Stainless Steel Sample Preparation

Introduction

Stainless steel is one of the most commonly analyzed materials in metallography. From common austenitic grades like 304 Stainless Steel to high-strength precipitation-hardening alloys, proper preparation is essential to reveal the true microstructure without introducing artifacts such as deformation, scratches, or contamination. This guide will walk you through the complete preparation process.

431 Stainless steel, Kallings no. 2 etchant, 400X magnification. This image demonstrates the proper microstructure revealed through correct preparation techniques.

Stainless steels can be challenging due to their work-hardening characteristics and varying hardness depending on the grade. Softer austenitic grades like 304and 316 work-harden during preparation, while harder martensitic and precipitation-hardening grades like 17-4 PH require more aggressive cutting and grinding. The key is to use appropriate abrasives and maintain consistent pressure throughout the process.

Sectioning

When sectioning stainless steel samples, use a slow cutting speed to minimize heat generation and deformation. A cutting speed of 100-200 RPM is typically appropriate for most stainless steel grades. Harder grades like 17-4 PH Stainless Steelmay require even slower speeds (80-150 RPM) to prevent excessive heat buildup.

MAX-VHS abrasive cut-off blades designed for hard and case-hardened stainless steels. For general stainless steel grades, MAX-D series blades are also suitable. Thin blades (0.5-1.0 mm) minimize heat generation and deformation.

• Use MAX-VHS series blades for hard and case-hardened stainless steels, or MAX-D series for general stainless steel grades
• Use a thin abrasive cut-off wheel (0.5-1.0 mm thickness)
• Apply steady, moderate pressure
• Use adequate coolant to prevent overheating
• Allow the wheel to do the cutting - avoid forcing

Mounting

Mounting provides edge retention and easier handling. For stainless steel, compression mounting with phenolic or epoxy resins works well. If edge retention is critical, consider conductive mounting materials.

Compression Mounting

1. Clean the sample thoroughly to remove cutting fluid and debris
2. Place sample in mounting press with appropriate resin
3. Apply pressure: 3000-4000 psi for phenolic, 2000-3000 psi for epoxy
4. Heat to 150-180°C and hold for 5-8 minutes
5. Cool under pressure to room temperature

Grinding

Grinding removes sectioning damage and prepares the surface for polishing. Start with coarse grits and progressively move to finer grits. For stainless steel, we recommend the following sequence:

Silicon carbide (SiC) grinding papers in various grit sizes (120, 240, 400, 600) for progressive grinding. Rotate sample 90° between each grit to ensure complete scratch removal.

Grinding Sequence

1. 120 grit: Remove sectioning damage (30-60 seconds per step)
2. 240 grit: Remove previous scratches (30-60 seconds)
3. 400 grit: Further refinement (30-60 seconds)
4. 600 grit: Final grinding step (30-60 seconds)

Important: Rotate the sample 90° between each grit to ensure complete removal of previous scratches. Use water as a lubricant and maintain light, consistent pressure.

Polishing

Polishing removes grinding scratches and prepares a mirror-like surface. For stainless steel, diamond polishing followed by oxide polishing typically yields excellent results.

Polycrystalline diamond compound provides aggressive cutting action ideal for hard materials like stainless steel.

Various polishing pads and cloths for different polishing stages. Select pad hardness based on material and polishing stage.

Diamond Polishing

1. 9 μm diamond: 3-5 minutes on a hard cloth (e.g., Texmet)
2. 3 μm diamond: 3-5 minutes on a medium-hard cloth
3. 1 μm diamond: 2-3 minutes on a soft cloth

Final Polishing

1. 0.05 μm colloidal silica: 1-2 minutes on a soft cloth
2. Rinse thoroughly with water and dry with compressed air

Use appropriate polishing lubricants and maintain consistent pressure. Over-polishing can introduce relief, especially around inclusions or second phases.

Etching

Etching reveals the microstructure by selectively attacking grain boundaries and phases. The choice of etchant depends on the stainless steel grade and what features you want to reveal.

431 Stainless steel etched with Kallings no. 2, 400X magnification (DIC). Proper etching reveals grain boundaries and phase structure without over-etching artifacts.

Common Etchants for Stainless Steel

• Vilella's Reagent: General purpose, reveals grain boundaries. Works well for martensitic grades like 431.
• Aqua Regia: For austenitic stainless steels like 304 and 316. Excellent for revealing grain boundaries and phases.
• Electrolytic Etching: For sensitive microstructures. Particularly useful for austenitic grades where chemical etching may be too aggressive.

Etching solutions and reagents for stainless steel. Common etchants include Vilella's Reagent, Aqua Regia, and electrolytic solutions. Etching time typically ranges from 5-30 seconds depending on the etchant and steel grade.

Etching Procedure

1. Ensure sample is clean and dry
2. Apply etchant with cotton swab or immerse sample
3. Etch for 5-30 seconds (time varies by etchant and grade)
4. Immediately rinse with water, then alcohol
5. Dry with compressed air

Tip: Start with shorter etching times and increase if needed. Over-etching can obscure fine details. Austenitic grades like 304typically require 10-30 seconds, while martensitic grades may need only 5-15 seconds depending on the etchant used.

Troubleshooting

Common Issues and Solutions

• Scratches remaining: Insufficient grinding/polishing time or skipped grits
• Relief around inclusions: Over-polishing or too soft a cloth
• Contamination: Clean between steps, use fresh abrasives
• Poor edge retention: Consider different mounting material or technique
• Over-etching: Reduce etching time or dilute etchant