Cast Iron Sample Preparation

Introduction

Cast iron preparation presents unique challenges that distinguish it from steel preparation. The defining characteristic of cast iron is its graphite structure, which must be preserved throughout the entire preparation process. Unlike steel, where the microstructure is revealed primarily through etching, cast iron's graphite is revealed by proper polishing - etching reveals the matrix structure (ferrite, pearlite, or bainite).

Nodular cast iron, 200X magnification. This image shows the characteristic spheroidal graphite structure and matrix. Proper preparation preserves graphite while revealing matrix microstructure through etching.

Cast iron types vary significantly in their graphite morphology and matrix structure:

• Gray Cast Iron: Flake graphite in pearlite or ferrite matrix (163-187 HB). Some grades contain steadite (Fe-Fe₃P phosphorus eutectic) as a wear-resistant network.
• Ductile Cast Iron: Spheroidal (nodular) graphite in pearlite or ferrite matrix (217 HB).
• Malleable Cast Iron: Temper carbon rosettes in ferrite or pearlite matrix (163 HB). Rosettes are friable and particularly difficult to retain during preparation.
• Austempered Ductile Iron (ADI): Spheroidal graphite in ausferrite (needle-like ferrite + carbon-saturated retained austenite) matrix (310 HB). Looks like bainite under brightfield but contains no carbides.
• Compacted Graphite Iron (CGI): Vermicular (compacted) graphite in pearlite matrix (230 HB). For acceptable CGI, ~80% of the graphite should be compacted (Type III/IV per ISO 945); the rest may be nodules. Flake graphite is unacceptable in CGI; flakes eliminate the strength and damping benefits the alloy is specified for.
• White Cast Iron: Pearlite + ledeburite (eutectic of pearlite and cementite), no free graphite. Very hard (~HV 600 / ~57 HRC for unalloyed; harder still for Ni-hard grades with martensitic matrix and chromium carbides). Sectioning and grinding require different abrasives than the graphitic grades.
• Austenitic Cast Iron: Cast iron with ≥20% Ni and 1-5.5% Cr, austenitic matrix with flake or spheroidal graphite. Used for corrosion-resistant pump housings and marine parts; particularly prone to smearing in preparation.

Each type requires similar polishing techniques, but sectioning and grinding paths differ substantially between graphitic grades and the very hard white/ADI grades. Spheroidal graphite (ductile iron) is generally easier to retain than flake graphite (gray iron), and temper carbon (malleable) is the hardest of all to keep in place.

Sectioning

Minimize sectioning damage at the source. The deeper the cut damage, the more material the grinding step must remove, and every removed micron is another chance to smear matrix metal over graphite. Use abrasive cut-off wheels matched to the cast iron type.

MAX-D abrasive cut-off blades for general ferrous metals including cast iron. MAX-I thin blades are an alternative for smaller sections.

Blade Selection

• Graphitic cast iron (grey, ductile, malleable, CGI, austenitic): MAXCUT abrasive blade, MAX-D series (general purpose for ferrous metals) or MAX-I series (industrial thin blade). Both are alumina-bonded and well-suited to cast iron.
• White cast iron (~HV 600, including Ni-hard grades): Use a harder alumina-bonded blade designed for hardened steels. MAX-VHS (alumina/resin-bonded for hard steels) is the PACE option; cubic boron nitride (CBN) cut-off wheels are the industry preference when available.

Cutting Parameters

• Thin abrasive cut-off wheel (0.5-1.0 mm).
• Apply steady, moderate pressure; let the wheel cut.
• Flood coolant. Graphitic cast irons corrode in water, so cut quickly and move to grinding/cleaning without delay.
• For hard grades (white cast iron, ADI), reduce feed and increase coolant flow to manage heat.
• For large white-cast-iron sections, automatic sectioning is more efficient and gives cleaner cuts than manual.

For more information on sectioning blades, visit our Abrasive Blades collection.

Mounting

Mounting provides edge retention and easier handling. For cast iron, compression mounting works well for routine work; quality-control samples taken directly from the casting line are often run unmounted for speed. Castable mounting is reserved for failure-analysis work where vacuum impregnation is needed to support a fragile microstructure.

Compression Mounting

Compression mounting with phenolic, epoxy, or diallyl phthalate (DAP) resins is suitable for most cast irons. Phenolic is fine for general work; DAP and epoxy give better edge retention, which matters when you are evaluating graphite or steadite near the mount/sample boundary.

1. Clean the sample thoroughly to remove cutting fluid and debris.
2. Place sample in mounting press with phenolic, epoxy, or DAP resin.
3. Apply pressure: 3000-4000 psi for phenolic, 2000-3000 psi for epoxy, ~4200 psi for DAP.
4. Heat to 150-180°C and hold 5-8 minutes (resin dependent).
5. Cool under pressure to room temperature.

Castable Mounting (also known as Cold Mounting)

Castable mounting with epoxy resin is also suitable and can provide better infiltration of open graphite cavities, improving graphite retention during preparation.

1. Clean the sample thoroughly
2. Place sample in mounting mold
3. Mix epoxy resin according to manufacturer's instructions
4. Pour into mold and allow to cure (typically 4-8 hours at room temperature)

For more information on mounting equipment, visit our Compression Mounting Equipment page.

Grinding

For cast iron, use alumina (Al₂O₃) grinding paper rather than silicon carbide. Alumina cuts cast iron cleanly with less matrix smearing over the graphite, which is the dominant cast iron preparation failure. Two paper steps are enough; longer SiC sequences (P120 → P240 → P400 → P600) commonly used for steel are not needed and actively work against graphite retention because each additional step is another chance to smear matrix metal into the graphite cavities.

Grinding Sequence

1. 500 grit alumina paper (P500): Plane-grind until the surface is flat and all sectioning damage is removed. Water lubricant, 5-10 lbs per sample, 200 RPM head and platen (complementary rotation).
2. 1200 grit alumina paper (P1200): Fine grind, 1 minute. Same parameters: water, 5-10 lbs, 200 RPM complementary.

Rotate the sample 90° between steps. Inspect the surface; at this point graphite should be visible but the matrix may still show fine grinding scratches. The diamond polish step removes these.

White Cast Iron and ADI

For very hard cast irons (white cast iron at ~HV 600, ADI at 310 HB), the alumina paper sequence above still works but is slow. An alternative is a rigid diamond grinding disc for plane grinding (~75 μm grit) followed by diamond suspension on a rigid disc, eliminating paper entirely. This is standard practice in cast-iron laboratories that handle white cast iron volume.

Polishing

Polishing is critical for cast iron because graphite is revealed by polishing, not etching. The goal is to achieve a mirror-like surface that reveals the graphite structure clearly while preparing the matrix for etching. Use gentle polishing techniques to avoid removing graphite.

Polycrystalline diamond compound for polishing cast iron. Use gentle pressure to preserve graphite.

Various polishing pads and cloths. Use softer cloths and lighter pressure for cast iron to preserve graphite.

Diamond Polishing

The cast iron polishing sequence is deliberately short. Adding intermediate 6 μm and 3 μm diamond steps does not improve graphite retention; it adds more opportunities to smear matrix metal over the graphite. Go directly from the 1200 grit alumina paper (P1200) to 1 μm diamond on a low-nap pad.

1. 1 μm DIAMAT polycrystalline diamond on GOLDPAD: 2 minutes, 5-10 lbs per sample, 200 RPM head and platen (complementary rotation), DIALUBE Purple Extender as lubricant.

Final Polishing

Cast iron's final polish uses alumina, not colloidal silica. 0.05 μm Nanometer alumina on a TRICOTE pad gives the matte finish needed to see graphite as grey at 500× without the chemical action of colloidal silica, which can attack interfacial regions and cause graphite pullout in soft-matrix grades.

1. 0.05 μm Nanometer alumina on TRICOTE: 1 minute, 5-10 lbs per sample, 100 RPM head and platen (complementary rotation).
2. Rinse with cold water, then ethanol; dry with warm air. Do not wipe the polished face; wiping can pull graphite.

Pad Map & Parameters Summary

For cast irons that corrode during polishing (some austenitic grades, free-machining grey iron), substitute a non-aqueous diamond suspension and lubricant at the 1 μm step. Rinse with anhydrous ethanol only (no water) and dry immediately.

After polishing, inspect at 500×. Graphite should be grey, not black. Black at 500× means the cavity is filled with smeared matrix or the graphite has pulled out and the cavity has re-filled with debris during subsequent polishing. The fix in either case is to return to the 1 μm diamond step on a fresh cloth.

For more information on polishing supplies, visit our Diamond Abrasives and Polishing Pads collections.

Etching

Cast iron is unusual among metallographic samples in that the most important examination happens before any etchant touches the sample. Graphite morphology, size, and distribution are evaluated on the as-polished surface against standard reference charts. Only after that evaluation is the matrix revealed by etching.

Examination Protocol

1. Examine unetched at 100× and 500×. At 100×, evaluate graphite morphology, size, and distribution against ISO 945-1 or ASTM A247 reference charts. At 500×, verify graphite retention; properly polished graphite is grey, not black.
2. Photograph the as-polished view first. For ductile iron QC, count nodularity and measure nodule size per ISO 945-4 or ASTM E2567 on this image; etched images can shift apparent nodule boundaries.
3. Then etch to reveal the matrix.

Nodular cast iron after etching, 100X. The matrix structure (ferrite/pearlite) is revealed by etching, while the spheroidal graphite is preserved from polishing.

Standard Black-and-White Etchants

• 2% Nital: General-purpose etchant for most cast iron matrices. Reveals ferrite grain boundaries and pearlite structure (2 mL HNO₃ in 98 mL ethanol). Swab; 5-15 seconds.
• 3% Nital: For harder grades like ADI, white cast iron, alloyed cast irons. Same procedure as 2% Nital but slightly more aggressive.
• 4% Picral: Excellent for revealing pearlite and cementite without attacking ferrite grain boundaries. Particularly useful when measuring pearlite content per ASTM E407 (4 g picric acid in 100 mL ethanol). Swab; 10-30 seconds.
• Sequential Nital → Picral: For complex matrices in alloyed cast irons (Nital first to reveal ferrite/pearlite contrast, brief Picral to differentiate carbides).

Color (Tint) Etchants

Color etching is the standard approach for distinguishing phases in ADI (ausferrite vs. retained austenite vs. martensite) and for revealing fine compositional banding in cast irons.

• Modified Beraha for cast iron: Stock solution = 1000 mL water + 200 mL HCl + 24 g ammonium bifluoride (NH₄HF₂). Working solution = 100 mL stock + 1 g potassium metabisulfite (K₂S₂O₅), prepared just before use. Immerse until the surface develops color (30 seconds to 2 minutes). The standard color etchant for ADI and for distinguishing α-ferrite from retained austenite.
• Klemm's II: 50 mL saturated aqueous sodium thiosulfate + 5 g potassium metabisulfite. Immerse 60-120 seconds; warming to ~40 °C enhances staining. Colors ferrite blue, martensite brown/black; retained austenite stays white. Useful alternative to Beraha for ADI.

Etching solutions and reagents for cast iron. Common etchants include 2% Nital and 4% Picral. Etching reveals the matrix structure, not the graphite.

Etching Procedure

1. Confirm sample is clean and dry. Residual polishing debris will give a mottled etch.
2. Apply by swab for Nital/Picral; immerse for Beraha/Klemm color etches.
3. Etch 5-15 seconds for Nital, 10-30 seconds for Picral, 30 s-2 min for color etches.
4. Rinse with cold water briefly, then ethanol. Do not leave the sample in water; cast iron corrodes quickly.
5. Dry with warm air immediately. Do not wipe.

Sample-conserving option: Photograph the as-polished view first, then over-etch slightly if needed and re-polish briefly to recover the as-polished surface. Or run two sections, one for as-polished imaging, one for etched matrix examination.

For more information on etchants, visit our Etchants collection.

Cleaning & Corrosion Protection

Cast iron corrodes quickly when wet. Even tap water left on the surface for a minute will leave a hazy oxide film that obscures graphite and contaminates the matrix view. Cleaning protocol:

1. Rinse with cold water briefly. No hot water, no soak.
2. Immediately rinse with ethanol or isopropanol.
3. Dry with a strong stream of warm air. Do not wipe.
4. Store finished samples in a desiccator; humidity will surface-stain cast iron within hours.

For cast irons that corrode aggressively during polishing (some austenitic grades and high-sulfur grey iron in particular), use water-free diamond suspensions and lubricants at the 1 μm step, and rinse with anhydrous ethanol only.

Troubleshooting

Common Issues and Solutions

• Graphite appears black at 500×: Smeared matrix metal is filling the graphite cavity (most common cast iron preparation error). Return to the 1 μm DIAMAT step on GOLDPAD; extend the time by 1-2 minutes; verify the pad is not loaded. If still black, the matrix smear was generated during grinding; re-grind on fresh alumina paper with lighter pressure.
• Graphite pullout: Excessive force during grinding or polishing, or a pad that is too soft for the abrasive. Reduce force to the lower end (5 lbs/sample); confirm GOLDPAD is used at 1 μm (not a high-nap pad). For temper-carbon malleable iron, even Don's procedure can struggle; try a rigid composite disc surface (SIRIUS or ORION) at the fine grinding step.
• Scratches remaining after 1 μm diamond: Pad loaded with debris, or 1200 grit paper step was too short. Use a fresh GOLDPAD and re-run 1 μm DIAMAT for 2-3 minutes. If scratches persist, re-grind at 1200 grit and confirm complete scratch removal before moving on.
• Relief around graphite: Over-polishing at the alumina final step, or too compliant a pad. Reduce final-polish time to 30-45 seconds; verify TRICOTE is the pad in use (not a high-nap final cloth).
• Surface staining/corrosion after polishing: Water was left on the sample. Rinse with ethanol immediately after water; dry with warm air; for corrosion-prone grades, eliminate water from the final steps.
• Poor edge retention: Use DAP or epoxy compression mount; phenolic is acceptable for routine work but rounds the edge.
• Over-etching: Reduce time or dilute the etchant. 2% Nital finishes in 5-15 seconds; 4% Picral in 10-30 seconds.
• Under-etching: Extend time, increase concentration to 3% Nital, or for hard alloyed grades try sequential Nital then Picral.
• ADI matrix details indistinct: Standard Nital cannot distinguish ausferrite from martensite from retained austenite. Switch to modified Beraha or Klemm's II color etch.

Additional Reading

• Zipperian, D.C. Metallographic Handbook, Section 11.3.2 (Cast Irons). PACE Technologies, Tucson, AZ. The house reference. The grinding, polishing, and pad recommendations in this guide come directly from this source.
• Weidmann, E. and Guesnier, A. "Metallographic Preparation of Cast Iron." Struers Application Notes. Source for the modified Beraha color etch, white-cast-iron diamond-wheel warning, and corrosion handling guidance.
• ASM Handbook, Vol. 9: Metallography and Microstructures. ASM International. Section on cast iron preparation, etchants, and graphite morphology classification.
• ISO 945-1 — Microstructure of cast irons — Part 1: Graphite classification by visual analysis (Forms I-VI).
• ISO 945-4 — Microstructure of cast irons — Part 4: Test method for evaluating nodularity in spheroidal graphite cast iron.
• ASTM A247 — Standard Test Method for Evaluating the Microstructure of Graphite in Iron Castings (graphite form, size, distribution charts).
• ASTM E2567 — Standard Test Method for Determining Nodularity And Nodule Count in Ductile Iron Using Image Analysis.
• ASTM E407 — Standard Practice for Microetching Metals and Alloys (cast iron etchant compositions).
• ASTM E3 — Standard Guide for Preparation of Metallographic Specimens.
• Petzow, G. Metallographic Etching, 2nd ed. ASM International. Comprehensive reference for Nital, Picral, Beraha, and Klemm etchant formulations.