Austempered Ductile Iron

Use abrasive cut-off wheel designed for steel (Al₂O₃ or SiC abrasive). Standard cut-off wheel (1.0-1.5 mm thickness) is appropriate. Use adequate coolant flow to prevent overheating - excessive heat can affect the microstructure. Cutting speed: 200-300 RPM for most cut-off saws. Apply steady, moderate pressure - the material allows for reasonable feed rates. Avoid forcing the cut which can cause wheel wear and sample damage. Leave adequate allowance (~2-3 mm) for grinding away the heat-affected zone from cutting.

Cold mounting with epoxy resin is preferred to avoid heat that could affect the microstructure. Use a low-shrinkage epoxy resin for best edge retention. Ensure complete cure before grinding to prevent edge rounding and maintain sample integrity.

Hot compression mounting is acceptable if the part tolerates ~150-180°C and moderate pressure (2000-3000 psi for phenolic). Use phenolic or epoxy-phenolic resins. Ensure proper cooling under pressure to minimize shrinkage.

The hardness (310 HB) of Austempered Ductile Iron requires careful grinding. Use standard SiC grinding papers with adequate water lubrication. Disc speed: 200-300 RPM. Apply light to moderate pressure (30-40 N per 30 mm sample) - the hard material may require longer grinding times. Use sharp, fresh grinding papers to minimize deformation.

Grinding sequence:

• 120 grit: Remove sectioning damage (40-90 seconds). Use moderate pressure to remove heat-affected zone.
• 240 grit: Remove previous scratches (40-90 seconds). Ensure complete scratch removal.
• 320 grit: Remove previous scratches (40-90 seconds). Ensure complete scratch removal.
• 400 grit: Remove previous scratches (40-90 seconds). Ensure complete scratch removal.
• 600 grit: Remove previous scratches (40-90 seconds). Ensure complete scratch removal.

Always rotate the specimen holder 90° between steps to ensure complete scratch removal. Use complementary rotation (platen and holder same direction, different speeds) rather than contra-rotation to minimize deformation. Adequate water lubrication is critical - avoid drying during grinding which can cause smearing.

The hardness requires careful polishing. Use diamond polishing with appropriate polishing pads for each stage. Apply light to moderate pressure throughout to prevent deformation.

Diamond polishing sequence:

• 6μm diamond: 2-4 minutes on a medium-hard synthetic pad (e.g., TEXPAN) with light to moderate pressure (30-40 N per 30 mm sample). Start with 6μm to minimize damage. The hard material may require longer polishing times.
• 1μm diamond: 2-4 minutes on a medium-hard synthetic pad (e.g., TEXPAN) with light pressure (25-35 N). Continue removing scratches from previous step.
• 0.05μm colloidal silica: 1-2 minutes on a high-napped final polishing pad (e.g., MICROPAD) with very light pressure. This removes any remaining fine scratches and prepares the surface for etching. Monitor for relief - reduce polishing time if excessive relief develops.

Use appropriate polishing lubricants. The hard material means polishing times should be sufficient but not excessive - avoid over-polishing which can cause relief and affect grain boundary revelation. Monitor the surface frequently under the microscope to check for smearing or excessive relief.

Austempered Ductile Iron (ADI) has a unique ausferrite matrix consisting of acicular (needle-like) ferrite and high-carbon retained austenite, with spheroidal graphite nodules. This microstructure is distinctly different from other cast irons — there is no pearlite, no lamellar structure. The ausferrite provides the combination of high strength and ductility that makes ADI valuable. Quantifying retained austenite content is critical for quality control. 3-5% Nital (Chemical Etching) — Primary choice for ADI:

• Composition: 3-5ml HNO₃ (concentrated), 95-97ml ethanol
• Preparation: Add nitric acid to ethanol slowly with stirring. Use 3% initially; increase to 5% if the ausferrite matrix does not etch adequately.
• Application: Immerse sample or swab for 10–30 seconds. ADI's high hardness (310 HB) and ausferrite matrix resist etching — standard 2% Nital is typically insufficient.
• Reveals: Ausferrite structure (acicular ferrite darkens, retained austenite remains lighter). Graphite nodules appear as dark spheroids. Prior austenite grain boundaries may also be revealed at higher concentrations.
• Rinse: Immediately with water, then ethanol. Dry with compressed air.
• Note: Prepare fresh when needed. Use in fume hood. Start with shorter etch times and increase — over-etching can obscure the fine acicular structure.

Klemm's II (Tint Etching) — For retained austenite identification:

• Composition: 50ml saturated Na₂S₂O₃ solution + 5g K₂S₂O₅
• Preparation: Prepare saturated sodium thiosulfate solution. Add potassium metabisulfite just before use.
• Application: Immerse for 60–120 seconds at room temperature. Warming to 40°C enhances staining intensity. Do not swab — tint film is delicate.
• Reveals: Phase-specific coloring — ferrite appears blue, martensite brown/black, retained austenite remains white/uncolored. Essential for quantifying retained austenite in ADI, which directly affects mechanical properties.
• Rinse: Gently rinse with water. Dry with warm air.
• Note: Tint etchants require a scratch-free, well-polished surface. Any remaining scratches will interfere with color development.

Etching Strategy:

• Start with 3% Nital for general ausferrite structure examination — increase to 5% if needed
• Use Klemm's II when retained austenite quantification is required — critical for ADI quality
• Do NOT use 2% Nital — it is too dilute for ADI's hard ausferrite matrix
• Spheroidal graphite should be visible as-polished before etching
• The acicular ausferrite structure is fine-scale — avoid over-etching which obscures the needle-like morphology

Safety: Nital produces toxic fumes — use fume hood. Klemm's II contains thiosulfate/metabisulfite — handle with standard PPE (gloves, safety glasses, lab coat).