Your microscope
What you see under the microscope · Metallography
What you see with a metallurgical microscope
A metal does not let light through: you look at it with reflected light, which comes down through the objective and returns from the surface. The piece of steel is mounted in resin, polished to a mirror finish and etched with nital, which reveals the grains.
Choose the model and the technique in the panel.
50×: the overall appearance
You see light areas and dark areas spread across the whole piece: it is a low-carbon steel.
100×: ferrite and pearlite
The light grains are ferrite and the dark ones, pearlite. In a 0.2% carbon steel, pearlite makes up about a quarter.
200×: the grain boundaries
Nital attacks the boundaries between grains more strongly, so they show up as dark lines. They are used to measure the grain size.
500×: pearlite, up close
Pearlite is made of alternating lamellae of ferrite and cementite, often so fine that the light microscope cannot resolve them: it appears as a dark, mottled area.
Which metallurgical microscope do you need?
For metals you need reflected light, coming from above through the objective. Metallurgical microscopes have it, and some add darkfield and polarisation.
Which microscope do I need?Frequently asked questions
What is the difference between a metallurgical microscope and a biological one?
The biological microscope illuminates from below and the light passes through the sample. The metallurgical one illuminates from above, through the objective itself, because a metal does not let light through.
What are ferrite and pearlite?
Two constituents of steel: ferrite appears light after the nital etch, and pearlite, made of lamellae of ferrite and cementite, appears dark.
What are darkfield and polarisation used for in metallography?
In darkfield the contrast is reversed and the boundaries appear light. With crossed polarisers, a steel looks uniformly dark because its structure is cubic.
How this simulation is made
It is not a photograph: it is a calculated model. Magnification, field of view and resolution come from each microscope's spec sheet; the sample measurements come from the sources cited below.
| What | How it is calculated | What is approximate |
|---|---|---|
| Magnification and field of view | Magnification = eyepiece × objective. Field of view = eyepiece field number ÷ objective magnification. | Exact according to the spec sheet. |
| Ferrite and pearlite | Light ferrite and dark pearlite with nital; pearlite ≈ 24% for a 0.2% carbon steel (lever rule). | Grain distribution generated at random. |
| Grain size | About 28 µm (ASTM 7, using the formula in standard E112). | There is no single published figure for this steel: chosen within the usual range. |
| Pearlite lamellae | Too fine for the light microscope: a dark, mottled area. | Spacing not verified. |
| Darkfield and polarisation | In darkfield the contrast is reversed; with crossed polarisers, the steel looks uniformly dark. | Intensities chosen. |
Sources
- Spec sheets of the Letscopes metallurgical microscopes (iScope IS.1053-PLMi).
- Nital etching: PACE Technologies. Reference micrograph: DoITPoMS, University of Cambridge.
- Pearlite fraction and resolution: H. Bhadeshia, Cambridge. Grain size: ASTM E112.
- Polarisation in steels: G. Vander Voort.
Other samples
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