Micro Vickers Hardness Testing of Microstructures: Principles, Standards, and Applications

Welded, Brazed, and Soldered Joints

Cross-weld sections of fusion welds, resistance welds, brazed joints, and dissimilar-metal assemblies are mapped across the fusion zone, heat-affected zone, and base metal to verify hardness distributions and check against maximum hardness limits specified in welding procedure qualification standards such as ISO 15614-1.

Small and Thin Specimens

Thin sheet and foil, fine wire, fasteners, springs, bearings, electrical contacts, watch components, additive-manufactured builds, and powder metallurgy parts, including individually mounted metal powders, are routinely characterized because their dimensions preclude macro hardness testing.

Specimen Preparation Requirements

Reliable microhardness results depend on rigorous metallographic preparation. Specimens should be sectioned with minimal heat and deformation, mounted with good edge retention, ground through successive abrasive grades, and polished to a scratch-free finish, typically with diamond suspensions of 3 micrometers or finer. The test surface must be flat and parallel to the opposite face so that the indentation is symmetrical. The specimen thickness must be at least ten times the permanent indentation depth, which corresponds to approximately 1.5 times the diagonal length. Etching should be light, or performed after indentation, because etch relief can distort the apparent diagonal length and bias the measured hardness.

Testing Items

Accredited third-party laboratories typically offer the following micro Vickers hardness measurements, all reported with test force, dwell time, applicable standard, number of indentations, mean value, and scatter:

• Micro Vickers hardness values at forces from HV0.01 to HV1
• Phase-specific hardness of ferrite, pearlite, bainite, martensite, austenite, carbides, and intermetallics
• Microhardness depth profiles from treated surfaces into the core
• Effective case depth (CHD) per ISO 2639, conventionally at the 550 HV limit
• Nitriding hardness depth (NHD) per ISO 6507-1, at the limit of core hardness plus 50 HV
• Total case depth and decarburization depth per ISO 3887
• Cross-weld and heat-affected zone hardness surveys and hardness maps
• Coating and layer hardness with indentation placement verification
• Hardness uniformity and segregation assessment on microstructural scales
• Complementary Knoop microindentation hardness per ASTM E384 for elongated features and brittle layers

Testing Methods

Testing begins with the selection of an appropriate test force. As a rule, the largest force that does not exceed the boundaries of the phase or layer being tested, does not distort the surface, and does not interact with adjacent features is preferred, because larger indentations reduce measurement uncertainty. The specimen is aligned so that the surface is perpendicular to the indenter axis, the force is applied smoothly without shock, and the dwell time, typically 10 to 15 seconds, is held constant and reported. Indentations must be spaced according to ISO 6507-1: for steels and copper alloys, the distance between indentation centers should be at least 3 times the diagonal length and at least 2.5 diagonal lengths from the specimen edge, while for light metals, lead, and tin, larger distances, up to 6 diagonal lengths, are required to avoid work-hardening interaction between indentations.

Both diagonals of each indentation are measured with a calibrated optical measuring system, generally at magnifications of 400 times or higher, and the average is used for the hardness calculation. A difference greater than 5 percent between the two diagonals indicates elastic anisotropy, surface tilt, or poor preparation and triggers an investigation. For depth profiling, as used in case depth determination, automated stages execute linear or zig-zag traverses with indentations spaced at intervals consistent with the governing standard, and the hardness is plotted against depth from the surface; the case depth is then determined at the specified hardness limit by interpolation. Testing machines are verified directly for force, indenter geometry, and measuring-device accuracy per ISO 6507-2 and verified indirectly at regular intervals using certified reference hardness blocks per ISO 6507-3. Vibration isolation, stable temperature, and repeatable focusing are essential for reproducible results at the smallest forces, and measurement uncertainty budgets account for force calibration, diagonal measurement, indentation spacing, and material homogeneity.

Applications

Micro Vickers hardness testing supports quality control, qualification, and research across virtually every industry that engineers metallic materials:

• Heat-treatment validation for carburizing, nitriding, induction hardening, and quench-and-temper processes on gears, shafts, camshafts, and bearings
• Weld procedure qualification and production weld surveillance, including heat-affected zone hardness limits for creep-resistant and pipeline steels
• Failure analysis to identify soft spots, decarburization, untempered martensite, grain-boundary precipitates, and abnormal microstructures
• Coating development and quality control for cutting tools, forming dies, and decorative or protective films
• Materials research, including phase transformation studies, aging behavior, additive-manufactured microstructures, and HIP-processed components
• Product qualification of fasteners, springs, electrical contacts, medical devices, and precision instrument components

Hardness values obtained by micro Vickers testing may also be compared with, or converted to, other hardness scales and to approximate tensile strength using conversion tables in ASTM E140 or ISO 18265, provided that such conversions are treated as estimates valid only for the material class and condition for which they were established.

FAQ

What is the difference between micro Vickers and conventional Vickers hardness testing?

The principle and indenter geometry are identical; the distinction lies in the test force. Micro Vickers testing uses low forces, generally 1 gram-force to 1 kilogram-force as covered by ASTM E384, producing indentations small enough to resolve individual phases, thin layers, and narrow zones, whereas conventional Vickers testing employs higher forces on a bulk scale.

Which test force should be selected for microstructural measurements?

The force is chosen so that the indentation is comfortably larger than the polishing texture but small enough to remain inside the constituent or layer of interest without interference from boundaries or adjacent phases of different hardness. Common choices are HV0.05 to HV0.2 for individual phases and HV0.5 to HV1 for case-depth traverses; the selected force must always be reported with the result.

How thin can a specimen be for valid micro Vickers results?

ISO 6507-1 requires the specimen thickness to be at least ten times the permanent indentation depth. Since the depth of a Vickers indentation is approximately one-seventh of its diagonal, a practical rule is that the thickness should be at least 1.5 times the expected indentation diagonal.

Can micro Vickers hardness values be converted to HRC or tensile strength?

Approximate conversions are possible using ASTM E140 or ISO 18265, but they are valid only for specific material classes and metallurgical conditions. Because phase-specific microhardness values are frequently higher than bulk hardness, conversions from microhardness data should be applied with particular caution.

How is case depth determined from microhardness data?

A hardness traverse is made from the treated surface inward at a defined test force, and hardness is plotted against depth. Effective case depth (CHD) per ISO 2639 is taken at the 550 HV limit, nitriding hardness depth (NHD) per ISO 6507-1 is taken where the hardness equals the core hardness plus 50 HV, and decarburization depth per ISO 3887 is evaluated using similar hardness criteria combined with metallographic examination.

Why is etching controlled so carefully in microhardness testing?

Etching reveals the microstructure but also attacks the surface and creates relief around the indentation, which can distort diagonal measurements and bias results. Best practice is to perform indentations on a lightly etched or unetched polished surface, or to complete the indentation campaign before applying the final etch for microstructural documentation.