D2 Tool Steel

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 (255 HB) of D2 Tool Steel 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:

• 9μm 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 9μm to minimize damage. The hard material may require longer polishing times.
• 3μm 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.
• 1μm diamond: 2-3 minutes on a medium-hard synthetic pad with lighter pressure (25-35 N). These pads provide gentle material removal.
• 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.

D2 Tool Steel has 11-13% chromium content that creates a passive surface resistant to many common etchants. Vilella's Reagent is the preferred etchant for characterizing the Cr₇C₃ primary carbides and carbide banding that define D2 quality. Nital is less effective due to the high chromium content but can provide general matrix contrast. Vilella's Reagent (Chemical Etching) - Preferred etchant for high-chromium tool steels:

• Composition: 1g picric acid, 5ml HCl (concentrated), 100ml ethanol
• Preparation: Dissolve picric acid in ethanol first, then add HCl. Prepare fresh — solution degrades within hours as HCl reacts with picric acid.
• Application: Immerse polished sample for 10-60 seconds. Swab gently for more uniform etching. Check under microscope and re-etch if needed — D2's high Cr content requires longer etch times than lower-alloy steels.
• Reveals: Cr₇C₃ primary carbides, carbide banding and segregation, prior austenite grain boundaries, tempered martensite structure, and retained austenite.
• Rinse: Immediately with ethanol, then water. Dry with compressed air.
• Note: Picric acid is shock-sensitive when dry — always keep moistened. Prepare fresh for each session. Use in fume hood.

2% Nital (Chemical Etching) - General-purpose etchant, limited effectiveness on high-Cr steels:

• Composition: 2ml HNO₃ (concentrated), 98ml ethanol
• Preparation: Add nitric acid to ethanol slowly with stirring. Prepare fresh for best results. Solution is stable for several days if stored properly.
• Application: Immerse sample for 10-30 seconds. D2's chromium content resists Nital — expect longer etch times than plain carbon steels. 5% Nital may be more effective.
• Reveals: General matrix contrast between martensite and retained austenite. Less effective than Vilella's for carbide characterization in high-Cr steels.
• Rinse: Immediately with water, then ethanol. Dry with compressed air or warm air to avoid staining.
• Note: Prepare fresh when needed. Shelf life: several days. Use in fume hood.

Etching Strategy:

• Start with Vilella's Reagent for carbide characterization — this is the primary etchant for D2
• Use Nital only for quick matrix overview; increase to 5% if 2% gives insufficient contrast
• Carbide banding assessment requires careful etching — slight under-etching preserves carbide morphology better than over-etching
• For retained austenite assessment, use light Vilella's etch (10-15 seconds) to preserve contrast between martensite and austenite

Safety: Picric acid is shock-sensitive when dry — always store moistened and handle with care. All etchants require proper PPE (gloves, safety glasses, lab coat) and fume hood use.