P20 Plastic Mold Steel

P20 is a medium-hard (30-36 HRC, ~285-340 HB) pre-hardened plastic mold steel. The tempered martensite plus bainite microstructure is relatively straightforward to prepare, similar to other medium-carbon low-alloy steels. The primary metallographic concerns are verifying uniform hardness across the mold cross-section, identifying banding (compositional segregation from hot rolling), and assessing the quality of the tempered martensite structure. Large samples from thick mold blocks may show a hardness gradient from surface to center.

Use an abrasive cut-off wheel designed for medium-hardness steel (alumina abrasive). Standard blade thickness (1.0-1.5 mm) is appropriate. Use adequate coolant to prevent tempering of the martensite near the cut surface. Cutting speed: 250-350 RPM with moderate feed rate. For large mold blocks, band saw sectioning may be needed for rough cuts before precision sectioning. Leave 2-3 mm allowance for grinding away the heat-affected zone.

Hot compression mounting with phenolic or epoxy-phenolic resin is standard. The pre-hardened condition (30-36 HRC) is stable at mounting temperatures (150-180°C), well below any tempering concern. Cold mounting with castable epoxy is also suitable. For edge retention when examining surface treatments (nitriding, chrome plating on mold surfaces), use edge-retaining compounds.

Standard SiC grinding is effective for this medium-hardness steel.

Grinding sequence:

• 120 grit SiC: Remove sectioning damage and heat-affected zone (30-60 seconds). Moderate pressure (25-35 N).
• 240 grit SiC: Remove previous scratches (30-45 seconds).
• 320 grit SiC: Refinement (20-40 seconds).
• 400 grit SiC: Further refinement (20-40 seconds).
• 600 grit SiC: Final grinding (20-40 seconds).

Disc speed: 250-300 RPM. Rotate specimen 90° between steps. Use complementary rotation. Adequate water lubrication to prevent heat buildup. The medium hardness allows good material removal rates without excessive smearing.

Standard diamond polishing produces excellent results on P20.

Diamond polishing sequence:

• 6 μm diamond: 3-5 minutes on a medium-hard synthetic pad (20-30 N). Good scratch removal from grinding.
• 1 μm diamond: 3-5 minutes on a synthetic pad (15-25 N). Monitor for consistent scratch removal across the full surface.

Final polishing:

• 0.05 μm colloidal silica: 1-3 minutes on a soft final polishing pad with light pressure. Produces an excellent surface for Nital or Picral etching. Avoid over-polishing which can round grain boundaries and obscure banding.

P20 responds well to standard ferrous etchants. The tempered martensite and bainite microstructure etches readily.

2-5% Nital (Chemical Etching) - Primary choice:

• Composition: 2-5 ml HNO3, 95-98 ml ethanol
• Application: Immerse for 5-15 seconds. Start with 2% and increase to 5% for more aggressive etching.
• Reveals: Tempered martensite lath structure, bainite regions, prior austenite grain boundaries (lightly), and compositional banding. The darker-etching bands indicate higher carbon/alloy content.
• Rinse: Ethanol, then dry with warm air.

Picral (Chemical Etching) - For martensite/bainite differentiation:

• Composition: 4 g picric acid, 100 ml ethanol
• Application: Immerse for 10-30 seconds.
• Reveals: Tempered martensite and bainite with excellent contrast. Does not reveal ferrite grain boundaries, making it easier to distinguish between martensite and bainite phases. Preferred when banding assessment is the primary objective.

Safety: Picric acid is an explosion hazard when dry. Always keep it wetted. Standard fume hood and PPE for all etching.