Maraging Steel C250

Maraging Steel C250 is a high-strength (50 HRC aged, ~513 HV), precipitation-hardened nickel steel with a density of 8.0 g/cm³. The microstructure consists of lath martensite with nanoscale Ni₃(Mo,Ti) intermetallic precipitates. Unlike conventional hardened steels, maraging steels contain virtually no carbon and no carbides. The lath martensite is relatively uniform in hardness, making grinding and polishing more straightforward than multi-phase steels, but the high hardness requires diamond abrasives for efficient preparation.

Use an abrasive cut-off wheel designed for hardened steel (aluminum oxide abrasive). Standard wheel thickness (1.0-1.5 mm) is appropriate. Cutting speed: 200-300 RPM with generous coolant flow. The high hardness (~50 HRC) resists cutting, so allow the wheel to do the work without forcing. Excessive pressure or inadequate coolant will generate a heat-affected zone that alters the precipitation state. Leave 2-3 mm allowance for grinding away the heat-affected zone from cutting.

Hot compression mounting with phenolic or epoxy-phenolic resin is acceptable. The material is thermally stable well above mounting temperatures (150-180°C). Cold mounting with castable epoxy is also suitable. Choose based on throughput and edge retention requirements. For aged specimens where the aging temperature (480°C) is well above mounting temperature, no microstructural change will occur during mounting. Standard mounting practices apply.

The high hardness (50 HRC, ~513 HV) in the aged condition requires proper grinding technique but is straightforward for this class of steel. 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 uniform hardness of the single-phase martensite means differential grinding is not a concern.

Standard polishing procedures for hardened steels apply. The uniform martensite structure polishes well without relief issues.

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 slight chemical-mechanical action that is very effective for revealing the lath martensite structure in maraging steels.

Maraging steels require specific etchants that reveal the martensitic lath structure and prior austenite grain boundaries. Standard Nital works but specialized etchants provide better results.

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

• Composition: 5 g CuCl₂, 40 ml HCl, 30 ml H₂O, 25 ml ethanol
• Application: Immerse for 10-30 seconds. Swab etching also effective.
• Reveals: Lath martensite structure, prior austenite grain boundaries, and any reverted austenite. Excellent contrast for maraging steels specifically.
• Rinse: Water, then ethanol. Dry with warm air.

2% Nital (Chemical Etching) - General purpose alternative:

• Composition: 2 ml HNO₃, 98 ml ethanol
• Application: Immerse for 5-15 seconds.
• Reveals: General martensitic structure and prior austenite grain boundaries. Less selective than modified Fry's.

Note: The Ni₃(Mo,Ti) precipitates are nanoscale (5-20 nm) and cannot be resolved by optical microscopy. TEM is required for precipitate characterization.