904L Super Austenitic 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 (160 HB) of 904L Super Austenitic 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.

904L Super Austenitic 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.

Electrolytic 10% Oxalic Acid (Electrolytic Etching) - Preferred for high-alloy austenitic grades:

• Composition: 10g oxalic acid dihydrate (H₂C₂O₄·2H₂O) in 100ml distilled water
• Preparation: Dissolve oxalic acid in water. Solution is stable for several months.
• Application: Use as electrolyte with stainless steel cathode. Sample is anode. Apply 6V DC for 30-90 seconds. 904L's high Mo (4.5%) and Ni (25%) content may require longer times or slightly higher voltage than standard austenitic grades.
• Reveals: Grain boundaries, sensitization (Cr-depleted zones), carbide precipitation, and intermetallic phases. Electrolytic method provides better control than chemical etching for highly alloyed grades.
• Rinse: Immediately with water after etching. Dry with compressed air.
• Note: Ensure good electrical contact. If no etching occurs, check connections and increase voltage slightly. Electrolytic etching is often preferred over chemical for super austenitic grades due to their extreme corrosion resistance.

Etching Strategy:

• Start with Glyceregia for general microstructure examination — may need 20-45 seconds due to 904L's high alloy content
• Use electrolytic 10% oxalic acid if Glyceregia gives insufficient contrast — often more effective for super austenitic grades
• 904L's extreme corrosion resistance (6% Mo, 25% Ni) makes it one of the most difficult austenitic grades to etch chemically
• For sensitization testing (ASTM A262), electrolytic oxalic acid is the standard method

Safety: Glyceregia requires fume hood (toxic fumes). Electrolytic oxalic acid is toxic if ingested — wear gloves and avoid skin contact. Both require proper PPE.