Microhardness Testing with Load Retention (Dwell Time): Principles, Standards, and Applications

Microhardness testing with load retention—also known as microhardness testing with dwell time—is a precision indentation method in which a diamond indenter is held at full test force for a controlled period before the force is removed and the indentation is measured. Governed by standards such as ISO 6507-1, ASTM E384, GB/T 4340.1, and JIS Z 2244, this technique provides reliable hardness values for case-hardened layers, thin coatings, welds, small components, and individual microstructural phases.

Technical Overview

Microhardness testing with load retention (dwell time) is a microindentation hardness measurement in which a diamond indenter—most commonly a Vickers four-sided pyramid with a 136° interfacial angle, or a Knoop elongated pyramid—is pressed into a carefully prepared specimen surface under a low test force, typically between 10 gf (0.098 N) and 1 kgf (9.807 N). The defining characteristic of the method is the load retention phase: once the indenter has reached full test force, the force is maintained at a constant level for a precisely controlled dwell period—normally 10 to 15 seconds—before being withdrawn. The diagonals of the resulting indentation are then measured with a calibrated optical system at magnifications of typically 400× or higher, and the hardness value is calculated from the applied force and the indentation size.

The dwell phase is not arbitrary. Many engineering materials, particularly soft non-ferrous metals such as lead, tin, aluminum, copper, and gold alloys, as well as polymers and some electrodeposited coatings, exhibit time-dependent plastic deformation (room-temperature creep). If the force is removed too quickly, the indentation is still growing and the measured diagonal will be too small, producing an artificially high hardness value. Holding the load for a standardized period allows creep-driven deformation to saturate, stabilizing the diagonal measurement and dramatically improving repeatability and reproducibility. For this reason, ISO 6507-1 and ASTM E384 specify a standard dwell window of 10 to 15 seconds, and any deviation must be recorded explicitly in the test report—for example, 640 HV1/30 denotes a Vickers hardness of 640 measured with a 1 kgf force held for 30 seconds.

Accredited third-party laboratories performing this test operate microhardness testers that are directly and indirectly verified in accordance with ISO 6507-2 or the verification provisions of ASTM E384, using certified reference blocks traceable to national metrology institutes. Testing is conducted at ambient temperatures within the range permitted by the standard (normally 10 °C to 35 °C) under an ISO/IEC 17025 quality management framework, with documented measurement uncertainty.

Testing Samples

Metallic Cross-Sections and Heat-Treated Components

Carburized, carbonitrided, nitrided, induction-hardened, and laser-hardened parts; through-hardened tool and die steels; quenched-and-tempered structural steels; stainless steels; and cast irons. Specimens are sectioned perpendicular to the treated surface, mounted in resin, ground, and polished so that hardness depth profiles can be traversed from the surface inward.

Thin Materials and Small Components

Thin sheet and foil, fine wire (tested on polished cross-sections), micro-machined parts, fasteners, springs, electrical contacts, watch components, and medical devices such as needles, orthodontic wires, and stents, where the volume of material available is too small for macro-scale hardness testing.

Coatings and Surface Layers

Electroplated deposits such as hard chromium and nickel, PVD/CVD hard coatings, thermal spray coatings, anodized layers, nitride compound layers, and hot-dip coatings, together with decarburized or carburized surface zones. For very thin films where even the lowest microhardness forces produce excessive penetration depth relative to coating thickness, instrumented indentation per ISO 14577-1 may be recommended as a supplementary method.

Ceramics, Welds, and Composite Materials

Technical ceramics, glass, sintered carbides, semiconductor materials, polymers, metal-matrix composites, solder joints, and welded joints (fusion zone and heat-affected zone). For brittle materials, the test force must be selected carefully to avoid radial cracking, which invalidates the measurement.

Testing Items

Typical deliverables of microhardness testing with load retention include:

• Vickers microhardness (HV) at test forces from HV0.01 to HV1 (10 gf to 1 kgf), including HV0.05, HV0.1, HV0.2, HV0.3, HV0.5, and HV1, each with standardized dwell time.
• Knoop microhardness (HK) where an elongated indentation is advantageous, such as very thin layers, elongated phases, or brittle ceramics.
• Effective case depth (CHD/ECD) determination per ISO 2639 or GB/T 9450, based on microhardness depth profiles to the specified hardness limit (for example, 550 HV1).
• Nitriding hardness depth (NHD) and total case depth evaluation from surface-to-core hardness traverses.
• Decarburization depth determination per ISO 3887.
• Weld and heat-affected zone (HAZ) microhardness surveys per ISO 9015-2.
• Hardness of individual microstructural constituents: ferrite, martensite, bainite, retained austenite, carbides, and inclusions.
• Hardness uniformity mapping, gradient profiling on actual components, and acceptance testing against material or heat-treatment specification limits.

Testing Methods

The test is performed in a controlled sequence designed to satisfy the requirements of ISO 6507-1 and ASTM E384:

• Specimen preparation: The area of interest is sectioned, mounted, ground with progressively finer abrasives, and polished to a mirror finish, typically with 1 μm or finer diamond suspension, so that the indentation corners are sharply resolved. Light etching may be applied to reveal microstructure, provided it does not degrade edge definition. Specimen thickness must be at least ten times the indentation depth; because a Vickers indentation is approximately d/7 deep, this limit is readily checked from the measured diagonal, and no visible deformation or bulging may appear on the reverse side.
• Machine verification: The tester is verified per ISO 6507-2 or ASTM E384 using certified reference blocks, and the optical measuring system is calibrated at the working magnification before use.
• Test setup: A test force appropriate to the layer thickness, specimen thickness, and phase size is selected. The force is applied perpendicular to the specimen surface without shock or vibration, reaching full value within the standard loading time, and is then held for the standard dwell period of 10 to 15 seconds. Extended dwell times (for example, 30 or 60 seconds) may be used for creep-sensitive materials and must be reported with the result.
• Indentation and measurement: After force removal, both diagonals are measured and averaged. Vickers hardness is calculated as HV ≈ 0.1891 × F / d², where F is the test force in newtons and d is the mean diagonal in millimeters. Knoop hardness is calculated analogously from the long diagonal using the geometric constant of the Knoop indenter.
• Spacing and edge rules: The distance between the centers of adjacent indentations is kept at least 2.5 to 3 times the mean diagonal (up to 6 times for soft metals such as aluminum, lead, and tin alloys), and each indentation is placed at least 2.5 diagonal lengths from the specimen edge to avoid work-hardening interaction and edge relaxation effects.
• Reporting: The test report states the hardness value (mean and scatter), test force designation, dwell time, number of indentations, the standard applied, surface condition, and—for depth profiles—the distance of each measurement point from the reference surface. Reports issued by accredited third-party laboratories also include measurement uncertainty in accordance with ISO/IEC 17025.

Applications

Microhardness testing with load retention is used across a wide range of industries and failure-analysis scenarios:

• Heat-treatment validation: verification of carburizing, nitriding, carbonitriding, and induction hardening through hardness depth profiles and effective case depth determination.
• Welding qualification: microhardness traverses across the fusion line and heat-affected zone for welding procedure qualification of steels, stainless steels, and nickel alloys.
• Surface engineering: hardness verification of electroplated coatings, PVD/CVD hard layers, anodizing, and thermal spray coatings.
• Failure analysis: detection of grind burns, decarburization, softening from overheating, hydrogen embrittlement effects, and abnormal microstructures.
• Metallurgical research and development: phase discrimination, homogeneity evaluation, alloy development, and correlation with strength properties.
• Electronics and medical devices: hardness evaluation of lead frames, contacts, thin films, needles, and miniature mechanical components where conventional hardness scales cannot be applied.

FAQ

What does load retention (dwell time) mean in microhardness testing?

Load retention refers to the interval during which the full test force is held constant on the specimen before removal—typically 10 to 15 seconds per ISO 6507-1 and ASTM E384. The hold allows time-dependent deformation to stabilize so that the measured indentation reflects a steady-state material response.

How does dwell time affect the measured hardness value?

Longer dwell times generally lower the apparent hardness of creep-prone materials such as soft non-ferrous metals, polymers, and some coatings, because the indentation continues to grow during the hold. For hard steels and ceramics at room temperature, the effect is small. Because dwell time influences results, it is fixed by the standard and must be reported whenever a non-standard duration is used.

Which test forces are used in microhardness testing?

Microhardness test forces range from HV0.01 to HV1 (10 gf to 1 kgf), with HV0.05, HV0.1, HV0.2, HV0.3, HV0.5, and HV1 being the most common settings. The force is chosen so that the indentation is large enough for accurate optical measurement yet small enough to remain within a thin layer or individual phase and to satisfy the minimum thickness requirement.

Which standards govern microhardness testing with load retention?

The principal standards are ISO 6507-1 (test method), ISO 6507-2 (machine verification), ASTM E384 (microindentation hardness of materials), ASTM E92 (Vickers and Knoop hardness of metallic materials), GB/T 4340.1, and JIS Z 2244. Application-specific standards include ISO 2639 and GB/T 9450 (effective case depth), ISO 3887 (decarburization depth), ISO 9015-2 (weld microhardness), and ISO 14577-1 (instrumented indentation for very thin coatings).

What sample preparation is required?

Test surfaces must be flat, mounted, and polished to a mirror finish so the indentation corners are crisp; etching, when used to reveal microstructure, must be light. Specimen thickness must be at least ten times the indentation depth (approximately 1.4 times the diagonal for a Vickers indentation), and indentation spacing and edge-distance limits specified by the standard must be observed.

How are results expressed?

Vickers results are written as hardness value, scale, and—where applicable—dwell time: 640 HV1 indicates a Vickers hardness of 640 with a 1 kgf force at the standard dwell, while 320 HV0.5/30 indicates a 0.5 kgf force held for 30 seconds. Depth-profile reports additionally list the distance of each measurement from the surface and derived quantities such as effective case depth.