Maraging Steel C300

Maraging Steel C300 is an ultra-high-strength (54 HRC aged, ~577 HV) precipitation-hardened nickel steel with a density of 8.0 g/cm³. The microstructure is the same type as C250 (lath martensite with Ni₃(Mo,Ti) precipitates, no carbides) but with a higher precipitate volume fraction producing greater hardness. Preparation is essentially the same as C250, with allowance for the increased hardness during grinding and cutting.

Use an abrasive cut-off wheel designed for hardened steel (aluminum oxide abrasive). Standard wheel thickness (1.0-1.5 mm). Cutting speed: 200-300 RPM with generous coolant flow. The higher hardness (~54 HRC) compared to C250 means slightly slower cutting rates. Do not force the cut; allow the wheel to work at its own rate. Excessive heat will alter the precipitation state. Leave 2-3 mm allowance for grinding away the heat-affected zone.

Hot compression mounting with phenolic or epoxy-phenolic resin is acceptable. The aging temperature (480°C) is well above mounting temperatures (150-180°C), so no microstructural change occurs during mounting. Cold mounting with castable epoxy is equally suitable. Standard mounting practices apply.

The higher hardness (54 HRC, ~577 HV) compared to C250 means slightly more abrasive wear during grinding, but the technique is the same. Use SiC grinding papers with abundant water lubrication. Disc speed: 250-300 RPM. Apply moderate pressure (25-35 N per 30 mm sample).

Grinding sequence:

• 120 grit: Remove sectioning damage and heat-affected zone (30-60 seconds). Moderate pressure.
• 240 grit: Remove previous scratches (20-40 seconds).
• 320 grit: Refinement (20-40 seconds).
• 400 grit: Further refinement (20-40 seconds).
• 600 grit: Final grinding (20-40 seconds).

Rotate specimen 90° between steps. Use complementary rotation. The single-phase martensite grinds uniformly without differential removal.

Same approach as C250. The uniform martensite structure polishes well without relief.

Diamond polishing sequence:

• 6μm diamond: 3-5 minutes on a medium-hard synthetic pad with moderate pressure (25-35 N per 30 mm sample).
• 1μm diamond: 2-4 minutes on a synthetic pad (20-30 N).

Final polishing:

• 0.05μm colloidal silica: 1-3 minutes on a soft final polishing pad (15-20 N). Colloidal silica provides effective chemical-mechanical action for revealing lath martensite in maraging steels.

Same etchants as C250. The microstructure is the same type but with a higher precipitate volume fraction.

Modified Fry's Reagent (Chemical Etching) - Primary choice:

• Composition: 5 g CuCl₂, 40 ml HCl, 30 ml H₂O, 25 ml ethanol
• Application: Immerse for 10-30 seconds or swab.
• Reveals: Lath martensite, prior austenite grain boundaries, reverted austenite.
• Rinse: Water, then ethanol. Dry with warm air.

2% Nital (Chemical Etching) - General purpose:

• Composition: 2 ml HNO₃, 98 ml ethanol
• Application: Immerse for 5-15 seconds.
• Reveals: General martensitic structure and prior austenite grain boundaries.

Note: As with C250, the Ni₃(Mo,Ti) precipitates are nanoscale and require TEM for direct observation.