Microhardness profile testing is a precision metallographic technique used to map hardness gradients across surface layers, hardened cases, coatings, and welded joints by plotting Vickers or Knoop hardness values against distance from a reference surface. Performed in accordance with internationally recognized methods such as ISO 6507, ASTM E384, and GB/T 4340, this testing provides quantitative determination of effective case depth, nitriding hardness depth, hardening depth, and decarburization depth. Accredited third-party laboratories rely on microhardness curve measurement for process validation, quality control, and failure analysis across the automotive, aerospace, tooling, and energy industries.
Technical Overview
Microhardness profile testing, also referred to as microhardness curve determination, is a quantitative hardness testing method in which a series of micro-indentations are placed at precisely defined positions across a polished cross-section of a material. The resulting hardness values are plotted against distance from a reference surface—typically the treated surface, the fusion line of a weld, or an interface between a coating and substrate—to generate a hardness distribution curve. Unlike single-point hardness testing, the profile method reveals how hardness varies with depth or position, making it the definitive technique for evaluating gradient-engineered surfaces.
The test is most commonly performed using the Vickers hardness principle (HV) with test forces ranging from HV0.01 (0.098 N) to HV1 (9.807 N), and in some cases HV2 or HV3, although higher forces fall within the macro-Vickers or low-force hardness regime. The Knoop indenter (HK) is an alternative elongated diamond pyramid that is advantageous for very thin layers, brittle materials, and steep gradients because its shallow penetration and elongated geometry reduce substrate influence and allow closer spacing near surfaces.
Microhardness profile testing is governed by a family of international and national standards. The fundamental indentation procedures are defined in ISO 6507-1, ASTM E384, ASTM E92, GB/T 4340.1, and JIS Z 2244, while the interpretation of hardness curves for specific engineering purposes is covered by ISO 2639 and GB/T 9450 (effective case depth after carburizing), ISO 4970 and GB/T 9451 (total surface hardening depth), ISO 3754 (hardening depth after induction or flame hardening), ISO 15787 (nitriding hardness depth), and ISO 3887 together with ASTM E1077 (decarburization depth). By combining careful metallographic preparation with controlled indentation geometry, the method delivers depth measurements with resolution on the order of 0.01 mm.
Testing Samples
Carburized and Carbonitrided Components
Gears, shafts, pinions, and bearings that have been carburized or carbonitrided exhibit a characteristic high-hardness surface case transitioning to a tougher core. Cross-sectional microhardness traverses are used to determine the effective case depth, typically defined as the depth at which hardness equals 550 HV1, as well as the total case depth where hardness becomes indistinguishable from the core level.
Nitrided and Nitrocarburized Components
Gas-nitrided, plasma-nitrided, or salt-bath nitrocarburized parts such as crankshafts, camshafts, valve stems, and die components possess thin, extremely hard compound and diffusion layers. Because these layers are often only 0.1 to 0.8 mm deep, low test forces such as HV0.5 or HV1 are applied at fine depth increments to resolve the nitriding hardness depth, defined per ISO 15787 as the depth at which hardness falls to a specified limit above the core hardness.
Induction-Hardened and Flame-Hardened Parts
Localized surface hardening of crankshaft journals, rails, axle shafts, and machine ways produces a defined hardened zone with a sharp transition. Hardness-depth curves measured perpendicular to the hardened surface establish the hardening depth, commonly referenced to a limit hardness such as 80% of the minimum surface hardness or an absolute value agreed between purchaser and supplier.
Welded Joints and Heat-Affected Zones
Cross-weld microhardness traverses characterize the hardness distribution across the weld metal, heat-affected zone (HAZ), and base metal. These profiles are essential for weld procedure qualification, detection of untempered martensite or softening in the intercritical HAZ, and verification that hardness remains below thresholds associated with hydrogen-assisted cracking in sour service pipelines.
Coated and Surface-Treated Materials
Thermal spray coatings, laser-clad overlays, PVD and CVD films, nitrided tool surfaces, galvanized layers, and anodized coatings can be evaluated by micro-indentation profiles. Knoop indentations oriented along or across the layer are frequently specified for coatings thinner than 0.1 mm to minimize substrate effects, and profiles across the coating-substrate interface reveal adhesion-zone hardness transitions.
Decarburized or Diffusion-Treated Steels
Hot-worked, forged, or heat-treated bars may exhibit surface decarburization that reduces fatigue performance. Microhardness traverses starting immediately below the surface quantify the depth over which hardness is depressed relative to the unaffected core, complementing metallographic assessments of ferritic and total decarburization depth.
Small Parts, Foils, and Miniature Sections
When components are too thin, too small, or too geometry-constrained for bulk Rockwell or Brinell testing, microhardness profiles on a mounted cross-section provide the only practical means of hardness characterization. Examples include fine wires, thin rolled strip, watch components, electronic contacts, and small medical devices.
Testing Items
Microhardness profile testing services typically cover the following measurable quantities, each reported with its associated test force, indentation spacing, and applicable standard:
- Micro Vickers hardness values (HV0.01, HV0.025, HV0.05, HV0.1, HV0.25, HV0.3, HV0.5, HV1) along a defined measurement path
- Knoop microhardness values (HK0.01 to HK1) for thin layers and steep gradients
- Effective case depth after carburizing or carbonitriding (CHD) at the 550 HV1 limit per ISO 2639 and GB/T 9450
- Total case depth determined by the asymptotic approach of the hardness curve to core hardness
- Nitriding hardness depth (NHD) per ISO 15787, referenced to an agreed limit hardness above the core
- Hardening depth (HD) after induction or flame hardening per ISO 3754
- Total surface hardening depth per ISO 4970 and GB/T 9451
- Decarburization depth, both total and ferritic, per ISO 3887 and ASTM E1077
- Hardness profiles across welds, including weld metal, fusion line, coarse-grained HAZ, fine-grained HAZ, intercritical zone, and base metal
- Coating and overlay hardness profiles, including interface transition characterization
- Hardness uniformity surveys, step-plane mappings, and gradient quantification
- Converted hardness equivalents to HRC, HB, or tensile strength estimates per ASTM E140 and GB/T 1172, where metallurgically appropriate
Testing Methods
Specimen Preparation
Accuracy of microhardness profiles depends fundamentally on specimen preparation. A representative section is cut strictly perpendicular to the surface of interest using abrasive cut-off wheels with copious coolant to avoid burning or tempering the microstructure. The specimen is then mounted—edge-retention mounts are preferred for surface layers—ground through successive abrasive grades, and polished to a mirror finish, typically with 1 µm or finer diamond suspension. Etching is optional and performed lightly if microstructural correlation is required, since heavy etching can round indentation corners and bias diagonal measurement. Any smearing, edge rounding, or relief at the surface will systematically distort the first data points of the curve.
Test Force Selection
The test force is chosen to balance indentation size against layer thickness. The indentation depth must not exceed approximately one tenth of the distance to an interface or free surface to avoid substrate effects, and the diagonal should be large enough to be measured reliably. For case depth work, ISO 2639 and GB/T 9450 specify HV1 as the reference force for the 550 HV limit, whereas thin nitrided layers may require HV0.5 or lower with proportionally larger measurement scatter.
Indentation Patterns and Spacing
Hardness traverses are laid out according to defined protocols. In the stepped-depth method, single indentations are placed at successive depths on a common axis normal to the surface. Because the center-to-center spacing between indentations must be at least 2.5 times the diagonal length, and the distance to the specimen edge at least 2.5 times the diagonal, a staggered or diagonal zig-zag arrangement is often used to place multiple effective depth readings close to the surface without violating spacing rules. For weld profiles, lines of indentations are positioned at specified offsets from the fusion line, often at 0.5 mm to 2 mm spacing depending on the standard and section thickness.
Measurement and Curve Construction
Indentations are made with a calibrated microhardness tester equipped with a diamond pyramidal indenter, an optical or digital measuring system, and frequently a motorized automatic stage with pattern-programming software. Diagonals are measured to the precision required by ISO 6507 or ASTM E384, hardness is computed for each indentation, and the results are plotted as hardness versus distance from the reference surface. The characteristic depth is obtained by interpolation between the two measurement points bracketing the specified limit hardness, and reported to the nearest 0.05 or 0.1 mm as required by the governing standard. Testers are verified periodically against certified reference hardness blocks, and test reports document the test force, indentation coordinates, the hardness-distance data table, and the plotted curve.
Quality Assurance
Reputable accredited third-party laboratories maintain their microhardness capabilities under ISO/IEC 17025 management systems, with documented traceability of test forces, indenter geometry, measuring optics, and reference blocks. Measurement uncertainty, repeatability checks on reference blocks, and duplicate traverses provide confidence in reported depth values, which are frequently used as acceptance criteria in supply contracts and qualification documents.
Applications
Microhardness profile testing serves a broad range of engineering, quality assurance, and research purposes across many industries:
- Validation of carburizing, carbonitriding, nitriding, and induction hardening processes during heat treatment qualification and production release
- Acceptance testing of automotive gears, camshafts, crankshafts, splines, and fasteners against drawing-specified case depth requirements
- Weld procedure qualification and welder performance documentation, including maximum hardness verification in the HAZ for pipeline and pressure vessel codes
- Assessment of hydrogen-assisted cracking susceptibility by confirming HAZ hardness limits in sour oil and gas service
- Fatigue and fracture investigations, where surface softening, decarburization, or insufficient case depth is a suspected root cause
- Qualification of thermal spray coatings, laser cladding, hard chrome replacements, and weld overlays for repair and remanufacturing
- Tooling and die steel evaluation, including nitride layer integrity and temper verification after grinding or reconditioning
- Research and development of gradient materials, surface alloying, additively manufactured parts, and dissimilar metal joints
- Reverse engineering and supplier benchmarking by comparing hardness depth signatures of competitor or legacy components
- Support of failure analysis reports with objective, standards-based evidence of through-thickness mechanical gradients
FAQ
What is the difference between effective case depth and total case depth?
Effective case depth (CHD) is the depth at which the hardness curve intersects an agreed limit hardness—550 HV1 for carburized cases per ISO 2639 and GB/T 9450—representing the functionally load-bearing hardened layer. Total case depth is the depth at which hardness becomes indistinguishable from the core hardness, and it is always greater than or equal to the effective case depth. Both values are read directly from the same microhardness profile.
Why is HV1 specified as the reference test force for case depth?
HV1 provides an indentation large enough to average over the microstructure and deliver good measurement reproducibility, yet small enough to resolve the case transition in typical carburized depths of 0.5 to 2 mm. Standards such as ISO 2639 and GB/T 9450 fixed HV1 as the reference condition precisely so that results from different laboratories are directly comparable.
What is nitriding hardness depth (NHD) and how is its limit defined?
Nitriding hardness depth is determined per ISO 15787 as the depth at which hardness falls to the limit hardness, defined as the actual core hardness plus an agreed additional value, commonly 50 HV. Because nitrided layers are thin, lower forces such as HV0.5 or HV1 and closely spaced indentations near the surface are used.
How is decarburization depth measured by microhardness?
A hardness traverse begins immediately below the polished surface, and the curve is compared with the unaffected core hardness level. The depth over which hardness remains below the core band indicates total decarburization, while metallographic examination distinguishes the ferritic decarburized rim. ISO 3887 and ASTM E1077 describe both the hardness and microstructural approaches.
What sample size and preparation are required for testing?
A representative section typically 10 to 25 mm across, cut perpendicular to the treated surface, is sufficient for most work. The laboratory mounts, grinds, and polishes the specimen to a mirror finish with good edge retention. Customers usually submit either a complete component, a cut segment, or a pre-sectioned blank, and the testing laboratory completes the metallographic preparation.
Can microhardness results be converted to Rockwell C values?
Conversions between HV and HRC are permissible for specific material classes using recognized conversion tables such as ASTM E140 or GB/T 1172, provided the conversion is not applied across a phase boundary or steep gradient. For contractual case depth values, the original HV measurement basis is retained as the authoritative result.
Why do indentation spacing rules matter in profile testing?
Each indentation plastically deforms and work-hardens the surrounding material. If indentations are placed too close together, strain fields from prior indentations distort subsequent readings. Standards therefore require center-to-center spacing of at least 2.5 diagonal lengths in Vickers testing, which is why staggered or diagonal indentation patterns are employed when fine depth resolution near a surface is needed.
How long does microhardness profile testing take?
For straightforward case depth determinations, most accredited third-party laboratories complete specimen preparation, indentation, measurement, curve construction, and reporting within three to seven working days. Complex multi-zone weld surveys, coating systems requiring Knoop indentation, or programs with many specimens may require additional time, and expedited schedules can usually be arranged with the laboratory in advance.
