Sintered 316L Stainless Steel (PM)

Sintered 316L stainless steel is a moderately hard (60-85 HRB, 75-110 HV) austenitic PM material with variable porosity depending on processing route. Conventional press-and-sinter parts have 8-15% porosity, while MIM parts have 2-5% porosity. The dual challenge is preserving porosity while managing the work hardening and smearing tendencies of austenitic stainless steel. Vacuum impregnation with epoxy is essential for conventionally sintered parts with significant porosity. MIM parts with low porosity may not require impregnation but benefit from it.

Use a standard abrasive cut-off wheel (alumina, bonded for stainless steel) or a low-speed diamond saw with continuous coolant. Cutting speed: 200-300 RPM for cut-off saws. Apply moderate feed pressure. The material is somewhat more fragile than wrought 316L due to the porosity, so moderate clamping and steady feed are important. For MIM parts with complex geometry, select section planes that capture features of interest (gates, knit lines, areas of suspected incomplete fill). Leave 2-3 mm allowance for grinding.

CRITICAL: Vacuum impregnation with low-viscosity castable epoxy is essential for conventionally sintered parts with significant porosity (>5%). Follow the same vacuum impregnation procedure as for sintered iron: apply low-viscosity epoxy, draw vacuum (25-29 inches Hg) for 2-5 minutes, release vacuum, repeat 2-3 times. For MIM parts with low porosity (<5%), vacuum impregnation is still recommended but less critical. Allow complete cure. Hot compression mounting is acceptable after vacuum impregnation for parts needing better edge retention, since the solidus (1375 degrees C) is well above mounting temperatures.

After vacuum impregnation, the austenitic matrix work hardens significantly during grinding. Use SiC papers or diamond grinding discs with moderate pressure. The work hardening tendency means excessive pressure causes smearing, while too little pressure causes glazing of the surface without material removal. Use moderate pressure (25-35 N per 30 mm sample). Disc speed: 200-300 RPM. Complementary rotation.

Grinding sequence:

• 240 grit: Remove sectioning damage (20-40 seconds). Moderate, steady pressure. Fresh, sharp paper essential.
• 320 grit: Remove previous scratches (20-40 seconds). Rotate specimen 90 degrees.
• 400 grit: Further refinement (20-30 seconds).
• 600 grit: Final grinding step (20-30 seconds). Inspect pore edges under the microscope.

Rotate specimen 90 degrees between steps. Use fresh papers at each step; worn papers worsen work hardening. Thorough ultrasonic cleaning between steps to remove debris from pore/epoxy interfaces.

The austenitic matrix work hardens during polishing. Use napless or low-nap cloths with steady, consistent pressure. Avoid intermittent or pulsing pressure which exacerbates work hardening artifacts.

Diamond polishing sequence:

• 6 micrometer diamond: 3-5 minutes on a medium-hard synthetic pad with moderate pressure (25-30 N per 30 mm sample). Use consistent, steady motion.
• 3 micrometer diamond: 2-4 minutes on a medium-hard pad (20-28 N). Monitor for pore edge quality and smearing.
• 1 micrometer diamond: 2-3 minutes on a napless pad (18-25 N).

Final polishing:

• 0.05 micrometer colloidal silica: 1-3 minutes on a soft pad. Colloidal silica provides chemical-mechanical action that is particularly beneficial for austenitic stainless steel, helping to remove the work-hardened surface layer. Vibratory polishing for 2-4 hours with colloidal silica gives the best results by removing deformation artifacts and providing excellent pore edge definition.

After polishing, pores should appear as clean, sharp-edged voids. The metal surface should be free of smearing and scratches.

Sintered 316L responds to standard austenitic stainless steel etchants. Electrolytic etching is preferred for revealing grain boundaries without attacking pore edges.

Electrolytic Oxalic Acid (Electrolytic Etching) - Primary choice for grain boundaries:

• Composition: 10 g oxalic acid (H2C2O4), 100 ml water (electrolyte)
• Application: 6 V DC, 15-30 seconds. Specimen as anode, stainless steel cathode.
• Reveals: Austenite grain boundaries, annealing twins, prior particle boundaries (PPBs). Sigma or chi phase at grain boundaries appears as heavily etched regions. Carbide precipitation at grain boundaries (sensitization) also revealed.
• Rinse: Water, then ethanol. Dry with warm air.

Electrolytic 10% NaOH (Electrolytic Etching) - For phase identification:

• Composition: 10 g NaOH, 100 ml water (electrolyte)
• Application: 3-5 V DC, 10-20 seconds.
• Reveals: Sigma phase and delta ferrite are selectively colored. Prior particle boundaries with oxide films become visible. Preferred for distinguishing between different secondary phases.

For porosity quantification: Image analysis is performed on the as-polished (unetched) surface, where pores appear as dark features. ASTM B328 provides the standard method for PM density determination.