This solid hardness testing specification defines the standardized requirements for sample preparation, instrument verification, test execution, and result reporting when measuring the hardness of metallic and non-metallic solids. Aligned with ISO, ASTM, GB, and JIS standards, it covers Brinell, Rockwell, Vickers, Knoop, Shore, and Leeb methods, serving as a practical reference for quality engineers, metallurgists, and accredited third-party laboratories. Following a unified specification ensures that hardness values are repeatable, comparable, and metrologically traceable across machines, operators, and facilities.
Technical Overview
Hardness is the ability of a solid material to resist localized plastic deformation, penetration, or scratching. A solid hardness inspection specification establishes uniform requirements for test method selection, specimen preparation, equipment calibration, indentation measurement, and result evaluation so that hardness values obtained by different operators, machines, or laboratories remain comparable and traceable. Because hardness correlates strongly with tensile strength, wear resistance, and machinability, hardness inspection is one of the most widely applied quality-control tests in heat treatment, metalworking, and component manufacturing.
This specification references the prevailing international and national hardness testing standards, including:
- ISO 6506 / ASTM E10 / GB/T 231.1 / JIS Z 2243 — Brinell hardness test
- ISO 6507 / ASTM E92 / GB/T 4340.1 / JIS Z 2244 — Vickers hardness test
- ISO 6508 / ASTM E18 / GB/T 230.1 / JIS Z 2245 — Rockwell hardness test
- ISO 4545 / ASTM E384 / GB/T 18449.1 — Knoop hardness test
- ISO 868 / ASTM D2240 / GB/T 531.1 — Shore durometer hardness for plastics and elastomers
- ISO 16859 / GB/T 17394.1 — Leeb rebound hardness test for portable applications
- ISO 14577 — Instrumented indentation test for coatings and thin layers
- ISO 2639 / GB/T 9450 — Determination and verification of effective case depth
Testing Samples
Metallic Materials
Typical samples include carbon and low-alloy steels, stainless steels, tool steels, cast irons, aluminum alloys, copper alloys, titanium alloys, and nickel-based superalloys in the form of bars, plates, tubes, forgings, castings, and powder metallurgy parts. Coarse-grained or microstructurally inhomogeneous materials, such as as-cast and annealed states, are preferably tested by the Brinell method with a large indentation, while finished hardened parts are tested by Rockwell or Vickers methods.
Welded Joints and Heat-Affected Zones
Cross-sections of butt and fillet welds are ground and etched, then tested by Vickers traverses (typically HV5 or HV10) across the weld metal, heat-affected zone, and base metal. On-site verification of weld caps may be performed with portable Rockwell or Leeb testers after local grinding to remove reinforcement irregularities and oxide.
Heat-Treated and Surface-Hardened Parts
Quenched and tempered components are tested with Rockwell C (HRC) or Vickers (HV10 to HV30). Carburized, nitrided, or induction-hardened parts require microhardness traverses from the surface inward at small test forces (HV0.5 to HV1) to determine hardness gradients and effective case depth.
Thin Sheets, Foils, and Coatings
Thin-gauge sheets, foils, electroplated layers, and PVD or CVD coatings are evaluated by micro Vickers (HV0.01 to HV0.5) or Knoop indentation. The elongated Knoop indent is preferred for very thin or anisotropic layers because its shallow penetration minimizes substrate influence.
Polymers, Elastomers, and Composites
Plastics and rubber materials are tested with Shore durometers (Shore A for soft elastomers, Shore D for rigid plastics) on flat specimens at least 6 mm thick, built up from plies if necessary per ISO 868. Glass-fiber-reinforced plastics may be rated by Barcol impressor per ASTM D2583.
Ceramics, Carbides, and Sintered Materials
Technical ceramics and cemented carbides are tested by low-force Vickers or Knoop methods because of their brittleness. Indentation corners shall be examined for cracking; readings from cracked indentations shall be rejected unless fracture toughness evaluation is the specific objective.
Fasteners and Finished Components
Bolts, screws, nuts, bearings, gears, camshafts, and valve seats are hardness tested as part of mechanical property verification. Fastener testing follows ISO 898-1 / GB/T 3098.1, which specifies hardness limits for each property class as well as the location of test points on the shank, head, or transverse section.
Testing Items
Depending on the material, geometry, and specification of the part, the following hardness parameters are measured and reported:
- Brinell hardness (HBW), e.g., HBW 10/3000, for coarse or soft metallic materials
- Rockwell hardness: HRA, HRB, HRC and superficial scales such as HR15N and HR30T
- Vickers hardness (HV0.2 to HV30) and micro Vickers hardness (HV0.01 to HV0.1)
- Knoop hardness (HK0.01 to HK1) for thin layers and brittle materials
- Shore hardness (Shore A, Shore C, Shore D) and Barcol hardness for polymers and composites
- Leeb hardness (HLD, HLC) for rapid on-site screening of large components
- Effective case depth (CHD) and total case depth of hardened surface layers, referenced to the 550 HV1 limit per ISO 2639
- Hardness uniformity across a component surface or across a production batch
- Hardness scale conversions and estimated tensile strength per ASTM E140 / GB/T 1172, reported as indicative values only
Testing Methods
Specimen Preparation
Scale, oxide, decarburized layers, grease, and paint shall be removed before testing. For Vickers and Knoop testing, the surface shall be ground and polished so that both indentation diagonals can be resolved clearly under the measuring microscope; a typical requirement is a surface roughness Ra of 0.4 micrometers or better. Rockwell and Brinell tests tolerate moderate grinding marks provided the surface is flat and clean. The support face shall be flat, free of burrs and oil, and seated firmly on the anvil without rocking. Polished cross-sections for microhardness may be mounted in resin for edge retention.
Minimum Thickness and Test Force Selection
The specimen thickness shall be at least ten times the permanent indentation depth for Rockwell and Vickers tests and at least eight times the indentation depth for Brinell tests, and no visible bulge or marking shall appear on the support side. The test force shall be selected so that indentation size falls within the valid range of the scale; for Brinell testing, the indentation diameter shall be between 0.24 and 0.6 times the ball diameter. The force shall be applied smoothly, without shock or overshoot, and held for a standard dwell time of 10 to 15 seconds, extended where necessary for materials exhibiting creep.
Indentation Placement Rules
To avoid work-hardening interference between indentations, the distance between indentation centers shall be at least three times the indentation diameter or diagonal, and the distance from an indentation center to the specimen edge shall be at least two and one-half times the diameter or diagonal. The exact spacing values in the governing standard shall take precedence. A minimum of three to five valid indentations shall be taken per specimen or test location, and the individual values, mean, and range shall be recorded.
Calibration and Verification of Equipment
Hardness testers shall undergo direct and indirect verification per the relevant parts of ISO 6506, ISO 6507, ISO 6508, ISO 4545, and their ASTM counterparts, using certified reference blocks traceable to national metrology institutes. Periodic verification intervals shall be defined in the laboratory quality system, supplemented by routine daily or weekly checks with reference blocks whose hardness values are close to the working range. Durometers shall be verified against calibrated spring forces and indenter geometry per ISO 868 and ASTM D2240. Testers shall be installed on rigid, vibration-free benches.
Environmental Conditions
Testing shall be performed at a stable ambient temperature between 10 and 35 degrees Celsius, with 23 plus or minus 5 degrees Celsius recommended for high-precision microhardness work. Strong vibration, dust, and drafts that affect optical measurement shall be excluded from the test area.
Procedure and Reporting
The general test sequence is: select the appropriate method and scale, prepare and seat the specimen, apply the force and hold for the specified dwell, remove the force, measure the indentation or read the indicated value, repeat per spacing rules, and calculate the mean. The test report shall identify the specimen, material designation, governing standard, hardness scale with force and dwell time, number and locations of indentations, individual readings, mean and range, any conversions applied, instrument identification, and, where required, a statement of measurement uncertainty.
Applications
Solid hardness inspection is applied across the full product lifecycle, from raw material acceptance to in-service failure analysis. Key applications include:
- Verification of heat treatment operations such as quenching, tempering, annealing, and normalizing against drawing or grade requirements
- Incoming material inspection of bars, plates, and castings against mill test certificates and purchased specifications
- Welding procedure qualification and production weld surveillance through hardness traverses across weld zones
- Case depth verification of carburized and induction-hardened gears, camshafts, and bearing surfaces
- Mechanical property compliance of fasteners per ISO 898-1 / GB/T 3098.1
- Tool and die maintenance programs, wear evaluation, and regrind acceptance checks
- Failure analysis, including detection of decarburization, soft spots, and abnormal hardness gradients
- Quality control in automotive, aerospace, railway, energy, construction steel structures, general machinery, medical devices, electronics, and plastics industries
FAQ
How do I select the right hardness test method?
Selection depends on material, microstructure, and specimen thickness. Use Brinell for coarse-grained or soft metals, Rockwell C for hardened steels and routine production checks, Vickers for laboratory work and weld traverses, micro Vickers or Knoop for thin layers and coatings, Shore for plastics and elastomers, and portable Leeb for large components that cannot be brought to a bench tester.
Can hardness values be converted between scales or to tensile strength?
Approximate conversions between scales, such as HRC to HV or HBW, are given in ASTM E140 and GB/T 1172 for specific material classes. Converted tensile strength values are indicative only and shall never replace a direct tensile test where the specification requires certified tensile properties.
Is hardness testing destructive?
Hardness testing is semi-destructive: it leaves small localized indentations that are usually acceptable on structural parts but may be prohibited on finished functional surfaces. Portable Leeb testing causes minimal surface marking, and where any indentation is unacceptable, hardness may be measured on a sacrificial witness coupon from the same lot and heat treatment batch.
What is the minimum required specimen thickness?
The thickness shall be at least ten times the permanent indentation depth for Rockwell and Vickers tests and at least eight times for Brinell tests, with no visible deformation on the support face. For thinner materials, a lower test force or a shallower scale, such as superficial Rockwell or micro Vickers, shall be selected.
How many indentations are required and how are results accepted?
A minimum of three to five valid indentations is standard practice, with mean and range reported. Acceptance criteria are defined by the product specification, drawing, or applicable standard, for example an HRC range for a property class, a maximum hardness spread across a batch, or a minimum effective case depth of 550 HV1 for carburized cases per ISO 2639.
How often should hardness testers be calibrated?
Testers require scheduled direct and indirect verification per ISO 6506, ISO 6507, ISO 6508, and their ASTM equivalents, using certified reference blocks traceable to national metrology institutes. Accredited third-party laboratories additionally perform routine checks with reference blocks at the working hardness level each day of use and document results in the equipment log.
What is effective case depth and how is it measured?
Effective case depth is the perpendicular distance from the hardened surface to the point where the hardness reaches a specified limit, commonly 550 HV1 for carburized cases per ISO 2639 and GB/T 9450. It is measured by a micro Vickers traverse on a polished and etched cross-section, with indentations spaced at defined intervals and the depth determined by interpolation between measured points.