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Determining Grain Size Distributions
Need help with grain size distributions?
This webinar was initiated due to concern over a statement in ASTM E1181 concerning the width of a normal, or Gaussian, grain size distribution as compared to a duplex or bi-modal distribution, of which there are several types defined in E1181. Due to the influence of the sectioning plane, the metallographer does not see only the maximum grain area for each grain but a wide range of apparent areas, even if the grains in 3-D were all of uniform size and shape.
Consequently, as the mean grain size becomes larger, we should observe a wider range of grain size classes over the distribution. So, one cannot use a number of ASTM grain size classes to define a normal grain size distribution, nor any type of non-normal or bi-modal distribution. Instead, it is necessary to calculate the skew and kurtosis of a distribution of grain areas (a plot of area percent per grain size class vs. ASTM grain size, G, per G class). If the kurtosis is >5, it is not a normal, Gaussian grain size distribution, but non-normal or perhaps bi-modal. Examples will be shown of normal, non-normal and bi-modal grain size distributions as a function of the mean grain area, standard deviation, skew and kurtosis.
Questions Discussed
· What should we measure to determine if the grain structure is normal or not? Intercept lengths or grain areas?
· How can we do these measurements? How can we reveal the grain structure properly?
· How do we plot the data? Should we use the number percent of grain area classes or the number percent of intercept length classes for the Y-axis and the grain area or intercept length for the x-axis?
· How many classes should we use on the x-axis? The example in E 1181 has 38. Is that good?
· Should the x-axis by linear or logarithmic? Does it matter?
· Should the x-axis be linear classes of intercept lengths or grain areas, or should these values by converted to ASTM grain size numbers plotted linearly?