Alloy Vickers Hardness Testing: Principles, Standards, and Determination Methods

Vickers hardness testing is one of the most versatile and widely applied methods for evaluating the mechanical properties of metallic alloys, offering a single continuous hardness scale that spans from very soft aluminum alloys to extremely hard tool steels and cemented carbides. This article provides a comprehensive technical guide to alloy Vickers hardness determination, covering testing principles, relevant international standards such as ISO 6507 and ASTM E384, sample preparation requirements, test force selection, and typical industrial applications.

Technical Overview

The Vickers hardness test is an indentation-based method in which a polished diamond indenter, shaped as a right pyramid with a square base and an angle of 136 degrees between opposite faces, is pressed into the surface of a test specimen under a defined static force. After the force is removed, the lengths of the two diagonals of the resulting square indentation are measured optically, and the Vickers hardness number (HV) is calculated from the applied force and the mean diagonal length.

The Vickers hardness value is defined by the relationship between the test force and the surface area of the indentation. When the force is expressed in newtons and the mean diagonal in millimeters, the hardness is calculated as HV = 0.1891 × F / d². Because the geometry of the indenter is constant across the entire force range, the Vickers scale is geometrically self-consistent, which means that, within the limits of indentation size effects, results obtained at different test forces are directly comparable.

Vickers hardness determination of alloys is governed by a family of internationally recognized standards, which define the test method, machine verification, reference block calibration, and hardness value tables:

• ISO 6507-1: Metallic materials — Vickers hardness test — Part 1: Test method
• ISO 6507-2: Verification and calibration of testing machines
• ISO 6507-3: Calibration of reference blocks
• ISO 6507-4: Tables of hardness values
• ASTM E92: Standard test methods for Vickers hardness and Knoop hardness of metallic materials (macro range)
• ASTM E384: Standard test method for microindentation hardness of materials
• GB/T 4340.1: Chinese national standard for Vickers hardness testing of metallic materials
• JIS Z 2244: Japanese industrial standard for Vickers hardness test of metallic materials

Accredited third-party laboratories routinely perform Vickers hardness testing under these standards to support material qualification, heat treatment verification, failure analysis, and research and development programs for a broad spectrum of engineering alloys.

Testing Samples

Ferrous Alloys

Vickers hardness determination is extensively applied to ferrous alloys, including carbon steels, low-alloy steels, stainless steels (austenitic, ferritic, martensitic, duplex, and precipitation-hardening grades), tool steels, high-speed steels, bearing steels, and cast irons. The method is particularly valuable for hardened and tempered steels whose hardness may exceed the practical range of Rockwell scales, and for evaluating case-hardened or nitrided surfaces where localized measurement is essential.

Nonferrous Alloys

Nonferrous engineering alloys tested by the Vickers method include aluminum alloys (wrought and cast, including 2xxx, 5xxx, 6xxx, and 7xxx series), copper alloys such as brass and bronze, titanium alloys including Ti-6Al-4V, nickel-based superalloys, cobalt alloys, magnesium alloys, zinc die-casting alloys, and precious metal alloys. The wide force range of the Vickers test makes it suitable for both soft annealed nonferrous alloys and heavily strengthened precipitation-hardened grades.

Special Sample Forms

Beyond bulk materials, Vickers hardness testing is routinely performed on cross-sectioned weldments for heat-affected zone profiling, thermally sprayed or electroplated coatings, nitrided and carburized layers, thin sheet and foil products, wires, small precision components, fasteners, and metallographic mounts containing individual microstructural constituents. For micro-scale work, testing is commonly conducted on polished and etched metallographic cross-sections.

Testing Items

Typical Vickers hardness testing services for alloys include the following determination items:

• Macro Vickers hardness testing at forces of HV10 (98.07 N), HV30 (294.2 N), HV50 (490.3 N), and HV100 (980.7 N) for general bulk hardness evaluation
• Low-force Vickers testing at HV0.5, HV1, HV2, HV3, and HV5 for small parts, thin sections, and moderate gradients
• Micro Vickers testing at HV0.01, HV0.05, HV0.1, and HV0.2 for individual microstructural phases, coatings, and fine gradients
• Effective case depth (CHD) determination for carburized and carbonitrided steels
• Nitriding hardness depth (NHD) determination for nitrided and nitrocarburized components
• Hardness traverse profiling across welds, brazed joints, and functionally graded zones
• Surface and core hardness verification after heat treatment processes such as quenching, tempering, solution annealing, and aging
• Hardness uniformity surveys and statistical scatter evaluation across production lots

Testing Methods

Sample Preparation

Proper specimen preparation is critical for reliable Vickers results. The test surface must be flat, smooth, and free of oxide scale, lubricants, and processing residues. For micro and low-force testing, the surface should be mechanically ground and polished to a metallographic finish; for etched cross-sections used in traverse work, etching should be minimized or performed after indentation marking to avoid obscuring the indentation corners. The thickness of the test piece, or of the layer under test, must be at least ten times the indentation diagonal, and no visible bulging or marking on the side opposite the indentation is permitted. Specimens must be rigidly supported so that no movement occurs during force application, and when mounting is required, the mount must hold the specimen normal to the indenter axis.

Test Force Selection and Procedure

The test force is selected according to the specimen thickness, the hardness of the alloy, and the purpose of the test. In general, the largest force that avoids violating the thickness criterion and excessive surface damage should be used to minimize measurement uncertainty, while smaller forces are reserved for thin layers, coatings, and microstructural constituents. The indenter is brought into contact with the specimen, and the force is applied smoothly, without shock or vibration, and held for a dwell time typically between 10 and 15 seconds as specified by the relevant standard. Longer dwell times may be required for materials exhibiting time-dependent deformation, and any deviation must be noted in the test report.

Indentation Spacing and Measurement

To avoid interaction between plastic deformation fields, the center of any indentation must be spaced at least 2.5 times the diagonal length from the specimen edge and from the center of adjacent indentations, with larger multiples typically applied in micro testing. After force removal, both diagonals of the indentation are measured using a calibrated optical measuring system, and the mean diagonal length is used to compute the HV value. The resolution of the measuring device must be adequate for the indentation size; for micro testing, a typical requirement is a measuring resolution on the order of 0.1 to 0.5 micrometers. A minimum number of valid indentations, commonly three to five, is performed, and the individual values, mean value, and scatter are reported along with the test force designation (for example, 640 HV10).

Quality Control and Traceability

Testing machines used for Vickers determination must be verified periodically using certified reference blocks in accordance with ISO 6507-2 or ASTM E384, with direct and indirect verification of force application, indenter geometry, measuring device accuracy, and testing cycle timing. Temperature, vibration, and lighting conditions in the laboratory must be controlled, and technicians must be qualified through documented training and interlaboratory comparison. Expanded measurement uncertainty should be evaluated and may be reported with the results, particularly for conformity assessment against specification limits.

Applications

Vickers hardness determination of alloys serves a wide range of industrial and scientific purposes:

• Heat treatment verification: confirming that quenching, tempering, annealing, solution treatment, and aging of alloy steels, aluminum alloys, and superalloys have achieved specified hardness targets
• Surface engineering assessment: measuring hard chromium, electroless nickel, thermal spray, PVD, and CVD coatings, and determining the uniformity and depth of engineered surface layers
• Case depth qualification: determination of effective case depth for carburized gears and bearings and nitriding depth for crankshafts, valve components, and die components
• Weldment evaluation: hardness traverses across base metal, heat-affected zone, and weld metal to detect local softening, hardening, or susceptible microstructures in pipeline, pressure vessel, and structural welds
• Incoming material inspection and supplier qualification of bars, forgings, castings, sheet, and foil products
• Failure analysis: detecting softening from overheating, decarburization, over-tempering, or hydrogen damage, and hardening from improper processing
• Research and development: mapping hardness gradients in new alloys, additively manufactured parts, and graded materials, and correlating microstructure with mechanical response
• Sector-specific applications in aerospace, automotive, power generation, oil and gas, medical devices, electronics, and precision tooling

FAQ

What is the difference between Vickers, Rockwell, and Brinell hardness testing?

Vickers testing uses a diamond pyramidal indenter and measures indentation diagonals optically, providing a continuous scale suitable for all alloys and for very small or thin specimens. Rockwell testing measures indentation depth using either a diamond cone or ball indenters and is faster for bulk production control but less suited to gradients and thin layers. Brinell testing uses large ball indenters and high forces, making it appropriate for coarse-grained and inhomogeneous materials such as castings but not for hardened or thin samples.

How do I choose the correct test force for my alloy?

The test force should be as high as possible while ensuring that the specimen or layer thickness is at least ten times the resulting diagonal, the indentation does not interact with edges or adjacent indentations, and the surface condition is compatible with reliable optical measurement. Heavily hardened alloys generally permit lower forces due to smaller indentations, while soft alloys require sufficient force to produce well-defined, optically measurable indentations.

Can Vickers hardness values be converted to other hardness scales or tensile strength?

Conversions between hardness scales for specific material classes are standardized in ASTM E140 and similar references, but conversions are approximate and must only be applied to the material families for which they were established. Estimating tensile strength from hardness is likewise an approximation; contractual or specification-critical determinations should be based on direct measurement of the required property.

What is the difference between macro and micro Vickers testing?

Macro Vickers testing, covered by ASTM E92 and the higher force ranges of ISO 6507-1, employs forces of 980.7 N and above in common practice, or from 49.03 N depending on the standard scope, and evaluates bulk material hardness. Micro Vickers or microindentation testing, covered by ASTM E384 and ISO 6507-1 low-force ranges, uses forces down to 9.807 mN and below for individual phases, coatings, and fine hardness gradients, demanding superior surface preparation and measuring resolution.

How should results be reported?

A complete Vickers hardness report should state the test standard used, the test force and designation, the dwell time, the number of indentations, individual and mean hardness values with scatter, the surface preparation method, the test location and direction for traverses, any deviations from the standard, and the measurement uncertainty where applicable. Results are expressed in the form of value, scale symbol, and force, for example 250 HV10, with the dwell time appended when it differs from the standard range.