Technical Information

Comprehensive guide to metallographic sample preparation

Overview

Sectioning

Mounting

Grinding

Polishing

Final Polishing

Etching

Analysis

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Overview

Step 1

Introduction to metallographic sample preparation

Metallographic sample preparation is a systematic process that transforms a raw material sample into a polished surface suitable for microscopic examination. The quality of preparation directly impacts the accuracy of microstructural analysis.

Key Objectives:

•Preserve the true microstructure

•Minimize preparation artifacts

•Achieve consistent results

•Ensure reproducibility

•Maintain sample integrity

Process Flow:

1. Sectioning - Cutting the sample

2. Mounting - Encasing for handling

3. Grinding - Initial surface preparation

4. Polishing - Intermediate surface refinement

5. Final Polishing - Achieving mirror finish

6. Etching - Revealing microstructure

7. Analysis - Examination and documentation

Quality Control:

•Regular equipment maintenance

•Standard operating procedures

•Documentation of parameters

•Calibration checks

•Reference sample verification

Tips

Maintain a clean work environment
Document all preparation steps
Use reference samples for comparison

Warnings

Follow safety protocols at all times
Handle samples with care to prevent damage

Sectioning

Step 2

The process of cutting a sample from a larger piece of material

Sectioning is the first and crucial step in metallographic sample preparation. It involves cutting a representative sample from a larger piece of material while minimizing damage to the microstructure.

Key Considerations:

- Use appropriate cutting methods based on material hardness:

• Abrasive cutting for hard materials

• Diamond saw for brittle materials

• Band saw for soft materials

- Maintain proper cutting parameters:

• Speed: 100-300 RPM for most materials

• Feed rate: 0.05-0.2 mm/s

• Coolant flow: 2-4 L/min

- Prevent thermal damage:

• Use adequate coolant

• Avoid excessive pressure

• Monitor cutting temperature

Best Practices:

•Always cut perpendicular to the surface of interest

•Maintain consistent cutting pressure

•Use fresh cutting wheels for each material type

•Document cutting parameters for reproducibility

Tips

Always wear appropriate PPE during sectioning
Keep cutting area clean to prevent contamination
Mark sample orientation before cutting

Warnings

Avoid excessive heat generation
Never force the cut - let the wheel do the work

Mounting

Step 3

Encasing the sample in a protective medium for handling and preparation

Mounting provides support and protection for the sample during subsequent preparation steps. The choice of mounting method and material significantly impacts the final results.

Mounting Methods:

- Hot Compression Mounting:

• Temperature: 150-180°C

• Pressure: 2000-4000 psi

• Time: 5-8 minutes

- Cold Mounting:

• Mix ratio: 100:10 (resin:hardener)

• Curing time: 4-8 hours

• Vacuum impregnation for porous materials

Material Selection:

•Phenolic resin for general use

•Epoxy for edge retention

•Conductive resin for SEM analysis

•Transparent resin for optical clarity

Best Practices:

•Clean sample thoroughly before mounting

•Ensure proper sample orientation

•Use appropriate mounting pressure

•Allow complete curing before proceeding

Tips

Use release agent for easy demounting
Label mounts clearly for traceability
Consider sample size when choosing mount diameter

Warnings

Handle hot mounts with care
Ensure proper ventilation during mounting

Grinding

Step 4

Initial surface preparation using progressively finer abrasives

Grinding removes the damaged layer from sectioning and prepares the surface for polishing. This step is critical for achieving a flat, scratch-free surface.

Grit Progression:

1. Coarse grinding (120-240 grit)

2. Medium grinding (320-400 grit)

3. Fine grinding (600-800 grit)

4. Final grinding (1200 grit)

Parameters:

•Speed: 150-300 RPM

•Pressure: 20-30 N

•Time per step: 30-60 seconds

•Water flow: Continuous

Best Practices:

•Clean between each grit change

•Use fresh grinding paper for each sample

•Maintain consistent pressure

•Check surface after each step

•Document grinding parameters

Tips

Use water spray to prevent overheating
Rotate sample 90° between grits
Inspect surface under microscope between steps

Warnings

Avoid excessive material removal
Never skip grit sizes

Polishing

Step 5

Intermediate surface preparation to remove grinding scratches

Polishing removes the fine scratches from grinding to prepare for final polishing. This step requires careful attention to detail.

Polishing Steps:

1. Rough polishing (9μm diamond)

2. Intermediate polishing (3μm diamond)

3. Fine polishing (1μm diamond)

Parameters:

•Speed: 100-150 RPM

•Pressure: 15-25 N

•Time: 2-5 minutes per step

•Lubricant: Water or special polishing fluid

Best Practices:

•Use appropriate polishing cloths

•Maintain consistent pressure

•Clean between steps

•Monitor surface quality

•Document polishing parameters

Tips

Use fresh polishing media for each step
Keep polishing area clean
Inspect surface under microscope

Warnings

Avoid contamination between steps
Never mix polishing media

Final Polishing

Step 6

Achieving a mirror-like finish for optimal analysis

Final polishing is the last mechanical preparation step, crucial for achieving a scratch-free surface suitable for microscopic analysis.

Final Polishing Steps:

1. Ultra-fine polishing (0.25μm diamond)

2. Oxide polishing (0.05μm alumina)

3. Colloidal silica (optional)

Parameters:

•Speed: 50-100 RPM

•Pressure: 10-15 N

•Time: 1-3 minutes per step

•Lubricant: Special polishing fluid

Best Practices:

•Use fresh polishing media

•Maintain gentle pressure

•Clean thoroughly between steps

•Inspect under microscope

•Document all parameters

Tips

Use fresh polishing cloths
Maintain consistent pressure
Check for remaining scratches

Warnings

Avoid excessive pressure
Prevent contamination

Etching

Step 7

Chemical treatment to reveal microstructural features

Etching enhances contrast to reveal the material's microstructure. The choice of etchant and etching parameters depends on the material and features of interest.

Common Etchants:

•Nital (2-5% HNO₃ in ethanol)

•Picral (4% picric acid in ethanol)

•Kalling's reagent (for stainless steels)

•Vilella's reagent (for titanium alloys)

Parameters:

•Etching time: 5-30 seconds

•Temperature: 20-25°C

•Agitation: Gentle swabbing

Best Practices:

•Clean sample thoroughly before etching

•Use fresh etchants

•Control etching time carefully

•Rinse immediately after etching

•Document etching parameters

Tips

Test etch on scrap material first
Use proper ventilation
Have neutralizing solution ready

Warnings

Handle etchants with extreme care
Wear appropriate PPE
Dispose of used etchants properly

Analysis

Step 8

Examination and characterization of the prepared sample

Analysis involves examining and characterizing the prepared sample using various microscopy techniques.

Analysis Methods:

1. Optical Microscopy:

• Magnification: 50x-1000x

• Lighting: Brightfield, darkfield, polarized

• Documentation: Digital imaging

2. Scanning Electron Microscopy:

• Magnification: Up to 100,000x

• Modes: SE, BSE, EDS

• Sample preparation: Conductive coating

3. Image Analysis:

• Grain size measurement

• Phase fraction determination

• Inclusion analysis

Best Practices:

•Calibrate equipment regularly

•Use appropriate magnification

•Document all observations

•Follow standard procedures

•Maintain proper records

Tips

Use consistent lighting conditions
Calibrate regularly
Document all parameters

Warnings

Handle samples with care
Follow safety protocols
Backup all data