ASTM E112 Standard Test Methods for Average Grain Size

These test methods cover procedures for estimating and rules for expressing the average grain size of all metals consisting entirely, or principally, of a single phase. The test methods may also be used for any structures having appearances similar to those of the metallic structures shown in the comparison charts. The three basic procedures for grain size estimation are:Comparison Procedure.

The comparison procedure does not require counting of either grains, intercepts, or intersections but, as the name suggests, involves comparison of the grain structure to a series of graded images, either in the form of a wall chart, clear plastics overlays, or an eyepiece reticle. There appears to be a general bias in that comparison grain size ratings claim that the grain size is somewhat coarser (1/2 to 1 G number lower) than it actually is (see X1.3.5). Repeatability and reproducibility of comparison chart ratings are generally ±1 grain size number.

These test methods cover the measurement of average grain size and include the comparison procedure, the planimetric (or Jeffries) procedure, and the intercept procedures. These test methods may also be applied to nonmetallic materials with structures having appearances similar to those of the metallic structures shown in the comparison charts. These test methods apply chiefly to single phase grain structures but they can be applied to determine the average size of a particular type of grain structure in a multiphase or multiconstituent specimen.

These test methods are used to determine the average grain size of specimens with a unimodal distribution of grain areas, diameter, or intercept length. These distributions are approximately log normal. These test method do not cover method to characterize the nature of these distributions. Characterization of grain size in specimens with duplex grain size distributions described in Test Method E1181. Measurement of individual, grain in fine grained matrix described in Test Method E930.

These test methods deal only with determination of planar grain size, that is, characterization of the two-dimensional grain sections revealed by the sectioning plane. Determination of spatial grain size, that is, measurement of the size of the three-dimensional grains in the specimen volume, is beyond the scope of these test methods.

These test methods describe techniques performed manually using either a standard series of graded chart images for the comparison method or simple templates for the manual counting methods. Utilization of semi-automatic digitizing tablets or automatic image analyzers to measure grain size is described in Test Methods E1382.

These test methods deal only with the recommended test methods and nothing in them should be construed as defining or establishing limits of acceptability or fitness of purpose of the materials tested.

The measured values are stated in SI units, which are regarded as standard. Equivalent inch-pound values, when listed, are in parentheses and may be approximate.

This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility of the user of this standard to establish appropriate safety and health practices and determine the applicability of regulatory limitations prior to use.

Test Methods E 112, one of the most widely cited ASTM standards, is chiefly concerned with the measurement of grain size when the grains are equiaxed in shape, that is, non-deformed, although it does contain some information about measurement of grain size when the grains have been elongated by processing. There are other situations where Test Methods E 112 is not helpful and other standards have been developed. For example, certain alloys may not exhibit a uniform distribution of grain sizes. Instead, a bimodal distribution may exist; several types have been observed. Two ASTM standard test methods deal with such structures. Standard E 930, Test Methods for Estimating the Largest Grain Observed in a Metallographic Section (ALA Grain Size), is used to measure the size of an unusually large grain in an otherwise uniformly fine grain size distribution, while standard E 1181, Test Methods for Characterizing Duplex Grain Sizes, is used to measure the grain size when the distribution is non-normal. With the growth of image analysis, test methods for performing measurements must be established and a new standard, E 1382, Test Methods for Determining the Average Grain Size Using Semiautomatic and Automatic Image Analysis, completed the balloting process in 1990. This standard describes a number of equivalent approaches for measuring grain size using both tablet digitizer systems and fully automatic systems.