AM 316L Stainless Steel (SLM)

AM 316L is a medium-hardness (85-95 HRB, ~210 HV as-built) austenitic stainless steel produced by selective laser melting. The as-built yield strength is significantly higher than wrought 316L due to rapid-solidification dislocation cell structures. The microstructure features melt pool boundaries (arc-shaped contrast lines), cellular/dendritic solidification substructure within melt pools, and possible delta-ferrite stringers. The material is austenitic and work-hardens during grinding, requiring fresh sharp abrasives. Preparation is similar to wrought 316L but the AM-specific features (melt pool boundaries, cellular substructure) require careful etching optimization. Always examine the as-polished surface for porosity characterization before etching. The build orientation relative to the cross-section plane must be documented. Extra grinding allowance may be needed for as-built surfaces, which have significant surface roughness (Ra 5-15 micrometers typical for SLM).

Use an abrasive cut-off saw with an alumina (Al2O3) blade designed for stainless steel. Apply generous coolant flow. Cutting speed: 200-300 RPM with moderate feed rate. Do not dwell or pause, as 316L work-hardens and a hardened layer will slow subsequent cutting. For AM build characterization, section in multiple orientations: XY plane (perpendicular to build) reveals melt pool cross-sections, scan track overlap, and scan strategy pattern; XZ or ZX planes (parallel to build) reveal columnar grain structure, melt pool depth, and layer banding. Leave 2-3 mm allowance for grinding. For as-built surfaces, leave extra allowance (3-5 mm) due to significant surface roughness.

Both cold and hot compression mounting are acceptable. The high melting point (1400C) and austenitic stability mean compression mounting at 150-180C will not alter the microstructure. Hot compression mounting with phenolic or epoxy-phenolic resin provides good edge retention. Cold mounting with low-shrinkage epoxy is preferred for examining as-built surface roughness, near-surface porosity, or surface-connected defects. For porosity analysis (density measurements, pore characterization), vacuum impregnation with fluorescent epoxy is recommended. Edge-retaining mounts are essential for near-surface examination.

AM 316L work-hardens during grinding (similar to wrought 316L). Use fresh, sharp SiC papers or rigid diamond grinding discs. Disc speed: 250-300 RPM. Apply moderate pressure (25-35 N per 30 mm sample). Avoid excessive pressure which promotes work hardening and subsurface deformation.

Grinding sequence:

• 120 grit SiC: Remove sectioning damage (30-60 seconds). Moderate, consistent pressure.
• 240 grit SiC: Remove previous scratches (30-45 seconds). Fresh paper essential.
• 320 grit SiC: Refinement (20-40 seconds). Rotate specimen 90 degrees.
• 400 grit SiC: Continue refinement (20-40 seconds).
• 600 grit SiC: Final grinding step (20-40 seconds). Ensure uniform scratch pattern.

Rotate specimen 90 degrees between steps. Thorough cleaning between steps. Water lubrication must be continuous. For as-built surfaces with significant roughness, start at 80 or 120 grit with extended grinding time to establish a flat plane through the rough surface layer.

Use diamond suspensions on appropriate pads. The work-hardening tendency requires consistent technique.

Diamond polishing sequence:

• 9 micrometer diamond: 3-5 minutes on a napless composite pad with moderate pressure (20-30 N per 30 mm sample).
• 3 micrometer diamond: 3-5 minutes on a napless synthetic pad with moderate pressure (15-25 N).
• 1 micrometer diamond: 2-3 minutes on a short-nap pad with light pressure (12-20 N).

Final polishing:

• 0.05 micrometer colloidal silica: 2-4 minutes on a soft chemical-mechanical polishing pad with light pressure. Colloidal silica provides excellent chemical-mechanical action on austenitic stainless steel, removing the work-hardened layer and producing a deformation-free surface. This is critical for subsequent EBSD analysis of solidification texture. Vibratory polishing with colloidal silica for 4-8 hours is recommended for the best surface quality, especially if EBSD or dislocation cell imaging (SEM) is planned.

For porosity analysis, stop after 1 micrometer diamond and image the as-polished surface before proceeding to final polishing and etching.

AM 316L responds to standard austenitic stainless steel etchants, but etching conditions must be optimized for the AM-specific features (melt pool boundaries, cellular substructure, delta-ferrite). Always examine as-polished surface first for porosity.

Electrolytic Oxalic Acid (Electrolytic Etching) - Primary choice for AM features:

• Composition: 10 g oxalic acid, 100 ml H2O
• Application: Electrolytic at 6 V DC for 15-30 seconds. Stainless steel cathode.
• Reveals: Melt pool boundaries with excellent contrast (the primary AM feature at optical scale). Also reveals cellular/dendritic solidification substructure within melt pools, delta-ferrite stringers, grain boundaries, and any sensitization. The melt pool overlap zones appear as distinct etching contrast regions.
• Notes: This is the preferred etchant for AM 316L because it selectively attacks the microsegregation at melt pool boundaries and cell walls, providing the best contrast for AM-specific features.

Glyceregia (Chemical Etching) - For general microstructure:

• Composition: 15 ml HCl, 10 ml glycerol, 5 ml HNO3 (mix fresh immediately before use)
• Application: Immerse for 30-90 seconds. The etchant must be freshly prepared; it degrades rapidly.
• Reveals: General austenite grain structure, grain boundaries, and melt pool boundaries. Less selective for AM features than electrolytic oxalic acid but useful for overall microstructural survey.
• Safety: Glyceregia is unstable and can become explosive if stored. Prepare fresh, use immediately, and dispose of excess promptly.

10% Ferric Chloride (Chemical Etching) - For delta-ferrite:

• Composition: 10 g FeCl3, 30 ml HCl, 120 ml H2O
• Application: Immerse for 10-30 seconds.
• Reveals: Delta-ferrite stringers along melt pool boundaries with good contrast. Useful for quantifying delta-ferrite content in AM builds, which can be higher than wrought 316L.

AM-specific etching strategy: For melt pool boundary mapping, electrolytic oxalic acid at 6V for 15-30 seconds gives the best results. For cellular substructure within melt pools, a shorter electrolytic etch (10-15 seconds) at slightly lower voltage (3-4V) preserves the fine details. For overall grain structure comparison (as-built vs solution-annealed), use Glyceregia. Sequential etching can reveal multiple features.

Safety: Use fume hood. Standard PPE. Glyceregia must be freshly mixed and never stored.