Metallography is the science of studying the microstructure of metals and alloys through microscopic examination. Correct specimen preparation is critical for obtaining reliable analytical results — any error in a preparation step can introduce artefacts and lead to misinterpretation of the material's true structure. This guide follows the standard preparation workflow, explaining the purpose, key parameters, and important considerations at each stage.
Preparation Workflow Overview
Sectioning is the first step in the preparation workflow. Its purpose is to cut a representative analytical cross-section from a large workpiece. The quality of the cut directly affects the difficulty of all subsequent steps and the final result.
Key considerations:
- Cut-off wheel selection: Choose the appropriate wheel based on material hardness and toughness — diamond wheels for high-hardness materials (cemented carbides, ceramics); aluminium oxide or silicon carbide wheels for steel and non-ferrous metals
- Feed rate: Excessive speed causes overheating and a heat-affected zone (HAZ); use OptiFeed or FixFeed modes to control feed rate
- Coolant: Continuous coolant supply removes heat and prevents microstructural changes (e.g., secondary tempering of martensite)
- Clamping: The specimen must be firmly secured to prevent vibration causing an uneven cut or wheel damage
Precision cutting machines (such as the Accutom series) are suitable where precise positioning and minimal thermal damage are required; high-throughput machines (such as the Discotom and Labotom series) are suitable for general cross-section preparation.
Mounting encapsulates the specimen in resin, serving two purposes: protecting specimen edges from rounding during grinding and polishing (edge retention), and providing a stable, easy-to-handle platform.
Mounting method comparison:
- Hot Mounting: Uses thermosetting or thermoplastic resin (e.g. Bakelite, Multifast) cured under high temperature and pressure. Fast, high hardness, excellent edge retention; suitable for high-volume samples. Equipment: CitoPress series
- Cold Mounting: Epoxy or acrylic resin cured at room temperature. Suitable for heat-sensitive specimens (electronic components, composites); longer cure time (10 minutes to several hours)
- Vacuum Impregnation: Vacuum is applied first, then low-viscosity epoxy is injected to penetrate fine pores in porous materials (casting pores, weld seams), preventing grinding fluid from becoming trapped. Equipment: CitoVac
Choose a resin whose hardness is as close to the specimen as possible. A large hardness mismatch causes differential polishing, producing unrealistic relief at the interface zone.
Inscribe or mark the specimen number on the side or base of the mount to prevent mix-ups during batch preparation. Automated systems (such as Xmatic) can integrate barcodes or RFID tracking with digital management systems such as SureScan, enabling complete preparation traceability records.
Plane grinding removes the surface damage layer introduced by cutting and brings the specimen cross-section to macroscopic flatness. Silicon carbide (SiC) abrasive paper or bonded grinding discs are used, progressing from coarse to fine (e.g. P120 → P320 → P800) to progressively remove damage.
- Clean the specimen between each abrasive grade change to prevent coarse particles contaminating the finer disc
- Rotating direction should be changed by 90° at each grade — the change in scratch direction confirms that the previous damage layer has been fully removed
- Apply uniform pressure to avoid specimen tilt that leads to an uneven cross-section (rounding effect)
Fine grinding uses diamond grinding discs or diamond suspension on cloth discs, typically at 9 μm → 3 μm, to eliminate the deep scratches remaining from plane grinding while further removing the damage layer to leave only a minimal deformation layer.
MD Magnetic Disc System: The Tegramin series grinder-polisher uses magnetically retained multi-function discs. With MD-Largo, MD-Allegro, MD-Primo, and other grinding discs, a single machine can perform from fine grinding through to final polishing, greatly reducing the number of disc changes.
Polishing is divided into two stages: diamond polishing and final polishing:
- Diamond polishing (1 μm, 0.25 μm): Diamond suspension on velvet cloth removes fine grinding scratches; the specimen surface becomes semi-mirror
- Oxide final polishing (OP-S / OP-U): Colloidal silica suspension on a soft cloth uses chemo-mechanical action to remove the final surface deformation layer, achieving a mirror finish with no artefacts
Polishing for too long may cause soft phases to develop excessive relief; too short leaves micro-scratches that affect subsequent etching uniformity. It is recommended to use a fully automated grinder-polisher to set precise time and force.
After polishing, the specimen surface appears as a mirror under an optical microscope. Most metallic phases (ferrite, pearlite, martensite) have very low contrast, so chemical etching is required to reveal the microstructure.
- Nital (2–4% nitric acid in ethanol): The most commonly used etchant for steel; reveals grain boundaries and phases
- Keller's Reagent: Standard etchant for aluminium alloys; reveals grains and precipitates
- Aqua Regia: For stainless steels and nickel-based alloys
- Electrolytic Etching: For difficult-to-etch materials such as austenitic stainless steels
After etching, immediately rinse with water, dehydrate with ethanol, blow dry, then observe and record microstructure images under an optical microscope (or SEM).
Common Defects and Solutions
| Defect | Probable Cause | Solution |
|---|---|---|
| Residual scratches on surface | Previous abrasive damage not fully removed; specimen not cleaned thoroughly before disc change | Return to previous step and regrind; ensure specimen is thoroughly cleaned before each disc change |
| Edge rounding (radius) | Specimen not mounted, or resin too soft; uneven pressure applied | Use a high-edge-retention resin (e.g. Multifast); use automated machine to control pressure |
| Pull-outs | Material contains hard phases (e.g. carbides); soft matrix phase is polished away | Shorten polishing time; switch to harder polishing cloth; use low-viscosity lubricant |
| Artefacts — inclusions | Abrasive particles remain embedded in specimen; polishing cloth contaminated | Thoroughly clean after each step; replace polishing cloth regularly |
| Over-etching | Etching time too long, or concentration too high | Shorten etching time; use diluted concentration; rinse immediately after etching |
| Heat-affected zone (HAZ) | Feed rate too fast during cutting; insufficient coolant | Reduce feed rate; verify coolant flow; use OptiFeed mode |
Advantages of Automated Preparation: Manual preparation depends heavily on operator skill, resulting in poor batch-to-batch repeatability. Using the Tegramin fully automated grinder-polisher combined with the Xmatic automated system, standardised preparation programmes can be set to ensure consistent results across every batch, with complete preparation parameter records for quality traceability.