CoCrMo Wrought (ASTM F1537)

CoCrMo wrought (ASTM F1537) is a hard, ductile cobalt-chromium alloy (35-45 HRC, ~340 HV) with a density of 8.3 g/cm³. Slightly harder than the cast F75 variant due to wrought processing. The microstructure features recrystallized equiaxed FCC grains with prominent annealing twin boundaries. Fewer carbides than F75 due to the low carbon specification, but those present are finer and more uniformly distributed. The alloy work-hardens aggressively, so sharp abrasives and controlled pressure are essential. Preparation approach is very similar to F75.

Use an abrasive cut-off wheel designed for hard non-ferrous or cobalt alloys with continuous coolant flow. The wrought alloy is tougher than the cast variant, so expect slightly slower cutting rates. For standard cut-off: 2500-3500 RPM with moderate feed pressure. For precision sectioning, a low-speed diamond saw at 200-400 RPM is preferred. Leave 2-3 mm allowance for grinding away the sectioning deformation zone.

Compression mounting with standard phenolic or epoxy compounds is appropriate. The high melting range (1350-1450°C) is unaffected by mounting temperatures. For implant retrieval specimens (hip stems, knee tibial trays), edge-retaining mounting is recommended if examining the bearing or articulating surface. Cold mounting with castable epoxy works well for porous-coated surfaces; vacuum impregnation fills the porous coating for better retention during grinding.

Very similar to F75. The wrought alloy is slightly harder and tougher, so expect marginally slower material removal. Diamond grinding discs preferred. Disc speed: 250-300 RPM. Apply 25-30 N per sample.

Grinding sequence:

• 240 grit: Remove sectioning damage and establish a flat plane (30-60 seconds). Firm, steady pressure.
• 320 grit: Remove previous scratch pattern (30-45 seconds).
• 400 grit: Continue refinement (30-45 seconds).
• 600 grit: Fine grinding (20-40 seconds).
• 800 grit: Final grinding step (20-40 seconds).

Rotate specimen 90° between steps. Complementary rotation. Abundant water lubrication throughout.

Use napless or low-nap pads. The more homogeneous microstructure (fewer carbides) makes relief less of a concern than with F75, but careful technique is still required.

Diamond polishing sequence:

• 9μm diamond: 3-5 minutes on a napless composite pad at 20-25 N per sample.
• 3μm diamond: 3-5 minutes on a napless synthetic pad at 20-25 N.
• 1μm diamond: 2-4 minutes on a napless pad at 15-20 N.

Final polishing:

• 0.05μm colloidal silica: 2-4 minutes on a porous chemical-mechanical pad at 10-15 N. The slight chemical attack of colloidal silica is particularly effective for revealing the twin boundaries characteristic of this alloy. Vibratory polishing (4-8 hours) is recommended for grain size analysis or EBSD preparation.

Same etchants as F75 work well, but the resulting microstructure appears very different due to the wrought processing and low carbon content.

Electrolytic 10% HCl (Electrolytic Etching) - Primary choice:

• Composition: 10 ml HCl (concentrated), 90 ml water
• Application: Electrolytic at 3-6 V DC for 5-15 seconds. Stainless steel cathode.
• Reveals: Equiaxed recrystallized grains with prominent annealing twin boundaries. Sparse, fine carbides visible along grain boundaries. No dendritic structure (unlike F75).
• Rinse: Water, then ethanol. Dry with warm air.

Mixed Acid HCl-H2O2 (Chemical Etching) - Alternative:

• Composition: 15 ml HCl, 10 ml H2O2 (30%), 100 ml water
• Application: Immerse for 10-30 seconds or swab.
• Reveals: Grain boundaries and twin boundaries. General microstructure.

Glyceregia (Chemical Etching) - For grain boundaries:

• Composition: 15 ml HCl, 10 ml glycerol, 5 ml HNO₃. Mix fresh.
• Application: Immerse for 15-60 seconds.
• Reveals: Grain boundaries and twins. Preferred for ASTM grain size measurement per ASTM E112.

Safety: Use fume hood. Standard PPE with acid-resistant gloves. DC power supply for electrolytic etching.