310 Stainless Steel

Use abrasive cut-off wheel designed for stainless 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 cause sensitization (chromium carbide precipitation) in the heat-affected zone. 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. For critical applications, ensure the mounting material is compatible with the intended use environment and provides adequate edge retention for corrosion analysis.

The medium hardness (217 HB) of 310 Stainless Steel requires careful grinding. Use standard SiC grinding papers with adequate water lubrication. Disc speed: 200-300 RPM. Apply light to moderate pressure (25-35 N per 30 mm sample) - avoid excessive force that could cause work hardening. Use sharp, fresh grinding papers to minimize deformation. The austenitic structure can work-harden, so use sharp abrasives and avoid excessive pressure.

Grinding sequence:

• 120 grit: Remove sectioning damage (30-60 seconds). Use moderate pressure to remove heat-affected zone.
• 240 grit: Remove previous scratches (30-60 seconds). Ensure complete scratch removal.
• 320 grit: Remove previous scratches (30-60 seconds). Ensure complete scratch removal.
• 400 grit: Remove previous scratches (30-60 seconds). Ensure complete scratch removal.
• 600 grit: Remove previous scratches (30-60 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 medium 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 (25-35 N per 30 mm sample). Start with 9μm to minimize damage. ensure complete scratch removal at each step.
• 3μm diamond: 2-4 minutes on a medium-hard synthetic pad (e.g., TEXPAN) with light pressure (20-30 N). Continue removing scratches from previous step.
• 1μm diamond: 2-3 minutes on a medium-hard synthetic pad with lighter pressure (20-30 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 medium 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.

310 Stainless Steel responds well to standard etchants for stainless steel materials. The austenite will reveal clearly with appropriate etchants. Glyceregia (Chemical Etching) - Primary choice for general microstructure:

• Composition: 10ml glycerol, 15ml HCl, 5ml HNO₃
• Preparation: Add acids to glycerol slowly with stirring. Prepare fresh for best results. The glycerol moderates the reaction rate.
• Application: Immerse sample or swab for 10-30 seconds. Classic general-purpose micro-etchant for austenitic stainless steels and nickel alloys.
• Reveals: Grain boundaries, grain structure, and twin boundaries clearly. Excellent for general microstructure examination.
• Rinse: Immediately with water, then ethanol. Dry with compressed air or warm air to avoid staining.
• Note: Prepare fresh when needed. Shelf life: several hours. Use in fume hood.

Aqua Regia (Chemical Etching) - For more aggressive etching when Glyceregia is insufficient:

• Composition: 3 parts HCl + 1 part HNO₃ (by volume), e.g. 75ml HCl + 25ml HNO₃
• Preparation: Add HNO₃ to HCl slowly (CAUTION: exothermic, produces toxic fumes). Prepare fresh immediately before use — solution is unstable.
• Application: Immerse sample or swab for 5-15 seconds. Very aggressive — use shorter times than Glyceregia and check frequently. 310's high Cr/Ni (25%/20%) may require slightly longer times than 304.
• Reveals: Grain boundaries, twin boundaries, and carbide precipitation from high-temperature service.
• Rinse: Immediately with water, then ethanol. Dry with compressed air.
• Note: Prepare fresh immediately before use — shelf life is minutes. Use in fume hood. Extremely corrosive.

Etching Strategy:

• Start with Glyceregia for general microstructure examination — better control than Aqua Regia
• Use Aqua Regia only if Glyceregia gives insufficient contrast or for faster results
• 310's high Cr/Ni content (25%/20%) creates a strong passive layer — expect slightly longer etch times than 304 (18%/8%)
• For high-temperature service specimens, look for sigma phase and Cr₂₃C₆ carbide precipitation at grain boundaries

Safety: Both etchants require proper PPE (gloves, safety glasses, lab coat) and fume hood. Glyceregia and Aqua Regia produce toxic fumes — prepare and use in well-ventilated fume hood only.