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1 These test methods are under the jurisdiction of ASTM Committee E04 on. Metallography and are E Test Methods for Determining Average Grain Size. Home; ASTM E(). $; Add to Cart. Printed Edition + PDF; Immediate download; $; Add to Cart. Designation: E − 97 (Reapproved ) Standard Test Methods for Determining Average Grain Size Using Semiautomatic andAutomatic Image Analysis 1.
A few notable exceptions exist where a coarse grain structure is desired. Alloy composition and processing must be controlled to achieve the desired grain size. Metallographers examine polished cross sections of specimens from appropriate locations to determine the grain size. Complications--Grain Characteristics Grain size measurement is complicated by a number of factors.
First, the three-dimensional size of the grains is not constant and the sectioning plane will cut through the grains at random. Thus, on a cross-section we will observe a range of sizes, none larger than the cross section of the largest grain sampled. Grain shape also varies, particularly as a function of grain size. One of the earliest studies of grain shape was made by Lord Kelvin in He showed that the optimum space-filling grain shape, with a minimum surface area and surface tension, is a polyhedron known as a tetrakaidecahedron, which has 14 faces, 24 corners, and 36 edges.
While this shape meets most grain criteria, it does not satisfy the required degree dihedral angles between grains where three adjacent grains meet at an edge, unless the faces exhibit a minor amount of curvature. Another ideal grain shape, the pentagonal dodecahedron, agrees well with observations of grains, but is not a space filling shape.
It has twelve five-sided faces. However, it must be recognized that we are sampling grains with a range of sizes and shapes. In most cases, the grains observed on a polished cross-sectional plane exhibit a range of sizes around a central mean and individual measurements of grain areas, diameters, or intercept lengths exhibit a normal distribution. In the vast majority of cases, we merely determine the mean value of the planar grain size, rather than the distribution.
There are cases where the grain size distribution is not normal but bimodal, or "duplex. Different product shapes, and different processing procedures, can produce a variety of non-equiaxed grain shapes.
This, of course, does influence our ability to measure the grain size. Grain size measurement is also complicated by the different types of grains that can be present in metals, although their fundamental shapes are the same.
For example, in body-centered cubic metals, such as Fe, Mo, and Cr, we have ferrite grains; in face-centered cubic metals, such as Al, Ni, Cu, and certain stainless steels, we have austenite grains. The grains exhibit the same shapes and are measured in the same way, but we must be careful in describing what kind of grains we are measuring. In the face-centered cubic metals, we may observe so-called twin boundaries within the grains see sidebar on grain types. Aluminum alloys, however, rarely exhibit twins.
When twins are present, they are ignored if we are trying to define the grain size. However, if we are trying to establish a relationship between microstructure and properties, for example, strength, we must consider twin boundaries as they influence dislocation movement, just as grain boundaries do. Hence, we must recognize the intent of the work being performed. In heat-treated steels, it is recognized that the grain size of the product of the heat treatment, usually martensite, is not measured or cannot be measured.
For low-carbon steel, the martensite forms in packets within the parent austenite grains. In high-carbon martensites, we do not observe any convenient structural shape that can be measured. In most cases, we try to measure the size of the parent austenite grains that were formed during the high temperature hold during the heat treatment. This is usually referred to as the "prior-austenite grain size" and it has been widely correlated to the properties of heat treated steels.
The most difficult process here is the etching procedure needed to reveal these prior boundaries. Sometimes they cannot be revealed, particularly in low-carbon steels.
In this case, it may be possible to measure the low-carbon lath martensite packet size, which is a function of the prior-austenite grain size.
ComplicationsDifferent Measures of Size Another complicating factor is the different measures of grain size. The planimetric method, described below, yields the number of grains per square millimeter area, NA, from which we can calculate the average grain area, A.
It is common practice to take the square root of A and call this the grain diameter, d, although this assumes that the cross sectional shape of the grains is a square, which it is not. The intercept method yields a mean intercept length, L3 ; its relationship to NA, A, or d is not exceptionally well defined.
A variety of planar grain size distribution methods have also been developed to estimate the number of grains per unit volume, Nv, from which the average grain volume, V, can be calculated. The relationship between these spatial measures of grain size and the above planar measures is also ill-defined. Although the NA, d, or L3, values had been used for many years as measures of grain size, the G values were adopted readily due to their simplicity.
As shown in Eq. Measurement Methods Although Committee E-4 was formed in for the express purpose of establishing standard magnifications for micrographs, its first standard, E T, Methods of Preparation of Micrographs of Metals and Alloys, was partly devoted to grain size measurement.
Two basic approaches to measure grain size were being developed at that time. In the United States in , Albert Sauveur published a "planimetric" approach, which was further developed by Zay Jeffries with two publications.
This approach measured grain size in terms of the number of grains visible on a cross section within a fixed area, the number per square inch at X, or the number per square millimetre at 1X, NA. From this value, the average cross-sectional area of the bisected grains can be computed. This is not an average of the maximum cross-sectional area of each grain because the sectioning plane does not intersect each grain at its maximum width.
In Germany in , Emil Heyn published an intercept approach for measuring grain size.
In this method, one or more lines are superimposed over the structure at a known magnification. The true line length is divided by the number of grains intercepted by the line. This gives the average length of the line within the intercepted grains. This average intercept length will be less than the average grain diameter but the two are interrelated. Many grain size raters expressed the need for simpler ways to estimate the grain size.
In some cases, such as heat clearance, grain size measurement is required. In many cases, it is required that G be 5 or greater i.
Hence, if the grain size is substantially finer than this, a quick method, which may not be as precise as an actual measurement, is adequate. A comparison chart method with examples of grain sizes meets this need adequately, as long as the grain size distribution is normal. Additionally, the specimens should be etched in the same manner as depicted on the chart. If the grain size is near the specification limit, an actual measurement is preferred due to the improved precision. The first grain size comparison chart was introduced in Methods E 2 in its revision; this chart was for copper.
Note that these methods are applied on the polished surface of the specimen, that is, on a plane that cuts through the three-dimensional grains. Thus, these are planar rather than spatial measures of the grain size. The planimetric, or Jeffries method, defines the grain size in terms of the number of grains per unit area, the average grain area, or the average grain diameter, while the Heyn intercept method defines it in terms of the average intercept length.
The comparison chart method expresses the grain size only in terms of G, except for the copper charts, which use d.
Evolution of Test Methods E Methods E T was only slightly more than three pages long and had three sections: standard magnifications, lenses, and grain size. The grain size section did not actually detail the measurement method, it merely suggested the method to apply depending on whether the grains were equiaxed Jeffries planimetric method or elongated Heyn intercept method.
Exces-sively deep etching with such etchants to bring out the fainterboundaries should not be done because deep etching createsexcessive relief deviation from planar conditions and willbias certain measurements, particularly grain intercept lengthsand grain areas, performed by automatic image analysis andalso measurements made with a digitizing tablet.
Consequently, the imaging system must bekept clean. Apparatus 7. For example, for optically anisotropic materialsthat are difficult to etch, crossed polarized light may berequired to observe the grain structure.
Such images exhibitgrain contrast or color differences between grains rather thangrain boundary delineation. These images, which usuallyexhibit low light intensities, can be measured using a digitizingtablet but may be more difficult to measure with automaticimage analyzers. A variety of approaches can be employed. Alternatively, the grid can be placed on aneyepiece reticle. A third approach is to transfer the microstructuralimage, test grid image and cursor image to a television monitor.
The microscopist moves the cursor across the tablet surfacewhile watching the monitor to make the appropriate measure-ments. For measuringintercept lengths, a test grid with a number of equally spacedstraight, parallel lines is used.
For automatic image analysis,the computer also controls all of the operations except,perhaps, focusing automatic focusing is optional. Graphical data may be produced with either a printer or plotter,as desired.
Verylow levels of humidity must also be avoided as static electricitymay damage electronic components. Vibrations, if excessive,must be isolated, particularly for automatic image analysis. Sampling 8. Sampling location andfrequency should be based upon agreements between manu-facturers and users. Test Specimens 9. Consequently, preparation of longitudinallyoriented specimens, where the plane-of-polish is parallel to thedeformation axis or grain elongation direction, is recom-mended.
ASTM E1382 - 97(2015)
If the grain structure is not equiaxed butelongated, then grain size measurements on specimens withdifferent orientations will vary.ASTM Committee E-4 and Grain Size Measurements Metals, except in a few instances, are crystalline in nature and, except for single crystals, they contain internal boundaries known as grain boundaries.
In this method, one or more lines are superimposed over the structure at a known magnification. For transmission electron characteristic radiationX-radiation of a particular set of microscopy, the illumination of an object so that it appears wavelengths, produced by and characteristic of a particular on a bright background.
In many cases, it is required that G be 5 or greater i. Balanced filters thus can be made unit volume of a substance per unit change in temperature to serve as a crude monochromator. Two basic approaches to measure grain size were being developed at that time.