Nitrided Steel Cross-Section

Nitrided steel cross-sections have a very hard surface (900-1100 HV compound layer) transitioning through a hard diffusion zone to the softer core (30-45 HRC tempered martensite). The compound layer is very thin (5-30 μm) and must be preserved intact for thickness measurement. Edge retention at the nitrided surface is critical. The hardness gradient through the diffusion zone is measured by microhardness traverse, which requires a flat, polished surface free of deformation artifacts that could affect hardness values.

Use an abrasive cut-off wheel with alumina abrasive designed for hardened steel. Generous coolant is essential to prevent thermal damage to the nitrided case. The compound layer is brittle and can crack or spall from thermal shock during cutting. Cutting speed: 250-350 RPM with moderate to low feed rate. Cut perpendicular to the nitrided surface to produce a true cross-section for case depth measurement. Leave 2-3 mm allowance for grinding. For precision work, use a low-speed diamond wafering saw to minimize heat generation.

Edge-retaining mounting is critical for preserving the thin compound layer. Use a hard epoxy mounting compound with mineral filler or a specialized edge-retaining resin. Vacuum impregnation is recommended to fill any cracks in the brittle compound layer. Orient the nitrided surface face-down in the mold. Cold mounting is preferred for maximum edge retention. For best results, apply electroless nickel plating (10-15 μm) over the nitrided surface before mounting. The nickel provides excellent mechanical support for the thin compound layer during grinding and polishing.

The hard nitrided surface (900-1100 HV) and softer core (300-450 HV) create a hardness gradient that must be managed during grinding.

Grinding sequence:

• 120 grit SiC: Remove sectioning damage (30-60 seconds). Moderate pressure (25-35 N). The hard nitrided case grinds slower than the core.
• 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). Very light pressure to avoid fracturing the compound layer.

Disc speed: 250-300 RPM. Rotate specimen 90° between steps. Diamond grinding discs (40-15 μm) can be used as an alternative for more uniform material removal across the hardness gradient. Monitor for chipping of the brittle compound layer at the sample edge.

Standard diamond polishing with attention to edge retention at the nitrided surface.

Diamond polishing sequence:

• 6 μm diamond: 3-5 minutes on a napless or low-nap synthetic pad (20-30 N). Use napless cloth near the surface to prevent rounding of the compound layer edge.
• 1 μm diamond: 3-5 minutes on a synthetic pad (15-25 N). The compound layer should appear as a bright, featureless band at the surface.

Final polishing:

• 0.05 μm colloidal silica: 1-3 minutes on a soft pad. Keep time minimal to preserve edge retention. For microhardness traverse specimens, a flat, deformation-free surface is essential; vibratory polishing (1-2 hours) may give better results for hardness accuracy.

Etching reveals the compound layer, diffusion zone, and core microstructure with distinct contrast.

2-3% Nital (Chemical Etching) - Primary choice:

• Composition: 2-3 ml HNO3, 97-98 ml ethanol
• Application: Immerse for 5-15 seconds.
• Reveals: The compound layer appears bright white (resists etching), hence its name "white layer." The diffusion zone etches darker than the core due to nitrogen-induced lattice strain. The core tempered martensite etches with normal contrast. The case/core boundary is visible as a color transition.
• Rinse: Ethanol, then dry with warm air.

Marble's Reagent (Chemical Etching) - For compound layer detail:

• Composition: 4 g CuSO4, 20 ml HCl, 20 ml water
• Application: Immerse for 5-15 seconds.
• Reveals: Compound layer sub-structure. Can differentiate gamma-prime (Fe4N) and epsilon (Fe2-3N) phases within the compound layer. The copper deposition darkens the Fe2-3N phase while Fe4N remains brighter.

Microhardness traverse note: For case depth measurement by microhardness (ASTM E384), the surface should be polished but NOT etched. Etching can affect apparent hardness values near the surface.