Microhardness Distribution Testing: Principles, Standards, and Applications

Microhardness distribution testing maps how hardness varies across the surface, cross-section, or hardened case of a metallic component, providing quantitative data on case depth, heat treatment uniformity, and weld integrity. Performed in accordance with standards such as ASTM E384, ISO 6507-1, and ISO 2639, this technique is essential for verifying carburized, nitrided, induction-hardened, and welded parts. This article explains the testing principles, sample requirements, methods, results interpretation, and typical industrial applications.

Technical Overview

Microhardness distribution testing is a destructive testing technique that measures hardness at microscopic intervals along a polished cross-section or across a component surface, producing a hardness-versus-position profile or a two-dimensional hardness map. Unlike single-point macro hardness checks, it reveals hardness gradients that are invisible to bulk measurements, making it the primary method for evaluating surface engineering processes and metallurgical transitions.

The technique uses a diamond pyramidal indenter pressed into the sample under low test forces, typically between 10 gf and 1 kgf (HV0.01 to HV1). Two indenter geometries are standard: the Vickers pyramid, which produces a square indentation and is the basis of the HV scale, and the elongated Knoop indenter, whose HK scale is preferred for very thin layers and shallow gradients because its shallow penetration reduces substrate influence. Indentation diagonals are measured optically at magnifications of 400× to 1000×, and hardness values are calculated from the applied force and the measured diagonal lengths.

The core value of the method lies in its ability to derive engineering parameters from the measured distribution, including effective case depth (CHD) after carburizing, nitriding hardness depth (NHD), effective hardening depth after induction or flame hardening, decarburization depth, and peak hardness in weld heat-affected zones. Because results depend strongly on specimen preparation, machine calibration, and indentation placement, testing is typically performed by accredited third-party laboratories following internationally recognized procedures.

Testing Samples

Carburized and Case-Hardened Components

Gears, shafts, camshafts, pins, and bearing components that have undergone carburizing, carbonitriding, or vacuum carburizing are sectioned perpendicular to the treated surface so that the full hardness gradient from surface to core can be traversed. Typical test pieces are ring or segment sections 10–20 mm in width, although complete teeth or small parts can be mounted whole.

Nitrided and Nitrocarburized Parts

Nitriding produces shallow, very hard cases, often less than 0.6 mm deep, frequently with a brittle compound (white) layer at the surface. Microhardness traverses with HV0.5 or HV1 indentations, spaced at 25–50 µm near the surface, are used to determine nitriding hardness depth and to assess compound layer integrity.

Induction and Flame Surface-Hardened Parts

Crankshaft journals, rails, axles, and wear tracks hardened by induction or flame heating exhibit sharp transitions between hardened zones and the substrate. Hardness profiles perpendicular to the hardened track define the effective hardening depth and the sharpness of the transition, which controls fatigue and spalling resistance.

Welded Joints and Heat-Affected Zones

Cross-sections of arc, laser, electron beam, and friction stir welds are surveyed with microindentation lines crossing base metal, heat-affected zone (HAZ), and weld metal. This reveals softening or hardening in the HAZ, checks against maximum hardness limits for sour-service pipelines under ISO 15156, and supports welding procedure qualification to ISO 15614 series requirements.

Coatings and Surface-Engineered Layers

Thin coatings such as electroplated layers, thermal spray coatings, chemical vapor deposition (CVD) and physical vapor deposition (PVD) films, and diffusion layers are tested with Knoop or very low-force Vickers indentations. For coatings thinner than a few micrometers, instrumented indentation to ISO 14577 may supplement conventional microhardness to isolate coating hardness from substrate effects.

Raw Materials and Failure Analysis Specimens

Bar stock, forgings, castings, and rolled plate are examined for decarburization depth (ASTM E1077, GB/T 224), segregation, banding, and through-thickness uniformity. Fractured or prematurely worn components are profiled to detect grinding burn, untempered martensite, or inadequate heat treatment.

Testing Items

Typical test items and reported parameters include:

• Micro-Vickers hardness at scales HV0.01, HV0.025, HV0.05, HV0.1, HV0.2, HV0.3, HV0.5, and HV1
• Knoop microhardness at scales HK0.01 through HK1 for thin layers and steep gradients
• Hardness distribution curves (hardness versus distance from surface or from the weld centerline)
• Effective case depth after carburizing (CHD), defined at the 550 HV1 limit per ISO 2639 and GB/T 9450
• Total case depth and surface hardness values, including subsurface peak hardness location
• Nitriding hardness depth (NHD) at the limit hardness of core hardness plus 50 HV per DIN 50190-3
• Effective hardening depth after induction or flame hardening at the 80 percent minimum surface hardness limit per DIN EN 10328 and ISO 3754
• Decarburization depth (total and ferritic) per ASTM E1077 and GB/T 224
• Weld and HAZ hardness mapping with line traverses per ISO 9015-1 and ISO 9015-2, including narrow-HAZ evaluation per ISO 22826
• Hardness uniformity surveys across surfaces, batches, or build directions
• Approximate hardness scale conversions per ASTM E140, reported with appropriate caution

Testing Methods

Specimen Preparation

Accurate distribution data begin with metallographic-quality preparation. The component is sectioned cleanly, using ample coolant and slow cuts to avoid tempering or burning the case, then mounted in resin with the region of interest protected for edge retention. Successive grinding and polishing stages must remove all deformation from previous steps and produce a scratch-free, flat surface; rounded or beveled edges invalidate near-surface measurements. A light etch, such as 2–3 percent nital for steels, reveals the case, hardened zone, or weld macrostructure so indentations can be positioned deliberately, while heavy etching is avoided because relief and staining degrade diagonal measurement.

Test Execution

Testing follows ISO 6507-1, ASTM E384, or the equivalent GB/T 4340 and JIS Z 2244 procedures. The test force is selected so that indentation size is large enough for precise optical measurement yet small enough to resolve the gradient; HV1 is standard for case depth work, with HV0.5 or lower for thin nitrided cases and weld microzones. Dwell time is typically 10–15 seconds. Indentation spacing must respect material interaction rules, generally at least three diagonal lengths between indentation centers for steels and at least 2.5 diagonal lengths from specimen edges, and specimen thickness must exceed roughly ten times the indentation depth.

Traverse Strategies

For case depth determination, ISO 2639 permits a step-by-step method, in which rows of indentations are placed at defined depth increments, and a single-traverse method, in which a tapered or sloping section allows one row of indentations to sample increasing depth through a known geometric correction factor. Weld hardness surveys follow the line arrangements of ISO 9015-1, placing traverses typically 2 mm below the surface and 2 mm above the root, with additional mid-thickness lines for thick joints, indenting across base metal, HAZ, and weld metal at close spacing to capture narrow zones.

Data Evaluation and Reporting

Measured hardness values are plotted against distance, and a smooth curve is fitted to the data. Derived depths are obtained at the intersection with the relevant limit hardness: 550 HV1 for carburized cases, core hardness plus 50 HV for nitrided cases, and 80 percent of the minimum specified surface hardness for surface-hardened parts. Reports include the test scale, indentation layout diagram, raw hardness values, the distribution curve, calculated depth parameters, and the applicable standard. Machines are verified and calibrated per ISO 6507-2 and ASTM E384, with certified reference blocks traceable to ISO 6507-3, ensuring the repeatability that customers of accredited third-party laboratories rely upon.

Applications

Microhardness distribution testing supports quality control, process development, and failure investigation across many industries:

• Automotive powertrain: verifying case depth and core hardness of transmission gears, differential components, crankshafts, camshafts, and constant velocity joints
• Bearings and power transmission: confirming raceway hardening depth, grinding integrity, and heat treatment uniformity
• Aerospace: qualifying carburized and nitrided landing gear and actuator parts, evaluating coatings on turbine components, and supporting process approvals
• Oil and gas: HAZ hardness surveys on pipeline girth welds against ISO 15156 sour-service limits, and hardfacing depth verification on valves and drill tool components
• Fasteners and springs: assessing nitrided, carburized, and induction-hardened products for depth and surface hardness compliance
• Tooling and dies: measuring nitrided layers and thin PVD or CVD coatings using Knoop and low-force Vickers methods
• Additive manufacturing: mapping hardness uniformity and anisotropy across build directions in laser and electron beam melted alloys
• Failure analysis: detecting grinding burn, decarburization, incomplete hardening, temper embrittlement effects, and weld softening in service failures

FAQ

What is the difference between microhardness distribution testing and a standard hardness test?

A standard hardness test provides one value representing the bulk or surface of a part. Microhardness distribution testing applies many small indentations at defined positions to reveal how hardness changes with depth or position, which is the only way to measure case depth, HAZ behavior, or hardness gradients in thin engineered layers.

How is effective case depth determined?

For carburized and hardened steel, effective case depth (CHD) is the depth at which hardness equals 550 HV1, measured on a polished cross-section per ISO 2639 or GB/T 9450. The measured profile is plotted, and the depth corresponding to the 550 HV1 limit is interpolated from the curve.

Which test force should be selected?

The force must balance resolution against measurement precision. HV1 is preferred for case depths above roughly 0.3 mm; HV0.5 suits thin nitrided cases and weld microstructures; HV0.05 to HV0.1 and Knoop scales are reserved for coatings and near-surface layers where the indentation must remain within the layer being measured.

Can small or oddly shaped parts be tested?

Yes. Specimens as small as a few millimeters can be sectioned and mounted, and modern testers with automated stages and pattern programming can follow curved gear flanks, tooth roots, fillets, and weld contours with consistent indentation placement.

Can Vickers results be converted to Rockwell HRC?

Approximate conversions are available through ASTM E140, but conversions between scales covering very different indentation sizes carry uncertainty, especially in hardened cases with steep gradients. Direct measurement in the scale specified by the product standard is always preferred.

Does sample preparation affect results?

Preparation quality is critical. Overheating during cutting, inadequate polishing, poor edge retention, or heavy etching can shift results by tens of HV or more, so competent laboratories apply controlled metallographic procedures and verify edge flatness before testing.

How long does testing take?

For a single specimen with a straightforward traverse, testing and evaluation can typically be completed within a few working days, including sectioning, mounting, polishing, etching, indentation, and reporting. Multi-line weld maps, grids, or full component qualification programs require additional time proportional to the number of indentations.