The measurement of grain size is performed nearly always on a metallographically prepared cross section, suitably etched to reveal the grain structure, using methods that pertain only to the grain cross sections. These measurements are termed planar and some may be converted mathematically into spatial estimates of the size of the three-dimensional grains. However, the vast majority of such work is planar where no assumptions about grain shape are required and grain size is described in one- or two-dimensional terms (intercept length, diameter, or area) based on sections through the grains. The most frequently used measurement methods are described in this paper and compared using the same images. These methods are the Jeffries planimetric method, the triple-point count method, and the Heyn intercept method. These methods base grain size on two-dimensional, zero-dimensional, and one-dimensional features of the microstructure, respectively (that is, areas, points, and lines).
Test lines can be utilized to count grain interceptions or grain boundary intersections; or, actual intercept length measurements may be made. Furthermore, the test lines may be straight or curved (circular test lines being commonly employed). Does the curvature of a circular test line cause a bias in either intercept/intersection counts or intercept length measurements? When a digitizing tablet is used with a circular test line, either chord lengths between sets of grain boundary intersections, or arc lengths between the grain boundary intersections can be measured. Does either practice cause bias, or are the results similar? How do averages of intercept lengths compare to the mean lineal intercept calculated from the reciprocal of the number of intercepts (or intersections) per unit length? Does the triple-point count method produce the same results as the planimetric results and how do these estimates of the ASTM grain size number compare to that from the intercept procedures? These questions are addressed.