Methods Of Grain size Determination


ASTM Method :
  1. Count the grains on the boundaries of the micrograph.
  2. Then count the grains inside the boundaries of the given photomicrograph.
  3. The grains on the boundaries i.e., partially visible grains are considered as half grains.
  4. The total no. of grains =1/2(Grain at boundaries) + Grains inside the boundaries
  5. Now, the total no. of grains per unit area is calculated by adding all the grains and dividing them by the area of the photomicrograph.
  6. For calculating ASTM grain size number, we require the no. of grains per square inch at 100x magnification(N100).
  7. If the magnification of photomicrograph is other than 100x then we need to calculate N100 which is given by N100 = [ Magnification of micrograph at hand/100]2xN(at given magnification)
  8. Now using formula n=2G-1 we get ASTM grain size number.
  9. The method with greater grain size number has more grains and smaller grains than the metal with smaller grain size number
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