Optical Metallography: Sample Preparation, Etching, and Microstructure Interpretation

Introduction to Optical Metallography

Optical metallography — the preparation and optical microscopic examination of metal sections — is the most fundamental analytical technique in metallurgy. Since its development by Henry Sorby in the 1860s, it has underpinned virtually every major advance in our understanding of metal microstructure. From failure analysis to quality control to research, the ability to prepare a representative, artefact-free metallographic section and correctly interpret what is observed under the microscope is an indispensable skill for any practising metallurgist.

This guide covers the complete metallographic preparation workflow, etchant selection for a wide range of alloys, and systematic microstructure interpretation for common engineering metals.

Step 1: Sectioning

The first step is obtaining a representative section from the sample without introducing preparation artefacts. Poor sectioning can produce:

• Thermal damage: Microstructural changes (recrystallisation, tempering of martensite, sensitisation) from excessive heat during cutting
• Mechanical damage: Deformation layer at the cut surface, smearing of soft phases, cracking of brittle phases
• Incorrect orientation: Transverse, longitudinal, or through-thickness sections reveal different microstructural features

Preferred sectioning methods:

• Abrasive cut-off wheel (coolant): General-purpose; correct wheel selection (Al₂O₃ for steels, SiC for non-ferrous) and generous coolant flow prevents thermal damage
• Precision saw (diamond wafering blade): Minimal deformation; preferred for soft alloys (aluminium, copper) and for EBSD or TEM specimen preparation
• Electrical discharge machining (EDM): Zero mechanical force; produces minimal deformation but creates a recast layer that must be removed

Step 2: Mounting

Mounting protects delicate surface layers (coatings, oxide layers, edge microstructures) and provides a convenient handling format for grinding and polishing. Two mounting methods:

Hot Compression Mounting

Thermosetting or thermoplastic resins (Bakelite, acrylic) moulded around the sample at 150–180°C under 25–30 kN in an automatic mounting press. Convenient; hard resin provides good support for grinding. Avoid for samples with heat-sensitive features (low-temperature tempered martensite, precipitates that dissolve at 150°C).

Cold Mounting

Epoxy or acrylic resins cure at room temperature. Suitable for heat-sensitive samples, porous materials (sintered PM parts, thermal spray coatings), and samples requiring vacuum impregnation. Longer cure time; generally lower hardness than hot-mounted resins.

Conductive Mounting

Carbon-filled or copper-filled resins for SEM/EDS applications. Provides electrical conductivity without additional coating of the sample.

Step 3: Grinding

Grinding removes the sectioning damage layer and achieves a flat, planar surface. It is performed in sequential stages of decreasing abrasive particle size:

At each stage, the sample is rotated 90° relative to the previous stage. Grinding continues until all scratches from the previous stage are removed — visible as all remaining scratches running in one direction. Water or oil coolant is applied continuously; interrupted dry grinding is a common cause of thermal damage.

Step 4: Polishing

Polishing removes residual grinding scratches to produce a scratch-free, mirror-bright surface suitable for microscopic examination. Two stages:

Diamond Polishing (9 µm → 3 µm → 1 µm)

Diamond suspension or paste on a rigid cloth (woven or non-woven). Diamond is suitable for all engineering metals. Each stage reduces scratch depth by 3–5×. The 1 µm diamond stage removes all residual 3 µm scratches and produces a surface suitable for etching in many cases.

Final Polish (0.05 µm OPS or alumina)

Oxide polishing suspension (OPS, colloidal SiO₂ at pH 9.8–10.2) on a chemomechanical pad. The slight chemical attack combined with mechanical action removes the thin deformed layer remaining after diamond polishing, producing a truly damage-free surface essential for EBSD and reliable hardness testing at low loads. For stainless steels and aluminium alloys, the OPS stage is particularly critical — the deformed layer left by diamond polishing can mask the true grain structure under the microscope.

Step 5: Etching

The polished surface reveals only inclusions, pores, and cracks in unetched condition. Etching selectively attacks different phases and grain boundaries, developing contrast for optical microscopy. Etchant selection depends on the material and the features of interest.

Common Etchants for Steel

Etching Technique

Apply etchant by immersion or swabbing with a cotton bud. Typical etch times: 2% nital on annealed steel, 3–10 seconds; 2% nital on hardened steel, 5–20 seconds. Monitor colour change — a light gold-brown indicates sufficient attack. Immediately rinse with ethanol (not water) then dry with warm air. Over-etching darkens the surface uniformly and obscures microstructural detail.

Microstructure Interpretation: Steel

Identifying Ferrite

Ferrite appears as equiaxed, light (white or cream) polygonal grains after nital etching. Grain boundaries are revealed as dark lines. In low-carbon steels (<0.3% C), ferrite is the dominant phase — a large proportion of the microstructure appears light.

Identifying Pearlite

Pearlite appears as dark colonies with a characteristic lamellar internal structure (alternating ferrite and cementite lamellae). At low magnification (100–200×), pearlite appears dark and may look featureless. At high magnification (500–1,000×), the individual lamellae become visible. Very fine pearlite (sorbite, troostite) appears dark and featureless even at 1,000×.

Identifying Martensite

Lath martensite (low/medium-carbon steels): fine, needle-like laths in packets, typically white or light after nital etch. At higher magnification, individual laths with thin austenite films between are visible. Plate martensite (high-carbon steels): broader, lenticular plates, often with midrib visible. Generally darker than lath martensite after nital etch.

Identifying Bainite

Upper bainite: acicular (needle-like) ferrite laths with carbides between them; dark grey appearance after nital. Lower bainite: finer, darker grey; difficult to distinguish from tempered martensite without TEM or hardness correlation. The key diagnostic is that bainite does not show the fine martensitic lath packet structure.

Grain Size Measurement (ASTM E112)

Grain size is reported as an ASTM grain size number G, where:

N = 2^(G−1) where N = number of grains per in² at 100× magnification

Three measurement methods per ASTM E112:

• Comparison method: Compare the etched microstructure at 100× with standard ASTM charts. Quick but subjective; accuracy ±1 grain size number.
• Planimetric (Jeffries) method: Count all grains within a known area and those intersecting the boundary (counted as ½). More accurate; ±0.5 grain size number.
• Intercept method: Count grain boundary intersections with a test line of known length. Most repeatable; ±0.25 grain size number.

Practical grain size numbers: ASTM 1–3 = coarse (cast, annealed); ASTM 4–6 = medium (normalised); ASTM 7–10 = fine (HSLA TMCP steel); ASTM 11–14 = ultrafine (nanostructured).

Frequently Asked Questions

Q: What causes “false” grain boundaries or artefacts in metallographic preparation?
A: Common artefacts include: scratches from incomplete polishing; smear (soft phase deformed over hard particles) from insufficient polishing; pull-out of inclusions or carbides from inadequate mounting or too aggressive etching; relief (soft/hard phases at different heights) from polishing with excessive pressure on soft backing cloths. Careful technique at each stage prevents these.

Q: How do I distinguish bainite from tempered martensite?
A: Under optical microscopy, both appear dark grey and can be difficult to distinguish. Key differences: tempered martensite retains the lath structure of the parent martensite; bainite shows a more random acicular morphology. Hardness correlation helps — tempered martensite at 500°C is typically 35–40 HRC; bainite at equivalent formation temperature is 35–45 HRC but has better toughness. TEM is needed for definitive identification.

Conclusion

Optical metallography requires precision at every stage — sectioning without thermal damage, progressive grinding to remove deformation layers, careful polishing to a deformation-free surface, and controlled etching to reveal the true microstructure. Correct interpretation of the resulting micrographs provides direct insight into heat treatment response, prior processing history, and failure mechanisms. See also: The Iron-Carbon Phase Diagram and Grain Refinement and Hall-Petch Strengthening.

References

• ASTM E3-11: Standard Guide for Preparation of Metallographic Specimens. ASTM International.
• ASTM E112: Standard Test Methods for Determining Average Grain Size. ASTM International.
• Vander Voort, G.F., Metallography: Principles and Practice. ASM International, 1999.
• Struers Technical Notes: Metalog Guide. Struers ApS, 2020.

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