# Metallographic examination

Planning, preparing and interpreting metallographic sections while controlling artifacts and measurement limits.

Reviewed: 2026-11-01

Tags: inspection, metallurgy, microscopy, quality-control

Select the specimen and section orientation for the structure you need to observe. ASTM E3-26 guides preparation. Acceptance requires its own criterion. [ASTM E3-26: scope and significance](https://store.astm.org/e0003-26.html)

A bounded example comes from Buehler's **Metallographic Preparation of Tool Steels**, TECHNotes volume 5, issue 3, updated August 2024. It explains that improper sectioning can temper martensite or, with excessive heating, re-austenitize the surface. Such damage is not necessarily removed easily by subsequent grinding. It also identifies edge retention as important when examining decarburization or heat-treated surfaces. These are tool-steel preparation concerns, not a cutting recipe for all alloys. [Buehler TECHNotes, updated 082024](https://www.buehler.com/assets/solutions/technotes/Metallographic-Preparation-of-Tool-Steels-TechNote-Volume-5-Issue-3.pdf)

## Begin with the inspection question

Specify what must be observed or measured before cutting: for example grain structure, inclusions, a coating interface, a weld region or a surface-affected layer. Leica identifies representative sampling as the first critical step and describes microscopy, imaging and quantitative analysis as parts of a larger examination workflow. A convenient piece of material is not necessarily representative of the feature under investigation. [Leica Microsystems: Metallography—an Introduction, January 8, 2020](https://www.leica-microsystems.com/science-lab/industrial/metallography-an-introduction/)

Record the sample's location and section orientation relative to the component and processing directions. ASTM E3 specifically notes that location and orientation affect interpretation and the number of specimens needed. For an investigation, distinguish a deliberately selected suspect region from material sampled to characterize a population; one cannot silently substitute for the other. Define any comparison specimen and preserve the connection between the original part and the final mount. [ASTM E3-26](https://store.astm.org/e0003-26.html)

PACE recommends photographing and identifying the as-received specimen before sectioning. Cutting conditions must minimize damage that could alter the structure. Its preparation guidance separates sectioning, mounting, grinding, polishing, etching and analysis; these stages have different purposes rather than being interchangeable ways to make the surface shiny. [PACE Technologies: specimen preparation basics](https://www.metallographic.com/basics)

## Prepare the surface without replacing the evidence

| Stage | Purpose and practical limit |
| --- | --- |
| Sectioning | Expose the intended plane while controlling heating and mechanical damage. |
| Mounting | Support handling and vulnerable edges; choose a resin and mounting process compatible with the specimen. |
| Grinding | Produce a plane surface and remove damage from cutting and previous abrasive steps. |
| Polishing | Remove remaining deformation while retaining particles and avoiding excessive relief. |
| Contrast development | Reveal the feature of interest using a suitable etch or optical technique. |

Struers describes progressive abrasive removal and retention of structural elements as preparation goals. Its Metalogram selects methods using **hardness and ductility**, not material name alone. Each stage must remove the preceding damage while introducing as little new damage as possible. Grit sizes, forces and times are not universal. [Struers: grinding and polishing guidance](https://www.struers.com/en/Knowledge/Grinding-and-polishing)

## Recognize preparation artifacts

Particle pull-out can leave holes resembling real pores. Smearing can cover features, differential removal can produce relief, and edge rounding can obscure an interface. Struers' troubleshooting guidance recommends checking specimens between stages to identify when damage develops. If a suspicious feature appears after preparation, consider re-preparation or examination by a complementary method rather than immediately calling it a manufacturing defect. Preserve the original observation and describe the check performed. [Struers: preparation artifacts and pull-out](https://www.struers.com/en/Knowledge/Grinding-and-polishing)

Etching provides contrast through selective attack and is material- and objective-dependent. PACE describes chemical and alternative etching methods and directs users to the applicable SDS and appropriate protective equipment. Select the actual reagent and handling procedure through the laboratory's chemical controls; this reference supplies no general mixing, storage or disposal recipe. [PACE Technologies: etching and handling](https://www.metallographic.com/basics)

Buehler's tool-steel example illustrates why “etched” is insufficient documentation: for spheroidize-annealed material, picral reveals ferrite–carbide interfaces, whereas nital also reveals ferrite grain boundaries and can obscure carbide shape. Etch choice therefore changes what can be interpreted or measured. Record reagent identity, concentration, application method, temperature and exposure where relevant, with the approved procedure controlling their values. [Buehler: tool-steel etching considerations](https://www.buehler.com/assets/solutions/technotes/Metallographic-Preparation-of-Tool-Steels-TechNote-Volume-5-Issue-3.pdf)

## Observe and measure at the appropriate scale

Reflected brightfield is common for metallography, but darkfield, differential interference contrast and other techniques reveal different features. A dark region can result from topography or optical contrast as well as a constituent's response. Choose overview and detail images that show both the feature and its context; do not identify a phase solely because it resembles a familiar image. [Leica: microscopy and contrasting techniques](https://www.leica-microsystems.com/science-lab/industrial/metallography-an-introduction/)

For dimensions, calibrate the actual imaging configuration. Leica's March 4, 2026 guidance uses a stage micrometer, repeats calibration at the relevant magnifications and checks the result by measuring the reference. Camera configuration and software scale matter along with the objective. A magnification label is not a substitute for a verified scale bar. Retain original images and measurement settings so later resizing does not silently change the reported scale. [Leica: microscope calibration for measurements](https://www.leica-microsystems.com/science-lab/industrial/microscope-calibration-for-measurements-why-and-how-you-should-do-it/)

## Quantification and acceptance have separate rules

Use the method appropriate to the characteristic. For example, ASTM **E112-25e1** covers average grain-size procedures including comparison, planimetric and intercept methods. It characterizes **two-dimensional grain sections**, not the sizes of three-dimensional grains in the specimen volume. Its comparison charts are not intended to measure an individual grain, and its methods do not establish material acceptability. Grain-size distribution and structure affect applicability; unusual or mixed distributions may require another method. [ASTM E112-25e1: scope and significance](https://store.astm.org/e0112-25e01.html)

As a practical reporting boundary, “no discontinuity observed in the examined sections” does not mean “the entire component contains no discontinuities.” State the sampled area, section locations, observation conditions and limitations. If a customer requires a numerical limit, cite that requirement separately from the preparation or measurement method. Additional sections, hardness testing, chemical analysis or electron microscopy should answer a defined unresolved question rather than merely add more images.

## Examination record and source versions

Include component and sample identities, section map, orientation, material condition, preparation procedure, mount, etchant, microscope configuration, calibrated scale, sampling plan, observations, calculations and applicable acceptance criteria. Separate direct observations from interpretations and identify preparation artifacts or unresolved ambiguities.

Sources were reviewed November 1, 2026. ASTM's catalog identifies E3-26 as active, updated April 30, 2026, and E112-25e1 as active, updated September 9, 2026. Buehler's PDF footer explicitly says updated 082024. The PACE and Struers pages cited here did not display revision dates.

[Inspection and quality documents](../../sourcing/documents/index.md)