Microhardness Cross-Section Testing: Principles, Procedures, and Industrial Applications

Microhardness cross-section testing is a precision metallographic technique that measures Vickers or Knoop hardness on a prepared cross-section to characterize hardness gradients beneath a surface. Performed in accordance with ISO 6507, ISO 4545, ASTM E384, ASTM E92, and related GB and JIS standards, it is the reference method for determining effective case depth, nitrided layer depth, decarburization depth, coating hardness, and heat-affected zone hardness. This article explains the testing principles, sample preparation, procedures, result evaluation, and typical industrial applications of cross-sectional microhardness testing.

Technical Overview

Microhardness cross-section testing, also known as cross-sectional micro-indentation hardness testing, involves pressing a diamond indenter into a polished metallographic cross-section under a small, precisely controlled test force and measuring the indentation diagonals optically to calculate hardness. The Vickers indenter is a square-based pyramid with a 136° face angle, while the Knoop indenter is an elongated rhombic pyramid that produces a shallow, elongated indentation well suited to thin layers and steep hardness gradients. Test forces typically range from 10 gf to 1 kgf (scales HV0.01 to HV1), with low-force Vickers tests up to HV3 also common for case depth work.

For the Vickers method, hardness is calculated as HV = 0.1891 × F / d², where F is the test force in newtons and d is the mean indentation diagonal in millimeters. Because the indentations are extremely small, hardness can be measured at defined depths below the surface, on individual microstructural constituents, or across narrow zones such as weld heat-affected zones (HAZ) and coating interfaces. A series of indentations made at increasing depth produces a hardness-depth profile, which is the basis for quantifying hardened layers, diffusion treatments, and surface degradation.

Surface hardness testing alone cannot reveal sub-surface gradients or the transition between a treated layer and the core. Cross-sectional microhardness testing fills this gap and is prescribed by numerous international standards, including ISO 6507-1, ISO 4545-1, ISO 2639, ISO 3754, ISO 9015-1, ISO 9015-2, ISO 15156, ASTM E384, ASTM E92, ASTM E1077, ASTM E140, GB/T 4340, GB/T 18449, GB/T 9450, GB/T 9451, GB/T 11354, GB/T 224, GB/T 27552, and JIS Z 2244.

Testing Samples

Suitable samples are solid materials, components, or joints from which a representative cross-section can be extracted and prepared with good edge retention. Common sample categories include:

Case-Hardened and Surface-Hardened Components

Carburized and carbonitrided gears, pinions, and sleeves; nitrided or ferritic nitrocarburized crankshafts, camshafts, valve seats, and dies; induction-hardened or flame-hardened shafts, axles, rails, and splines. Cross-sections are typically taken at functionally critical locations such as the tooth flank, tooth root, or journal fillet.

Coated and Surface-Engineered Materials

PVD and CVD hard coatings on cutting tools and molds, electroplated chromium layers, thermal spray and laser-clad overlays, anodized aluminum, diffusion aluminide coatings, and multilayer thin-film systems where coating, interlayer, and substrate hardness must be distinguished.

Welded, Brazed, and Clad Joints

Butt welds, fillet welds, dissimilar metal joints, surfacing welds, and brazed assemblies where hardness traverses across the weld metal, fusion line, HAZ, and base metal are required for procedure qualification or fitness-for-service assessment.

Decarburized and Process-Affected Steel Products

Hot-rolled bars, wire rod, forgings, castings, fasteners, and springs in which surface decarburization, overheating, or grinding damage must be quantified through microhardness depth profiles.

Fragments and Failure Analysis Specimens

Fractured components, wear surfaces, grinding-burn regions, and rehardened or tempered zones, sectioned to correlate hardness variation with microstructure and failure mechanism.

Testing Items

Typical test items and deliverables for cross-sectional microhardness evaluation include:

• Vickers microhardness testing from HV0.01 to HV1, and low-force testing up to HV3 where required
• Knoop microhardness testing (HK0.01 to HK1) for very thin layers and steep gradients
• Surface hardness and core hardness verification on the cross-section
• Hardness-depth profiles (hardness gradient curves) with tabulated data and plots
• Effective case depth after carburizing or carbonitriding (CHD) at the 550 HV1 criterion per ISO 2639 and GB/T 9450
• Effective depth of hardening after flame or induction hardening per ISO 3754 and GB/T 9451
• Nitrided and nitrocarburized layer depth, surface hardness, and brittleness-related assessments per GB/T 11354
• Total hardening depth and decarburization depth per ASTM E1077 and GB/T 224
• Weld metal, fusion line, and HAZ hardness surveys per ISO 9015-1, ISO 9015-2, and GB/T 27552
• Coating, interdiffusion zone, and substrate hardness for single- and multi-layer systems
• Two-dimensional hardness mapping across selected regions, interfaces, or defects
• Indicative hardness conversions to HRC, HRB, or HB per ASTM E140 and GB/T 1172, where metallurgically appropriate

Testing Methods

Sampling and Cross-Section Preparation

Samples are cut with abrasive or precision sectioning under abundant coolant to avoid heating or deformation damage that would alter the true hardness gradient. Specimens are mounted, preferably with hot-mounting resins providing excellent edge retention, then ground through progressively finer silicon carbide abrasives and polished with diamond suspensions down to 1 µm or finer. Etching, for example with 2–4% nital for steels, may be used to reveal microstructural features and locate measurement positions; hardness measurements should be made on carefully prepared, lightly etched or unetched surfaces so that etch relief does not distort diagonal measurement. A flat, scratch-free cross-section with preserved edges is essential for accurate near-surface readings.

Load Selection and Indentation Placement

The test force is selected so that the substrate or adjacent layers do not support the indentation. A widely applied rule is that indentation depth should not exceed about one tenth of the layer thickness; since a Vickers indentation is roughly one seventh as deep as its diagonal is wide, the diagonal must be kept well below the layer thickness. For very thin coatings, Knoop indentations or instrumented indentation per ISO 14577 are preferred. Per ISO 6507-1, the distance between indentation centers should be at least three times the mean diagonal, and the distance to the specimen edge at least 2.5 times the diagonal. Dwell times of 10 to 15 seconds are standard. Note that at very low forces the indentation size effect causes apparent hardness to rise; comparisons should therefore be made between values obtained at the same test force.

Hardness Traverse and Depth Determination

Rows of indentations are made perpendicular to the surface at defined depth increments. For carburized cases, ISO 2639 and GB/T 9450 specify HV1 testing with indentation spacing not exceeding 0.1 mm, beginning close to the surface; the effective case depth is obtained by interpolation at the 550 HV1 limit. For flame- and induction-hardened parts, ISO 3754 and GB/T 9451 define the boundary at an agreed hardness limit, commonly 0.8 times the specified minimum surface hardness. Nitrided layer depth per GB/T 11354 is determined from an HV0.5 or HV1 profile at the specified limit hardness above the core. Decarburization depth is assessed by microhardness profiling or microstructural methods per ASTM E1077 and GB/T 224. For welds, ISO 9015-1 uses HV5 or HV10 traverses and ISO 9015-2 uses microhardness traverses, with measurement lines typically located 1 to 2 mm below the surface; acceptance limits such as 320 HV10 or 380 HV10 depending on steel delivery condition, or approximately 248 HV (22 HRC) for sour-service welds per ISO 15156, may apply.

Equipment, Calibration, and Verification

Testing is performed on microhardness testers with verified force application and optical systems, often equipped with motorized stages, digital imaging, and automatic diagonal recognition for high-throughput traverses and hardness mapping. Direct and indirect verification of machines, indenters, and measuring devices is performed per ISO 6507-2 and ISO 6507-3, ISO 4545-2 and ISO 4545-3, and the verification provisions of ASTM E384 and ASTM E92, using certified reference hardness blocks. Accredited third-party laboratories maintain traceable calibration programs and documented measurement uncertainty for reported results.

Data Evaluation and Reporting

Test reports identify the applied standard, test force and scale, dwell time, indentation positions, individual and mean hardness values, the hardness-depth plot, and derived quantities such as CHD or layer depth with the interpolation method used. Statements on conversions, deviations, and measurement uncertainty provide the technical basis for acceptance decisions and process control.

Applications

Cross-sectional microhardness testing supports quality control, qualification, and failure analysis across virtually every metals-intensive industry:

• Automotive: case depth verification on carburized gears, hardening depth on induction-treated camshafts and crankshafts, nitrided valve train components, and fastener decarburization checks
• Aerospace: hardness profiling of landing gear, bearings, and shafts, coating qualification, and weld procedure qualification to recognized welding standards
• Oil, gas, and energy: HAZ hardness surveys on pipeline and pressure vessel welds, sour-service hardness compliance per ISO 15156, and hardfacing overlay evaluation
• Tooling and machining: PVD and CVD coating hardness on cutting tools, dies, and laser-clad or repaired surfaces
• Metallurgical failure analysis: detection of grinding burn, rehardening, temper damage, decarburization-induced fatigue, and improper heat treatment
• Research and development: optimization of carburizing, nitriding, laser hardening, welding, cladding, and additive manufacturing processes
• Supply chain control: incoming inspection, vendor qualification, and dispute resolution through accredited third-party laboratories

FAQ

What is the difference between microhardness and conventional hardness testing?

Microhardness uses much smaller test forces, typically 10 gf to 1 kgf, producing indentations only tens of micrometers across. This allows hardness to be measured at specific depths, in individual microstructural constituents, and in thin layers that conventional macrohardness methods such as HRC or HB testing would average out or destroy.

Why is the 550 HV1 criterion used for effective case depth?

The 550 HV1 limit specified in ISO 2639 and GB/T 9450 historically corresponds to the hardness associated with an approximately 50% martensitic structure in carburized carbon and alloy steels, marking the boundary of the load-bearing hardened case. Using a single defined force (HV1) and limit hardness makes results reproducible between laboratories.

What is the difference between effective case depth and total case depth?

Effective case depth (CHD) is the depth at which hardness equals 550 HV1 and represents the engineering case. Total case depth extends to the point where hardness reaches the unaffected core hardness and includes the transition zone that contributes little load-bearing capacity. Both are obtained from the same cross-sectional hardness profile.

How thin a layer can be measured reliably?

Reliability depends on keeping the indentation small relative to the layer. As a practical guideline, indentation depth should not exceed about one tenth of the layer thickness, so for a Vickers test the diagonal should be well below the layer thickness. Knoop testing, with its shallow elongated indentation, extends capability to thinner layers, and instrumented indentation per ISO 14577 is used for very thin films.

Can microhardness values be converted to HRC or tensile strength?

Indicative conversions between HV, HRC, HB, and HRB are given in ASTM E140 and GB/T 1172, and for many steels the tensile strength is roughly three times the Vickers hardness value in MPa. However, conversions are approximations influenced by composition, microstructure, and work-hardening history, and should never replace direct testing for acceptance purposes.

What sample preparation is required, and how are results delivered?

A representative section is cut under coolant, mounted with good edge retention, ground, and polished to a mirror finish, with etching only as needed to locate features. Results are typically delivered within a few working days as a report containing the hardness table, depth profile plot, derived case or layer depth, applied standard, test conditions, and calibration traceability information.