The International Scene of Grain Size
Work on grain size has been followed closely by other industrial countries and the lnternational Organization for Standardization (ISO). Many countries have adopted one or more of the grain size charts of ASTM Test Methods E 112. Some countries have also developed very useful charts. For example, for rating McQuaid-Ehn carburized specimens, most U.S. raters etch the pearlitic matrix dark as depicted in Plate IV of Test Methods E 112.
As the sidebar on grain structures demonstrates, it is easier to see the intergranular carbide phase if we use an etchant that darkens the grain boundary cementite. The French grain size standard, NF A04-102, contains a rating chart where the grain boundary cementite was darkened with alkaline sodium picrate. The German SEP 1510 grain size standard also contains a very useful chart.
It illustrates non-twinned grains (such as ferrite grains) that are equiaxed or deformed (elongated 2 to 1 and 4 to 1) by cold working. Eq. 1 described the approach used to compute ASTM grain size numbers which, de- veloped in the United States in the late 1940s, was based on English units rather than metric units.
Countries that used the metric system at that time developed an alternate equation that produces nearly identical grain size numbers: (Equation 2)
m = 8(2Gm)
where:
m = the number of grains per mm2 at 1 X, and
Gm = the metric grain size number.
Gm is slightly greater than G but the difference is negligible. Eq. 2 is used in the Swedish (SIS 11 11 01), ltalian (UNI 3245), Russian (GOST 5639), French (NF A04-102), and ISO (ISO 643) standards.
The German standard (SEP 15l0)also uses the metric system, but a different equation is employed : (Equation 3)
K= 3.7 + 3.33Log(Z)
where:
K = the photomicrograph serial number (same as G), and
Z = the number of grains per cm2 at 1OOX.
In this case, K equals G. Japanese standard JIS G 0551 and G 0552 also use the metric system, with a slightly different equation than Eq. 2 (but equivalent) that produces the same values as Eq. 2: (Equation 4)
m = 2(Gm+3)
where m and Gm are defined as before.
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