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Letter to the Editor
Grain Size Standard

This letter is in response to a section of an article written by Li Yuguang in the January 2003 issue of SN. In the article, titled “Adoption and Utilization of ASTM Standards at Baosteel,” Li states that some ASTM standards, specifically, ASTM E 112, Test Methods for Determining Average Grain Size, are not updated promptly. The article continues by asserting that an equation used in planimetric method of E 112 is incorrect, based on a paper presented at the 1995 International Metallography Symposium. The author finishes this section of the paper with an admonishment that the standard has yet to be updated.

As chairman of ASTM Committee E04 on Metallography, which has jurisdiction over E 112, I would like to offer my response to Li’s assertion. First, simply because a paper was presented at an internationally respected forum such as the Metallography Symposium, does not automatically qualify its content as proof that an ASTM standard requires changing. One of the major strengths of any ASTM standard is the critical review of its contents by a broad range of people knowledgeable in the related field of study. Second, any changes made to a standard must be proposed, balloted and accepted by this peer group.

The equation, to which Li refers, is used to determine the number of grains per unit area on a polished and etched cross-section of material, usually steel. Currently, in this procedure, a circle or rectangular overlay is used to define the area of interest on a photomicrograph or on the ground glass of a microscope’s viewing screen. The number of grains within the overlay plus the number of grains intersecting the overlay are then counted. The sum of grains completely included within this area, plus one half the number of grains intersected by the circumference of the area, yields an equivalent number of whole grains per unit area at a given magnification.

As Li states, the equation currently used in E 112 may not be completely accurate due to some inherent bias found in the procedure; however, the correction she cites also contains bias. Consider the straight-line sides of a rectangular overlay. For grains intersecting these lines, on average, half of the area of these grains would be within the rectangle, while half the area of the grains would be outside the rectangle. When curvature is introduced, such as with the use of circular overlay, a bias is introduced because the sizes of grain areas inside and outside the grid are not equal, on average. Based on information published in the book, Stereometric Metallurgy, S. A. Saltykov showed that as the number of grains within the circle decreases, the bias increases, thereby creating a bias that is not constant. Since the bias is not constant, simply subtracting one grain in the equation, as Li suggests, will not be completely adequate when using a circular grid. Fortunately, if the number of grains counted wholly within the overlay is over 50, the bias introduced by a circular overlay has been found to be minimal.

Unfortunately, there is bias (also very minimal) introduced by counting the grains intersecting the corners of the square or rectangular overlay. At the corners, instead of splitting the grain areas in half, on average, as is the case along the straight sides, only one quarter of each of the grain areas will be included. In this case, as Li asserts, the procedure does overestimate the total number of grains by one. This point was recognized several years ago, and changes to the planimetric procedure have been balloted. As often happens in the balloting process, opinions on how to best change the document differed and negative votes were cast. The latest attempt to satisfy all concerned parties will hopefully be balloted in the next several months.

While change to our standards may not happen as quickly as we sometimes would like, it is a small price to pay to ensure that the best and most accurate documents are eventually published. As can be seen by this one example, even what appears to be a simple equation describing a simple procedure can become complicated when more than one point of view is advanced. Committee E04 welcomes comments concerning any of the standards under our jurisdiction, and is very willing to investigate any and all suggested changes.

So, no — we will not change any of our standards simply because a differing opinion has been published somewhere in the literature, and, yes — “some ASTM standards are not updated promptly.” However, at times, that is an advantage, not a problem.

Robert C. Nester
Chairman, Committee E04
on Metallography

Copyright 2003, ASTM