How to Calculate Grain Size

Grain size is the average size of the particles in a metal. All metals are made up of tiny crystals fused together. When viewed under a microscope, the boundaries between these crystals can be counted, allowing the grain size to be calculated. Metals with smaller grains tend to be stronger, though large grains can sometimes be useful as well. Testing grain size is thus useful for quality control in the development and production of alloys.

Metal specimen
Microscope
Stage micrometer
Metal cutter
Rag
Metal polish

Instructions

- 1
  Cut a thin slice of the metal to be tested and polish it smooth. Place the metal slice on the microscope stage and adjust the zoom until you can see the grain divisions clearly enough to count them. Remove the metal.
- 2
  Place the stage micrometer on the microscope stage. Rotate the ocular micrometer by turning the eyepiece that controls it and adjust the position of the stage micrometer until the two are aligned. Compare the divisions of the two to determine how wide a distance the ocular micrometer can measure. Remember that the divisions on the stage micrometer are 10μm each. Remove the stage micrometer.
- 3
  Return the metal specimen to the stage. Count the number of grain boundaries that intersect the line of the ocular micrometer. Move or rotate the piece of metal and count again. Repeat this several times until you can determine an accurate mean.
- 4
  Divide the distance across the ocular micrometer by the mean number of grain boundaries to find the grain size.

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