C93200 Leaded Tin Bronze

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 C93200 Leaded Tin Bronze 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.

C93200 Leaded Tin Bronze (Cu-7Sn-7Pb-3Zn) has a complex cast microstructure consisting of alpha dendrites, delta phase (Cu₃₁Sn₈) eutectoid, and dispersed lead globules. Lead is insoluble in copper and appears as discrete dark particles — these are visible in the as-polished condition before any etching is applied. Copper No. 2 (Chemical Etching) — Preferred etchant for leaded tin bronzes:

• Composition: 10g FeCl₃ + 50ml HCl (conc.) + 200ml H₂O
• Application: Immerse or swab for 5–15 seconds at room temperature.
• Reveals: Alpha grain boundaries, coring, and delta phase eutectoid. Lead particles remain unaffected by etching and appear as dark voids or retained globules against the etched copper matrix.
• Rinse: Immediately with water, then ethanol. Dry with compressed air.

Dichromate Etchant (Chemical Etching) — For delta phase characterization:

• Composition: 2g K₂Cr₂O₇ + 8ml H₂SO₄ (conc.) + 4ml saturated NaCl solution + 100ml H₂O
• Application: Immerse for 5–20 seconds at room temperature.
• Reveals: Excellent contrast for delta phase (Cu₃₁Sn₈) eutectoid against the alpha matrix. Particularly useful for quantifying delta phase content, which affects the alloy's brittleness and machinability.
• Rinse: Immediately with water, then ethanol. Dry with compressed air.
• Note: Contains hexavalent chromium — use in fume hood with appropriate PPE. Dispose as hazardous waste.

Etching Strategy:

• Examine as-polished first — lead particles and porosity are visible without etching
• Use Copper No. 2 for general microstructure assessment including coring and grain boundaries
• Use Dichromate Etchant for precise delta phase quantification
• Lead particles may pull out during polishing, leaving voids — do not confuse with porosity; check as-polished condition carefully

Safety: FeCl₃/HCl generates fumes — use fume hood. Dichromate contains Cr⁶⁺ — handle as hazardous. Wear gloves, safety glasses, and lab coat for all etchants.