C71500 Cupronickel 70-30

Use slow-speed diamond saw or abrasive cut-off wheel designed for non-ferrous materials. Standard cut-off wheel (1.0-1.5 mm thickness) is appropriate. Use adequate coolant flow to prevent overheating and minimize deformation. Cutting speed: 150-250 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 softness (65 HB) of C71500 Cupronickel 70-30 requires careful grinding. Use standard SiC grinding papers with adequate water lubrication. Disc speed: 200-300 RPM. Apply light to moderate pressure (20-30 N per 30 mm sample) - the soft material is prone to smearing if too much pressure is applied. Use sharp, fresh grinding papers to minimize deformation.

Grinding sequence:

• 240 grit: Remove sectioning damage (20-40 seconds). Use moderate pressure to remove heat-affected zone.
• 320 grit: Remove previous scratches (20-40 seconds). Ensure complete scratch removal.
• 400 grit: Remove previous scratches (20-40 seconds). Ensure complete scratch removal.
• 600 grit: Remove previous scratches (20-40 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 softness 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 soft synthetic pad (e.g., TEXPAN) with light to moderate pressure (20-25 N per 30 mm sample). Start with 6μm to minimize damage. Monitor constantly for smearing and reduce pressure if any deformation is observed.
• 3μm diamond: 2-4 minutes on a soft synthetic pad (e.g., TEXPAN) with light pressure (15-20 N). Continue removing scratches from previous step.
• 1μm diamond: 2-3 minutes on a soft synthetic pad with lighter pressure (12-18 N). These pads provide gentle material removal.
• 0.05μm colloidal silica: 1-2 minutes on a soft 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 soft 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.

C71500 Cupronickel 70-30 is more etch-resistant than pure copper or brass due to its 30% nickel content. The nickel in solid solution stabilizes the surface, requiring stronger etchants or longer times than standard copper alloys. Two etchants are recommended:

Copper No. 2 / FeCl₃ + HCl (Chemical Etching) - Primary choice for Cu-Ni alloys:

• Composition: 5-10g FeCl₃ + 25-50ml HCl + 100-120ml H₂O. Higher FeCl₃ and HCl concentrations are needed for this high-nickel alloy — start with 10g FeCl₃ and 50ml HCl.
• Application: Immerse for 15-45 seconds. Cu-Ni alloys require significantly longer etch times than pure copper or brass. Check under microscope and re-immerse in 10-second increments if needed.
• Reveals: Grain boundaries, annealing twins, and any second-phase particles in the α Cu-Ni solid solution.
• Rinse: Immediately with water, then ethanol. Dry with compressed air.

Ammonium Persulfate (Chemical Etching) - Alternative for lighter etching:

• Composition: 10g (NH₄)₂S₂O₈ in 90ml H₂O. Prepare fresh — solution decomposes within 1-2 days.
• Application: Immerse for 15-30 seconds — longer than for pure copper due to nickel content. May need multiple applications.
• Reveals: Grain boundaries. Less aggressive than Copper No. 2, useful when gentle etching is desired.
• Rinse: Immediately with water, then ethanol. Dry with compressed air.

Etching Strategy:

• Start with Copper No. 2 (FeCl₃ + HCl) — it is the most reliable etchant for Cu-Ni alloys
• Expect longer etch times than pure copper or brass — the 30% Ni increases corrosion resistance
• If Copper No. 2 over-etches, switch to ammonium persulfate for gentler grain boundary revelation
• Always clean and degrease before etching

Safety: FeCl₃ + HCl produces fumes — use in fume hood. Ammonium persulfate is an oxidizer. Wear gloves, safety glasses, and lab coat for all etchants.