Metallography: A look deep inside

When analysing the martensite structure, which is intended, for instance, for screws in accordance with DIN EN ISO 898-1, it is possible to determine whether both transformations have taken place in the entire component. If complete austenitisation has not taken place, some of the original ferrite is still visible in the polished section. If quenching is too slow, other phases form in addition to martensite. In both cases, the hardness of the component is reduced. If the C level in the atmosphere of the oven is incorrect, the carbon content can change in the surface area of the components. If the C level is too low, the carbon contained in the steel at the edges of the components diffuses into the atmosphere. This carbon-depleted area is referred to as decarburised and exhibits reduced strength and hardness. In a polished section, this is noticeable by a light-coloured seam at the edge of the component when etched with diluted alcoholic nitric acid. If the C level is too high, additional carbon from the furnace atmosphere diffuses into the component, a process known as carburisation. The increased carbon content in the surface area leads to a brittle layer, which has a negative effect on fatigue strength, among other things. The resulting dark outer area can be determined in a polished section.

There is a whole range of other parameters that we can determine using metallography. We will be happy to advise you on which features are relevant for your products.

The functional principle of metallography

Careful sample preparation of the components is essential to visualise the microstructure. The sample is first cut at a suitable position and usually embedded in plastic (i.e. caked with liquid plastic and solidified into a standard-sized "polished section"). Embedding facilitates further handling of the sample.

The cut surface is heavily deformed by the cutting process. To uncover the original structure, the deformation must be removed by grinding and polishing in several increasingly fine stages. The final polishing step is carried out with a diamond suspension with a grain size of 1-3 µm (depending on the application and the sample hardness).

The reflective and nearly undeformed section surface created in this way must be etched in the next step to create a contrast between the different structural components. Depending on the material and the property to be examined, different acids and alkalis are used.

The analysis is then carried out under a reflected-light microscope (depending on the case, also under a scanning electron microscope). Here, our materials scientists and material testers examine, for example, the crystal structure, grain sizes, phase distribution and layer thicknesses.