When ASTM standard E 2 was published in 1917, ASTM Committee E-4 on Metallography’s first standard, it described the planimetric method for measuring grain size based upon publications by Zay Jeffries, a founding member of E4; but, E 2 only briefly mentioned the intercept method developed in Germany in an appendix at the end of the standard.

The intercept method suggested by Heyn in 1903 [1] is considerably faster to perform manually which has made it popular, despite the fact that there is no direct mathematical connection between the mean lineal intercept length and G. Both straight lines and circles have been used as templates, plus other shapes.

When ASTM standard E 2 was published in 1917, ASTM Committee E-4 on Metallography’s first standard, it described the planimetric method for measuring grain size based upon publications by Zay Jeffries, a founding member of E-4.

Jeffries was a graduate student under the famous Harvard professor, Albert Sauveur. Sauveur published a paper in 1894 where he defined grain structures in terms of the number of grains per square mm at 1X. But, he did not develop details on his method. This method is more tedious to use than the Heyn intercept method because a count of the grains must be made by physically marking the grains as they are counted, when done manually. Experiments were conducted to determine the influence of the number of grains counted per grid application using the Jeffries planimetric procedure of ASTM E 112 with a single test circle of varying size. Results show that this is a viable test method and produced good data down to relatively low count numbers per grid application. Bias was not observed at low counts, only data scatter.

Grain size is probably the most frequent microstructural measurement due to its influence on properties and behavior/service performance. Grain size can be determined by several methods. Chart comparison ratings are probably the most often performed, as this method is fast and simple. But its accuracy is at best ± 1 G value.

An ASTM E-4 interlaboratory round robin test using Plate I of ASTM E 112 showed that chart ratings were biased with the rating being ½ to 1 G value coarser than the actual measured grain size. Similar studies have not been conducted with Plates II or III. Actual measurements of grain size are done by either the planimetric or the intercept methods, as defined in E 112. These are unbiased methods, as long as the grain boundaries were properly delineated by the etchant. Experience has shown that measuring the grain size of BCC metals is much easier than measuring the grain size of FCC metals and alloys that contain annealing twins. ASTM E 112 has two comparison charts for such metals; Plate II for specimens that exhibit a so-called “flat” etch appearance and Plate III for those that exhibit a grain contrast etchant response. Plate III was developed using copper specimens and the images are at 75X, while the other E 112 charts are at 100X. To further confuse the issue, Plate III expresses grain size in terms of d, the average grain diameter, calculated by taking the square root of the average grain area (which is the reciprocal of the number of grains per mm²), rather than as an ASTM grain size number, G.

Cold working is well known to change the properties of metals and alloys. Deformation increases the strength of metals but usually reduces it toughness and leads to anisotropy of properties, that is, directionality. Hot working also produces similar affects, the microstructural results after hot work with low finishing temperatures may appear to be the same as from cold working.

Hot rolling of shapes, plate or bar, for example, elongates the nonmetallic inclusions in the deformation direction, which will reduce the isotropy of mechanical properties. Hot working can also lead to segregation being elongated parallel to the deformation direction, which also reduces isotropy. Reducing the finishing temperature, that is, the temperature of the steel at the last deformation pass, will promote “banding” – parallel alignment of the constituents into layers, such as alternate bands of ferrite and pearlite. This also promotes anisotropy of mechanical properties, chiefly toughness and ductility. Strength is not usually affected to a significant degree by banding, compared to toughness and ductility.