C36000 Free-Cutting Brass

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 (80 HB) of C36000 Free-Cutting Brass 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.

C36000 Free-Cutting Brass is a leaded alpha brass (61.5Cu-35.5Zn-3Pb). The lead particles appear as dark globular inclusions in the as-polished condition and do not etch. Etching reveals the alpha brass grain structure and annealing twins around the lead particles. Two primary etchants are recommended:

Copper No. 1 (Chemical Etching) - Most common etchant for brass:

• Composition: 125ml HNO₃ (conc.), 125ml H₂O
• Application: Immerse for 3-10 seconds, or swab for more control. Very fast-acting — start with short times. Lead particles remain dark and unetched.
• Reveals: Grain boundaries, annealing twins, and lead particle distribution. Lead particles remain as dark globular inclusions.
• Rinse: Immediately with water, then ethanol. Dry with compressed air.
• Note: CAUTION — exothermic when mixing. Add acid to water. Produces toxic NOx fumes — use in fume hood.

Ammonium Persulfate (Chemical Etching) - Gentler alternative:

• Composition: 10g (NH₄)₂S₂O₈ in 90ml H₂O
• Application: Immerse or swab for 10-30 seconds. Prepare fresh — solution decomposes within 1-2 days.
• Reveals: Grain boundaries and twins. Less aggressive than Copper No. 1. Lead particles remain visible as dark inclusions.
• Rinse: Immediately with water, then ethanol. Dry with compressed air.

Safety: Contains lead — avoid generating dust. Copper No. 1 produces toxic NOx fumes — fume hood mandatory. Wear gloves, safety glasses, and lab coat for all etchants.