From polished section to microstructure: What is metallography?

Metallography plays a central role both in the services provided by our accredited testing laboratory and in our continuing education programmes. But what does the term actually mean?

Metallography at a glance

Metallography is a branch of materials testing and materials science concerned with examining the internal structure of metals and alloys. When non-metallic materials are examined, the term materialography is often used. The aim of metallographic examination is to make a material’s microstructure visible and suitable for analysis.

This provides a basis for drawing conclusions about, for example:

• mechanical properties
• corrosion resistance
• the effects of manufacturing processes
• the effects of heat treatments

The tasks of metallography

• Analysing the internal structure: Metallography enables the examination of a material’s internal microstructure and, in a sense, its “life history”. Phases, grains and inclusions, for example, can be identified.
• Determining grain size: Grain size has a considerable influence on a material’s mechanical properties, such as its yield strength or strength. This information is therefore particularly important for design engineers, for example.
• Determining phase fractions: Many people remember the iron–carbon diagram from their vocational training or studies – some fondly, others perhaps with less enthusiasm. In practice, however, understanding the distribution of phases in a material is crucial for numerous applications.
• Assessing inclusions: Metallographic examinations also allow inclusions or impurities that may impair a material’s quality and performance to be identified and assessed.

Sample preparation

Material samples generally cannot be analysed directly because their surfaces may be uneven, contaminated or unsuitable for microscopic examination. Every metallographic examination therefore begins with careful specimen preparation.

If the component is too large for examination, a representative sample section is first cut off. It is then deburred, cleaned and dried.

If the sample is very small or difficult to handle, it is mounted. For example, the sample is embedded in epoxy resin or acrylate to stabilise it and make it easier to handle during further preparation.

Grinding and polishing

After the initial preparation, the sample is the right size for examination under the microscope, but its surface is not yet suitable for analysis. It therefore undergoes several grinding stages using progressively finer grit. This removes irregularities and produces a flat surface. The sample is then polished using diamond suspension and cleaned regularly between stages. At the end of this process, the surface is mirror-smooth, flat and clean, making it suitable for microscopic examination.

Initial examinations under the light microscope

After preparation, the sample can be examined under the light microscope for the first time. Numerous features can already be identified at this stage, including certain layers or coatings, inclusions, shrinkage cavities and pores, as well as macroscopic features of welds. Different objectives and magnifications provide an initial insight into the material’s structure. For more detailed microstructural analysis, however, the polished surface is often still too reflective. Another important preparation step therefore usually follows.

Etching – making the microstructure visible

A material’s microstructure often only becomes visible after controlled etching of its surface. Acids or alkalis are used to attack the various microstructural constituents to different degrees. This creates a microscopic surface topography that changes how light is reflected under the microscope. This effect allows the microstructural constituents to be clearly distinguished and evaluated.

Metallography: Conclusion

Metallographic examinations are an essential part of modern materials testing, both in industrial quality assurance and in research and damage analysis. They provide a means of systematically deepening both practical skills and sound background knowledge of metallographic analysis.

W.S. Werkstoff Service – Expertise in metallography

We offer several options for anyone who would like to explore this subject in more depth:

• Metallography as a continuing education course
• Metallographic examinations as a service provided by our accredited testing laboratory