Metallographic preparation is a systematic process used to examine the microstructure of metals and alloys under a microscope. The quality of the sample preparation directly affects the accuracy of the microstructural analysis. The preparation process involves several key steps: sectioning, mounting, grinding, polishing, and etching.
The first step is to cut a small representative piece of the metal without altering its microstructure.
Tools Used:
Abrasive saws
Diamond or carbide-cutting wheels
Wire electrical discharge machining (EDM) (for delicate samples)
Precautions:
Avoid excessive heat generation to prevent microstructural changes.
Use coolants or lubricants to minimize heat and deformation.
The sample is mounted to support small or irregular pieces and provide an easy-to-handle shape.
Types of Mounting:
Hot Mounting: Uses thermosetting resins (e.g., Bakelite, epoxy) under heat and pressure.
Cold mounting: Uses liquid resins that cure at room temperature (good for heat-sensitive materials).
Advantages of Mounting:
Protects the sample edges.
Makes handling easier for grinding and polishing.
Grinding removes surface irregularities and prepares the sample for fine polishing.
Process:
Done using silicon carbide (SiC) or alumina abrasive papers.
Progresses from coarse (e.g., 240 grit) to fine (e.g., 1200 grit) in a sequence.
Sample is rotated 90° between each stage to remove previous scratches.
Precautions:
Apply uniform pressure to avoid uneven wear.
Use water as a lubricant to reduce heat.
Polishing removes fine scratches left by grinding and enhances clarity for microstructural observation.
Types of Polishing:
Rough Polishing: Uses diamond suspensions (6–1 μm) or alumina paste.
Final Polishing: Uses colloidal silica (0.05 μm) or alumina suspension for an ultra-smooth surface.
Precautions:
Avoid contamination between different polishing stages.
Use a soft cloth and correct polishing compounds.
Etching selectively attacks grain boundaries, phases, and microstructural features, making them visible under a microscope.
Common Etchants:
Nital (2-5% nitric acid in ethanol) – for steels.
Keller’s reagent – for aluminum alloys.
Picral (Picric acid solution) – for ferritic and martensitic steels.
Precautions:
Avoid over-etching, as it can obscure fine details.
Rinse with alcohol or water immediately after etching to stop the reaction.
After preparation, the sample is analyzed under optical microscopes (OM), scanning electron microscopes (SEM), or electron backscatter diffraction (EBSD) to study its microstructure.