirements
• Wind energy and heavy machinery: validation of carburized and nitrided gearbox components subject to high contact fatigue loading
• Tool and die manufacturing: qualification of nitrided punches, dies, and molding inserts and assessment of coating integrity
• Railway transportation: measurement of induction-hardened rail and wheel tread depths for wear-life prediction
• Oil and gas: evaluation of hard-facing layers on valve seats, drill tool joints, and pump components
• Fasteners and springs: confirmation of hardened-layer depth on carburized or induction-hardened products
• Failure analysis: detection of insufficient case depth, decarburization, grinding burn, and overheating that contribute to premature fatigue or wear failures
• Heat-treatment audits and process capability studies, including first-article inspection and production part approval submissions

FAQ

What is effective case depth (CHD)?

Effective case depth is the distance from the hardened surface to the depth at which the Vickers hardness equals 550 HV when measured with a 9.807 N (HV1) test force. It represents the layer that contributes meaningful load-bearing and fatigue capacity and is the standard acceptance criterion for carburized and carbonitrided parts under ISO 2639 and GB/T 9450.

What is the difference between total case depth and effective case depth?

Total case depth is the distance from the surface to the point where hardness becomes indistinguishable from the unaffected core, whereas effective case depth is defined at a fixed limit hardness, such as 550 HV1. Effective case depth is always the smaller of the two values and is the quantity normally specified on drawings, because it directly reflects functional performance.

How is nitriding hardness depth (NHD) different?

Nitrided layers are shallower and show a steeper gradient than carburized cases, so NHD is defined relative to the core rather than at an absolute hardness. Under ISO 15787 and GB/T 11354, NHD is the depth at which hardness falls to the core hardness plus 50 HV, usually measured at HV0.5.

How thin a layer can be measured?

With shallow-case standards such as ISO 4970 and GB/T 9451, layers down to roughly 0.05 mm can be characterized using small test forces such as HV0.1, provided edge retention is excellent. For very thin PVD or CVD films, individual coating hardness is measured at HV0.01 to HV0.05, and the effective supporting depth is assessed separately.

Why is specimen preparation so critical to the result?

The depth calculation depends directly on the hardness values nearest the surface. Edge rounding displaces the apparent surface location, work hardening from coarse grinding raises hardness artificially, and heating from abusive cutting can temper or transform the layer. Rigorous metallographic preparation with verified edge retention is therefore a prerequisite for a valid depth result.

How accurate and repeatatable is the method?

Indentation positioning on modern testers is accurate to within a few micrometres, and interpolated depths are typically reproducible to within about 0.05 mm when preparation and force selection are properly controlled. Repeatability is confirmed by performing multiple traverses, and measurement uncertainty is routinely evaluated by accredited third-party laboratories in accordance with the applicable testing standards.

Can cylindrical or complex surfaces be tested?

Yes. The specimen is sectioned perpendicular to the local surface at the location of interest, for example at a gear tooth flank, a fillet radius, or a journal diameter. For small radii, the measured depth is corrected for curvature, or several sections are taken around the feature to characterize depth uniformity, which is a common requirement in production audits.