W1 Water-Hardening Tool Steel

W1 tool steel in the quenched and tempered condition has a hard surface (60-65 HRC, ~770 HV) transitioning to a softer core (25-35 HRC for pearlitic core). This hardness gradient across the cross-section creates differential polishing and etching challenges. The hard martensitic surface layer grinds and polishes slower than the softer pearlitic core. Cross-section preparation from surface to center reveals the full hardening profile. In the annealed condition, W1 is a simple spheroidized or lamellar pearlite microstructure at ~200 HB.

Use an abrasive cut-off wheel with alumina abrasive for the hard quenched material. Generous coolant is critical to prevent tempering the martensitic surface layer. Cutting speed: 250-350 RPM with moderate feed rate. For round bars, cut a diametral cross-section to reveal the full hardening depth profile from surface to center. Leave 2-3 mm allowance for grinding. If the part is in the annealed condition (spheroidized pearlite), standard non-ferrous blades can also be used since the material is much softer.

Hot compression mounting with phenolic or epoxy-phenolic resin is standard. The tempered martensite is thermally stable at mounting temperatures (150-180°C) if the temper temperature was above 200°C. For low-temper parts (tempered at 150-175°C), cold mounting is safer to avoid any additional tempering during mounting. Ensure the full cross-section from surface to center is exposed for hardness traverse analysis.

The hard martensitic surface and softer pearlitic core grind at different rates. Use consistent technique across the full cross-section.

Grinding sequence:

• 120 grit SiC: Remove sectioning damage (30-60 seconds). Moderate pressure (25-35 N). The hard surface grinds slower; ensure the full surface is planarized.
• 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. The transition zone between hard and soft regions may show slight relief; minimize this by using fresh papers and consistent light pressure.

Standard diamond polishing works well. Monitor for differential polishing between the hard surface and softer core.

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). Check for consistent quality across both hard and soft zones.

Final polishing:

• 0.05 μm colloidal silica: 1-3 minutes on a soft final polishing pad. The surface should be uniformly scratch-free from the hard surface to the soft core.

Etching reveals the dramatic microstructural transition from martensite (surface) through the transition zone to pearlite (core).

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

• Composition: 2-5 ml HNO3, 95-98 ml ethanol
• Application: Immerse for 5-15 seconds. The martensitic surface will etch darker than the pearlitic core.
• Reveals: Surface zone: fine tempered martensite (dark etching). Transition zone: mixed martensite + pearlite with a visible boundary. Core: lamellar or spheroidized pearlite (lighter etching). Undissolved carbides appear as bright particles in the martensite.
• Rinse: Ethanol, then dry with warm air.

Picral (Chemical Etching) - For carbide detail:

• Composition: 4 g picric acid, 100 ml ethanol
• Application: Immerse for 10-30 seconds.
• Reveals: Pearlite lamellar structure with excellent resolution. Undissolved cementite particles in the martensite zone. Preferred for quantitative pearlite analysis.

For the best demonstration of the hardening transition, etch a full cross-section (surface to center to opposite surface) with 5% Nital. The martensite/pearlite boundary is typically visible to the naked eye as a lighter zone (pearlite core) surrounded by a darker zone (martensite case).