Metallographic Preparation: Sectioning, Mounting, Grinding and Etching

In metallography you are not looking at the surface of a material but at its internal microstructure. To see through a piece of metal you first have to take it through a complete preparation sequence: sectioning, mounting, grinding and polishing, etching. Prepare badly and even an excellent metallurgical microscope will show nothing useful. The sequence looks simple, but every step has its rules.

1. Sampling and sectioning

Take a representative location, avoiding defects and the deformed zone at an edge. The critical rule in cutting is that the sample must not heat up: the heat of a fast abrasive cut changes the microstructure and falsifies the result. Use a cut-off machine with cooling, control the feed rate, and aim for a flat cut face with no burning. Get this wrong and everything after it is wasted.

2. Mounting

Small parts, thin sections, edge specimens and anything whose edge must be protected are normally mounted. Hot mounting uses resin under heat and pressure: quick, but with possible thermal effects. Cold mounting uses room-temperature curing resin and suits temperature-sensitive materials. Mounting quality has a direct effect on how flat the subsequent grinding and polishing will be.

3. Coarse and fine grinding

Grinding proceeds step by step from coarse to fine. The usual practice is a sequence of abrasive papers from coarse grit to fine, and at every paper change the specimen is rotated 90 degrees so that the previous scratches are removed. The guiding principle is to remove only the marks left by the previous step without introducing new scratches that are harder to remove. Keep the process wet to prevent overheating and embedded abrasive.

4. Polishing

Polishing removes the fine scratches left by grinding and produces a mirror surface. Mechanical polishing uses cloths with diamond paste or alumina suspension; electrolytic and vibratory polishing suit particular materials. Timing has to be controlled: too little and scratches remain, too much and hard phases are smeared while soft phases such as graphite are polluted. This is the step that most depends on feel.

5. Etching

A polished specimen is a mirror and the microstructure is still invisible, so it has to be etched. A suitable etchant, nital being usual for iron-carbon alloys, attacks grain boundaries and different phases at different rates, and the contrast appears under the microscope. Under-etching leaves the structure unclear, over-etching produces artefacts, and the timing comes with experience. Non-ferrous alloys and stainless steels have their own dedicated etchants.

6. Observation and common defects

After etching the specimen can be examined, working from low to high power. Common preparation defects include residual scratches, polishing pits, smearing, artefacts from over-etching and gaps at the edge of the mount. When something looks suspicious, rule out preparation problems first and only then draw a conclusion.

The essence of metallographic preparation is to reveal the interior of a material truthfully, not to manufacture a good-looking picture. Following the procedure and comparing regularly against reference specimens is the shortest route to better preparation. On the equipment side, a cooled cut-off machine, a mounting press, a grinding and polishing machine and a metallurgical microscope are the basic kit of any materials or quality laboratory.