420 Stainless Steel

420 Stainless Steel is a medium-hard (217 HB) martensitic stainless steel (13% Cr, 0.15% C) with a martensitic microstructure when quenched and tempered. The material is magnetic and can be hardened by heat treatment, making it commonly used for cutlery and surgical instruments. The medium hardness allows for standard preparation procedures. The martensitic structure may show different etching behavior depending on heat treatment condition (quenched vs. tempered). Use appropriate abrasives and sufficient time per step. Edge retention is critical for cutlery and surgical instrument applications - ensure proper mounting and avoid excessive relief.

Use abrasive cut-off wheel designed for stainless steel (Al₂O₃ or SiC abrasive). Standard cut-off wheel (1.0-1.5 mm thickness) is appropriate. Use adequate coolant flow to prevent overheating - excessive heat can affect the martensitic microstructure and cause tempering or over-tempering in hardened material. Cutting speed: 200-300 RPM for most cut-off saws. Apply steady, moderate pressure - the medium hardness allows for reasonable feed rates. Avoid forcing the cut which can cause wheel wear and sample damage. Leave adequate allowance (~2-3 mm) for grinding away the heat-affected zone from cutting.

Cold mounting with epoxy resin is preferred to avoid heat that could affect the martensitic microstructure, especially if the material has been hardened. Use a low-shrinkage epoxy resin for best edge retention. Ensure complete cure before grinding to prevent edge rounding and maintain sample integrity.

Hot compression mounting is acceptable if the part tolerates ~150-180°C and moderate pressure (2000-3000 psi for phenolic). Use phenolic or epoxy-phenolic resins. The medium-hard material requires careful handling during mounting to avoid deformation. Ensure proper cooling under pressure to minimize shrinkage. Avoid excessive heat that could affect the martensitic structure or cause over-tempering in hardened material.

For critical cutlery and surgical instrument applications, ensure the mounting material provides adequate edge retention for microstructural analysis.

The medium hardness (217 HB) of 420 Stainless Steel allows for standard grinding procedures. Use standard SiC grinding papers with adequate water lubrication. Disc speed: 200-300 RPM. Apply moderate pressure (25-35 N per 30 mm sample) - the material can tolerate standard pressure levels. Use sharp, fresh grinding papers to minimize deformation.

Grinding sequence:

• 120 grit: Remove sectioning damage (30-60 seconds). Use moderate pressure to remove heat-affected zone from cutting.
• 240 grit: Remove previous scratches (30-60 seconds). Ensure complete scratch removal.
• 320 grit: Further refinement (30-60 seconds). Monitor for complete scratch removal.
• 400 grit: Refinement (30-60 seconds). Prepare surface for polishing.
• 600 grit: Final grinding step (30-60 seconds). Ensure uniform scratch pattern.

Always rotate the specimen holder 90° between steps to ensure complete scratch removal. Use complementary rotation (platen and holder same direction, different speeds) for best results. Adequate water lubrication is critical - avoid drying during grinding. The medium hardness means grinding times are standard - ensure complete scratch removal at each step before proceeding.

The medium hardness allows for standard polishing procedures. Use diamond polishing with appropriate polishing pads for each stage. The martensitic microstructure is generally uniform, so relief should be minimal with proper polishing.

Diamond polishing sequence:

• 6μm diamond: 3-4 minutes on a medium-hard synthetic pad (e.g., TEXPAN) with moderate pressure (25-35 N per 30 mm sample). Start with 6μm for medium-hard materials. Standard synthetic pads work well for this hardness level.
• 3μm diamond: 3-4 minutes on a medium-hard synthetic pad (e.g., TEXPAN) or non-woven intermediate pad with moderate pressure. Continue removing scratches from previous step.
• 1μm diamond: 2-3 minutes on a low-napped pad designed for fine polishing (e.g., GOLD PAD) with lighter pressure (20-30 N). These pads provide consistent material removal and flatness control.

Final polishing:

• 0.05μm colloidal silica: 1-2 minutes on a high-napped final polishing pad (e.g., MICROPAD) with light pressure. High-napped pads are recommended for colloidal silica and produce a mirror finish. This removes any remaining fine scratches and prepares the surface for etching. Monitor for relief - reduce polishing time if excessive relief develops.

Use appropriate polishing lubricants. The medium hardness means standard polishing times work well - ensure complete scratch removal at each step. Monitor for any carbides or inclusions that may require additional polishing time.

420 Stainless Steel responds well to standard martensitic stainless steel etchants. The martensitic structure will reveal grain boundaries and martensitic lath structure clearly with appropriate etchants. Two primary etchants are recommended:

Vilella's Reagent (Chemical Etching) - Primary choice for general microstructure:

• Composition: 1g picric acid, 5ml HCl, 100ml ethanol
• Preparation: Dissolve picric acid in ethanol, then add HCl. Prepare fresh for best results. Solution is stable for several weeks if stored properly.
• Application: Immerse sample or swab for 10-20 seconds. Classic general-purpose micro-etchant for martensitic stainless steels. Provides good grain boundary and martensitic structure contrast.
• Reveals: Grain boundaries, martensitic lath structure, and carbides clearly. Excellent for general microstructure examination. Good contrast for martensitic structure. Reveals prior austenite grain boundaries in quenched material.
• Rinse: Immediately with water, then ethanol. Dry with compressed air or warm air to avoid staining.
• Note: Prepare fresh when needed. Shelf life: several weeks if stored properly. Use in fume hood. Picric acid is explosive when dry - keep moist and handle with care.

2% Nital (Chemical Etching) - Alternative etchant for martensitic stainless steels:

• Composition: 2ml HNO₃ (concentrated), 98ml ethanol
• Preparation: Add nitric acid to ethanol slowly with stirring. Prepare fresh for best results. Solution is stable for several days.
• Application: Immerse sample or swab for 10-30 seconds. Standard etchant for martensitic and ferritic stainless steels. Provides good contrast for martensitic structure.
• Reveals: Grain boundaries, martensitic structure, and carbides. Good for revealing martensitic lath structure and prior austenite grain boundaries.
• Rinse: Immediately with water, then ethanol. Dry with compressed air.
• Note: Prepare fresh when needed. Shelf life: several days. Use in fume hood. Less aggressive than Vilella's reagent, good for fine microstructural details.

Etching Strategy:

• Start with Vilella's reagent for general microstructure examination - it provides good contrast and reveals martensitic structure clearly
• Use 2% Nital if Vilella's is too aggressive or for revealing fine microstructural details
• Etching time may vary significantly with heat treatment condition - quenched material may etch differently than tempered material
• For quenched material, prior austenite grain boundaries will be revealed clearly
• For tempered material, the martensitic lath structure and carbides will be visible
• Always clean and degrease before etching
• Use short initial etch times (a few seconds), check under the microscope, repeat if needed
• Check etching progress frequently - over-etching can obscure fine details
• For cutlery and surgical instrument applications, ensure proper surface preparation to reveal any microstructural changes from heat treatment or service
• The higher chromium content (13% vs 12% in 410) may make 420 slightly more corrosion resistant, but etching behavior is similar

Safety: Both etchants require proper PPE and fume hood. Vilella's reagent contains picric acid which is explosive when dry - keep moist and handle with extreme care. Nital produces toxic fumes.