Preparation Procedures / Stainless Steel

Step-by-Step Guide to Stainless Steel Sample Preparation

Stainless steels—ranging from austenitic and ferritic to martensitic and duplex grades—are used across critical industries such as medical, food processing, petrochemical, and power generation. Their corrosion resistance, phase stability, and microstructural complexity require meticulous metallographic preparation. This guide outlines the best practices for preparing stainless steel specimens for reliable microscopic analysis.

Stainless Steel Microstructure
Key Challenges in Stainless Steel Preparation
  • Retention of deformation zones and work-hardened layers
  • Differential etching across phase boundaries
  • Carbide pull-out or smearing in martensitic types
  • Phase transformation during polishing (especially in duplex)
  • Intergranular corrosion artifacts from improper prep
Technical Considerations
  • Hardness range varies significantly by grade (150–700 HV)
  • High alloy content requires controlled etching for phase visibility
  • Proper cooling during grinding prevents thermal damage
  • Electrolytic polishing and etching often recommended for best contrast

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Step 1: Sample Selection and Planning

Essential Steps

  • Identify the region of metallurgical interest and mark it accordingly
  • Note sample orientation and weld locations or heat-affected zones if applicable
  • Measure dimensions to evaluate suitability for hot or cold mounting
  • Degrease and clean surface to remove machining lubricants or corrosion films
  • Plan use of abrasive media compatible with stainless steel hardness and corrosion resistance
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Expert Guidance

Pre-Planning Checklist

  • Orientation to grain flow, weld lines, or machining direction
  • Purpose of analysis (e.g., delta ferrite, carbide distribution, sensitization)
  • Grade and family of stainless steel (austenitic, martensitic, etc.)
  • Mounting technique suited to avoid edge rounding or inclusion loss
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Technical Parameters

  • Record heat treatment and service conditions if known
  • Note surface conditions (passivation, pickling, etc.)
  • Account for corrosion-sensitive phases or precipitates
  • Ensure adequate edge retention for duplex or martensitic structures

Step 2: Sectioning

Essential Steps

  • Choose a cutting wheel rated for high-alloy or hardened materials
  • Optimize speed, feed, and coolant settings to minimize heat-affected zones
  • Ensure sample stability using soft jaws or rubber inserts to avoid vibration
  • Orient the cutting path perpendicular to the surface of interest
  • Rinse and dry sectioned surface immediately to prevent staining or rust
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Expert Guidance

Sectioning Best Practices

  • Minimize heat generation to avoid chromium carbide precipitation
  • Use moderate pressure to prevent martensitic transformation in metastable grades
  • Use fresh coolant with rust inhibitors for corrosion control
  • For duplex stainless, minimize mechanical and thermal distortion near cut edge
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Technical Parameters

  • Cutting speed: 150–300 RPM for most stainless grades
  • Feed rate: 0.5–1.5 mm/min depending on hardness
  • Coolant flow: ≥3 L/min with corrosion inhibitors
  • Recommended wheels: Al2O3 or diamond blades for hardened martensitic grades

Step 3: Mounting

Essential Steps

  • Thoroughly clean and degrease the stainless steel sample before mounting
  • Select a mounting compound suited to hard, corrosion-resistant materials
  • Use hot compression mounting or cold mounting based on sample geometry and thermal sensitivity
  • Ensure full cure or cool-down before grinding begins
  • Surface-grind mount flush to expose clean, undistorted metal
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Expert Guidance

Mounting Selection Tips

  • Use thermoset resins for high edge retention on hard stainless steels
  • Low-shrinkage epoxies are preferred for preserving welds or interfaces
  • Cold mounting with vacuum impregnation is ideal for duplex or porous surfaces
  • Use conductive mounting compounds for SEM or electrolytic polishing workflows
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Technical Parameters

  • Compression pressure: 250–350 bar for hot mounting
  • Curing temperature: 160–180°C (typical for phenolic or diallyl phthalate resins)
  • Cold mount cure time: 8–12 hours at room temperature or 30–60 minutes accelerated at 40–60°C
  • Edge retention: Enhanced with mineral-filled or glass-reinforced compounds

Step 4: Grinding

Essential Steps

  • Start with 120 or 180 grit SiC paper (P120 or P180) for removing welds or surface defects
  • Follow a progressive grit sequence: 180 → 240 → 320 → 400 → 600 (P180 → P240 → P320 → P400 → P600)
  • Apply generous water coolant to minimize heat and prevent smearing
  • Clean the specimen and holder thoroughly between steps to avoid abrasive cross-contamination
  • Inspect surface under oblique light or low-power magnification to confirm scratch removal
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Expert Guidance

Grinding Best Practices

  • Use PACE SiC or alumina grinding papers with continuous water flow to avoid abrasive loading
  • Apply consistent, moderate force to prevent work hardening or deformation
  • Rotate the sample 90° between steps to differentiate and eliminate prior scratches
  • When working with duplex stainless steels, limit pressure and thermal input to preserve phase integrity
  • Change papers when cutting efficiency decreases to ensure surface integrity
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Technical Parameters

Step Abrasive Time Force (N) RPM
1 180 grit SiC (P180) 2–3 min 35 300
2 240 grit SiC (P240) 2 min 30 300
3 320 grit SiC (P320) 2 min 25 300
4 400 grit SiC (P400) 1–2 min 20 300
5 600 grit SiC (P600) 1–2 min 15 300

Note: These settings are optimized for semi-automated systems like the NANO-1000S with an Semi-Automatic Polishing Head. Adjust force downward for manual prep to avoid over-grinding.

Step 5: Polishing

Essential Steps

  • Begin with rough polishing using 9µm diamond suspension on a napless cloth (e.g., chemotextile)
  • Continue with intermediate polishing using 3µm diamond suspension on the same or a fresh napless cloth
  • Refine with 1µm diamond suspension to reduce micro-scratches and deformation
  • Perform final polishing with colloidal silica (OP-S) on a high-nap or suede-like cloth
  • Thoroughly rinse and clean the specimen and holder between each polishing stage to eliminate abrasive carryover
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Expert Guidance

Polishing Strategy

  • Use high-quality diamond suspensions in the sequence: 9µm → 3µm → 1µm
  • Opt for firm napless polishing cloths (e.g., silk or satin) in early stages to maintain flatness
  • Transition to soft cloths for final polishing to enhance relief contrast without inducing rounding
  • Monitor surface integrity under a metallurgical microscope between steps for scratch inspection and phase clarity
  • Use lubricants/extenders to maintain consistent abrasive dispersion and cooling
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Technical Parameters

Step Media Time Force (N) RPM
1 9µm diamond on napless cloth 3–4 min 20 150
2 3µm diamond on napless cloth 2–3 min 15 150
3 1µm diamond on napless cloth 2 min 10 150
4 Colloidal silica on synthetic suede 1–2 min 5 100

Note: These values are ideal for systems like the NANO-1000S polisher. For manual setups, use lower force and ensure thorough rinsing.

Step 6: Cleaning and Drying

Essential Steps

  • Rinse thoroughly with deionized (DI) water immediately after polishing
  • Degrease with isopropyl alcohol or ethanol
  • Dry using clean, oil-free compressed air
  • Visually inspect under light or microscope for residue or water spotting
  • Store in a desiccator if analysis or etching is delayed
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Expert Guidance

Cleaning Protocol

  • Use ultrasonic cleaners (40 kHz) for complex or mounted parts
  • Always follow with alcohol to displace water and reduce drying spots
  • Ensure air lines are filtered to avoid oil or debris on the sample
  • Store cleaned samples in low-humidity chambers if not etching immediately
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Technical Parameters

  • Ultrasonic frequency: 40 kHz
  • Cleaning duration: 2–3 minutes
  • Compressed air: 2–3 bar, oil-free
  • Storage humidity: < 40% RH

Step 7: Etching

Essential Steps

  • Select a stainless steel-specific etchant from PACE Technologies
  • Always prepare fresh etchant in a well-ventilated fume hood
  • Apply using swabbing or immersion, depending on surface area and sample geometry
  • Closely monitor under a microscope to avoid over-etching and preserve structural contrast
  • Rinse immediately with DI water, follow with alcohol, and dry with compressed air
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Expert Guidance

Etchant Selection Tips

  • Vilella's Reagent: Ideal for general grain boundary and weld structure contrast
  • Glyceregia: Preferred for duplex stainless steels to reveal both ferrite and austenite phases
  • Oxalic Acid (electrolytic): Suitable for corrosion testing or ASTM A262 Practice A
  • Always etch under controlled lighting and time intervals for reproducibility
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Technical Parameters

  • Vilella's: 10–30 seconds (room temp)
  • Glyceregia: 30–60 seconds, swab application
  • Oxalic acid: 10% solution, 6 V for 60 seconds
  • Temperature range: 20–25°C

Step 8: Microscopic Analysis

Essential Steps

  • Begin with low magnification (50–100x) overview
  • Use higher magnification (200–1000x) to inspect grain boundaries, twins, or phases
  • Capture and annotate micrographs of key features
  • Record all preparation parameters for repeatability
  • Store sample in dust-free container with labeled ID
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Expert Guidance

Imaging Techniques

  • Brightfield: for general phase and grain size examination
  • Polarized light: enhances grain boundaries in austenitic and duplex steels
  • Differential Interference Contrast (DIC): for revealing strain fields or light topography
  • Use calibrated scales for dimensional measurements
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Technical Parameters

  • Magnification: 50–100x (overview), 200–500x (details), up to 1000x (fine structure)
  • Image resolution: ≥2048x2048 pixels
  • Recommended lighting: Kohler illumination
  • Storage: Labeled slide or sealed sample box

Troubleshooting Guide

Common Issues and Solutions
  • Smearing: Reduce polishing pressure and time
  • Scratches: Check for embedded abrasive, clean cloths
  • Pull-out: Use gentler polishing steps
  • Over-etching: Reduce etching time, monitor progress
  • Contamination: Clean between steps, use fresh consumables
Class 5 specimen preparation chart

Class 5 Procedure

See the standard metallographic approach for stainless steels.