White Cast 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 (450 HB) of White Cast 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.

White Cast Iron contains no graphite — all carbon exists as cementite (Fe₃C), forming a continuous network of hard carbide throughout the matrix. Unlike other cast irons, there is nothing to see in the as-polished condition since there are no graphite particles. Etching reveals the full microstructure: cementite network, pearlite colonies, and any free ferrite. 4% Picral (Chemical Etching) — Preferred etchant for white cast iron:

• Composition: 4g picric acid, 100ml ethanol
• Preparation: Dissolve picric acid in ethanol with gentle warming and stirring. Prepare fresh for best results.
• Application: Immerse sample or swab for 10–60 seconds.
• Reveals: Cementite network remains white/unetched while pearlite darkens — providing excellent contrast for assessing cementite morphology, continuity, and distribution. Superior to Nital for white cast iron because it does not attack ferrite grain boundaries, giving cleaner cementite definition.
• Rinse: Immediately with water, then ethanol. Dry with compressed air.
• Note: Picric acid is explosive when dry — keep moist and handle with extreme care. Use in fume hood.

2% Nital (Chemical Etching) — General purpose etchant:

• Composition: 2ml HNO₃ (concentrated), 98ml ethanol
• Preparation: Add nitric acid to ethanol slowly with stirring.
• Application: Immerse sample or swab for 5–15 seconds. Standard etchant for cast iron.
• Reveals: Pearlite structure and ferrite grain boundaries. Cementite remains unetched (white). More aggressive than Picral — attacks ferrite boundaries which can reduce cementite clarity.
• Rinse: Immediately with water, then ethanol. Dry with compressed air.
• Note: Prepare fresh when needed. Shelf life: several days. Use in fume hood.

Etching Strategy:

• Start with 4% Picral — it is the preferred etchant for white cast iron because it reveals cementite networks without the boundary attack that Nital causes
• Use 2% Nital when general matrix structure is more important than cementite definition
• The very high hardness (450 HB) means longer etch times may be needed compared to other cast irons
• No graphite is present — do not expect dark particles in the as-polished condition

Safety: All etchants require proper PPE (gloves, safety glasses, lab coat) and fume hood. Picric acid is explosive when dry — keep moist, store in water-wet condition.