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)
• Ductile Cast Iron: Spheroidal (nodular) graphite in pearlite or ferrite matrix (217 HB)
• Malleable Cast Iron: Temper carbon nodules in ferrite or pearlite matrix (163 HB)
• Austempered Ductile Iron (ADI): Spheroidal graphite in bainitic matrix (310 HB, very hard)
• Compacted Graphite Iron (CGI): Vermicular (compacted) graphite in pearlite matrix (230 HB)

Each type requires similar preparation techniques, but the graphite morphology affects how easily it can be preserved. Spheroidal graphite (ductile iron) is generally easier to preserve than flake graphite (gray iron), which can be more prone to pullout.

Sectioning

When sectioning cast iron samples, use standard cutting techniques similar to carbon steel. Cast iron is generally softer than hardened steel, so standard abrasive cutoff wheels work well. The key is to minimize damage that could affect graphite preservation later in the process.

MAX-D abrasive cut-off blades designed for general ferrous metals including cast iron. For very hard cast irons like ADI, MAX-VHS series blades may be more appropriate.

• Use MAX-D series blades for general cast iron grades, or MAX-VHS series for very hard cast irons like ADI
• Use a standard abrasive cut-off wheel (0.5-1.0 mm thickness)
• Apply steady, moderate pressure
• Use adequate coolant to prevent overheating
• Allow the wheel to do the cutting - avoid forcing

Note: For very hard cast irons like Austempered Ductile Iron (310 HB), use slower cutting speeds similar to hardened steel to prevent excessive heat buildup.

Mounting

Mounting provides edge retention and easier handling. For cast iron, both compression and cold mounting work well. The choice depends on whether edge retention is critical and the hardness of the specific cast iron grade.

Compression Mounting

Compression mounting with phenolic or epoxy resins is suitable for most cast irons. Phenolic provides better edge retention, which can be important for preserving graphite near edges.

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

Cold Mounting

Cold mounting with epoxy resin is also suitable and avoids any thermal effects. This is particularly useful if you're concerned about any potential effects of heat on the graphite structure.

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)

Grinding

Grinding removes sectioning damage and prepares the surface for polishing. For cast iron, use gentle grinding techniques to avoid damaging or removing graphite. The graphite is soft and can be easily pulled out or smeared during grinding.

Silicon carbide (SiC) grinding papers in various grit sizes (120, 240, 400, 600) for progressive grinding. Use lighter pressure than for steel to preserve graphite.

Grinding Sequence

1. 120 grit: Remove sectioning damage (30-60 seconds per step, lighter pressure)
2. 240 grit: Remove previous scratches (30-60 seconds, lighter pressure)
3. 400 grit: Further refinement (30-60 seconds, lighter pressure)
4. 600 grit: Final grinding step (30-60 seconds, lighter pressure)

Critical Guidelines:

• Use lighter pressure than for steel - graphite is soft and can be removed
• Rotate the sample 90° between each grit to ensure complete removal of previous scratches
• Use water as a lubricant and maintain consistent, light pressure
• Monitor the surface - if graphite starts to pull out, reduce pressure further
• For very hard cast irons like ADI, you can use slightly more pressure, but still be gentle

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

1. 6 μm diamond: 2-4 minutes on a medium-hard cloth (e.g., Texmet), lighter pressure
2. 3 μm diamond: 2-4 minutes on a medium-hard cloth, lighter pressure
3. 1 μm diamond: 2-3 minutes on a soft cloth, lighter pressure

Final Polishing

1. 0.05 μm colloidal silica: 1-2 minutes on a soft cloth, very light pressure
2. Rinse thoroughly with water and dry with compressed air

Critical Guidelines:

• Use lighter pressure throughout polishing - graphite is soft and can be removed
• Use softer cloths than you would for steel to reduce the risk of graphite pullout
• Monitor the surface - graphite should be clearly visible after polishing
• Avoid over-polishing - extended times can remove graphite
• For very hard cast irons like ADI, you can use slightly more pressure, but still be gentle

After polishing, examine the sample under the microscope. The graphite should be clearly visible as dark areas (flakes, nodules, or compacted shapes depending on the cast iron type). If graphite is missing or appears damaged, you may need to reduce pressure or use softer cloths.

Etching

Etching reveals the matrix structure (ferrite, pearlite, bainite) but does not reveal graphite - graphite is visible in the polished state. The same etchants used for carbon steel work well for cast iron matrices. Nital (nitric acid in ethanol) is the most common etchant.

Cast iron microstructure after proper etching. The matrix structure (ferrite/pearlite) is revealed by etching, while graphite is visible from polishing.

Common Etchants for Cast Iron

• 2% Nital: General purpose for most cast irons. Reveals ferrite grain boundaries and pearlite structure (2% HNO₃ in ethanol)
• 4% Picral: Excellent for revealing pearlite and cementite without attacking ferrite boundaries. Particularly useful for pearlitic cast irons (2-4g picric acid per 100ml ethanol)
• Higher Nital concentrations: For harder cast irons like ADI, 3-5% Nital may be needed

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. Ensure sample is clean and dry after polishing
2. Examine polished surface first - graphite should be visible
3. Apply etchant with cotton swab or immerse sample
4. Etch for 10-30 seconds (time varies by cast iron type and hardness)
5. Immediately rinse with water, then alcohol
6. Dry with compressed air

Important Notes:

• Graphite is not revealed by etching - it should already be visible after polishing
• Etching reveals the matrix structure (ferrite, pearlite, bainite)
• Start with shorter etching times (10-15 seconds) and increase if needed
• For softer cast irons like Gray Cast Iron and Malleable Cast Iron, use 2% Nital
• For harder cast irons like Austempered Ductile Iron, use 3-5% Nital
• Picral is excellent for revealing pearlite structure in pearlitic cast irons

Troubleshooting

Common Issues and Solutions

• Graphite pullout or missing graphite: Too much pressure during grinding or polishing. Reduce pressure, use softer cloths, and monitor the surface carefully. Graphite is soft and easily removed.
• Graphite appears smeared: Over-polishing or too aggressive polishing. Reduce polishing time and pressure, use softer cloths.
• Scratches remaining: Insufficient grinding/polishing time or skipped grits. Ensure complete scratch removal at each step, but use lighter pressure.
• Graphite not visible after polishing: May have been removed during preparation. Start over with lighter pressure throughout the process.
• Relief around graphite: Over-polishing or too soft a cloth. Reduce polishing time or use slightly harder cloth, but still maintain light pressure.
• Contamination: Clean between steps, use fresh abrasives, and ensure proper sample cleaning.
• Poor edge retention: Consider using phenolic mounting material or different mounting technique.
• Over-etching: Reduce etching time or use lower Nital concentration. Start with shorter times (10-15 seconds).
• Under-etching: Increase etching time or use higher Nital concentration. Ensure sample is clean before etching.
• Matrix structure not revealed: May need longer etching time or different etchant. Try Picral for pearlitic structures.