321 Stainless Steel

321 Stainless Steel is a soft (123 HB) titanium-stabilized austenitic stainless steel (18% Cr, 10% Ni, 0.4% Ti) with an austenitic microstructure containing TiC particles. The titanium addition prevents sensitization (chromium carbide precipitation) during welding and high-temperature service, making it ideal for welded structures and high-temperature applications. The softness requires careful preparation to avoid smearing and deformation. Use low to moderate pressure throughout grinding and polishing to prevent deformation of the soft austenitic matrix. The TiC particles are stable and will be visible in the microstructure after proper etching. Standard stainless steel preparation procedures work well. Monitor for smearing during grinding and polishing - reduce pressure if any deformation is observed. Commonly used in exhaust systems, heat exchangers, pressure vessels, and welded structures requiring high-temperature stability.

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 - while titanium stabilization prevents sensitization, excessive heat can still affect the microstructure. Cutting speed: 200-300 RPM for most cut-off saws. Apply steady, moderate pressure - the soft 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. For welded structures, ensure the section includes both base metal and weld metal for complete analysis.

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. The soft material requires careful handling during mounting to avoid deformation. Ensure proper cooling under pressure to minimize shrinkage.

For high-temperature applications and welded structures, ensure the mounting material is compatible with the intended use environment. Edge retention is important for examining TiC particles and weld interfaces.

The softness (123 HB) of 321 Stainless Steel requires careful grinding to avoid smearing and deformation. 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:

• 120 grit: Remove sectioning damage (30-60 seconds). Use moderate pressure to remove heat-affected zone.
• 240 grit: Remove previous scratches (30-60 seconds). Monitor for smearing.
• 320 grit: Further refinement (30-60 seconds). Ensure complete scratch removal.
• 400 grit: Refinement (30-60 seconds). Continue scratch removal.
• 600 grit: Final grinding step (30-60 seconds). Prepare surface for polishing.

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. For welded samples, ensure complete scratch removal to reveal weld interfaces and TiC particles clearly.

The softness requires careful polishing to avoid smearing and excessive relief. 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 medium-hard synthetic pad (e.g., TEXPAN) with light to moderate pressure (20-25 N per 30 mm sample). Start with 6μm rather than 9μm to minimize damage to the soft material. Monitor for smearing and reduce pressure if needed.
• 3μm diamond: 2-4 minutes on a medium-hard synthetic pad (e.g., TEXPAN) with light pressure. Continue removing scratches from previous step.
• 1μm diamond: 2-3 minutes on a soft synthetic pad with lighter pressure (15-20 N). These pads provide gentle material removal suitable for soft stainless steel.

Final polishing:

• 0.05μm colloidal silica: 1-2 minutes on a soft final polishing pad (e.g., MICROPAD) with very light pressure. Soft pads are recommended for colloidal silica on soft materials and produce a mirror finish. This removes any remaining fine scratches and prepares the surface for etching. Monitor for relief - reduce polishing time if excessive relief develops. TiC particles may show slight relief after polishing - this is normal and will be visible after etching.

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. For welded samples, ensure proper polishing to reveal weld interfaces and TiC particle distribution.

321 Stainless Steel responds well to standard austenitic stainless steel etchants. The titanium stabilization and TiC particles are key microstructural features that should be revealed. The high chromium and nickel content can make the material more difficult to etch than carbon steels, but several effective etchants are available. Two primary etchants are recommended:

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. Provides good grain boundary contrast and reveals TiC particles.
• Reveals: Grain boundaries, grain structure, twin boundaries, and TiC particles clearly. Excellent for general microstructure examination. Good contrast for austenitic structure. TiC particles appear as small, dark particles in the austenitic matrix.
• 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. The glycerol moderates the reaction - check progress frequently.

Aqua Regia (Chemical Etching) - For aggressive etching and revealing TiC particles:

• Composition: 3 parts HCl, 1 part HNO₃ (by volume)
• Preparation: Mix acids just before use. Prepare fresh - solution is unstable and loses effectiveness quickly.
• Application: Immerse sample or swab for 5-15 seconds. Very aggressive etchant - use with caution. Good for revealing TiC particles and grain structure. Effective for difficult-to-etch samples.
• Reveals: Grain boundaries, structure, and TiC particles. Very aggressive - can over-etch quickly. TiC particles are stable and will be clearly visible. Use for samples that resist other etchants or when TiC particle distribution is critical.
• Rinse: Immediately with water, then ethanol. Dry with compressed air.
• Note: Prepare fresh immediately before use. Shelf life: minutes. Use in fume hood with excellent ventilation. Very corrosive - handle with extreme care.

Etching Strategy:

• Start with Glyceregia for general microstructure examination - it provides good contrast and reveals grain boundaries and TiC particles clearly
• Use Aqua Regia when TiC particle distribution is critical or when Glyceregia is insufficient
• Etching time may vary with heat treatment condition - solution-annealed material may etch differently than material exposed to high-temperature service
• For welded structures, ensure proper etching to reveal weld interfaces, heat-affected zones, and TiC particle distribution
• TiC particles are stable and will be visible as small, dark particles in the austenitic matrix - they do not dissolve during normal heat treatment
• Always clean and degrease before etching
• Use short initial etch times (a few seconds), check under the microscope, repeat if needed
• Check etching progress frequently - over-etching can obscure fine details and TiC particles
• For high-temperature applications, ensure proper surface preparation to reveal any microstructural changes from service exposure
• The titanium stabilization prevents sensitization, so chromium carbide precipitation should not be observed in properly heat-treated material

Safety: All etchants are highly corrosive and produce toxic fumes – full PPE (gloves, safety glasses, lab coat), proper fume hood, and appropriate safety measures are mandatory. Aqua Regia is particularly dangerous - use with extreme caution.