ASTM E 112ASTM E 112

STANDARD TEST METHOD FOR STANDARD TEST METHOD FOR DETERMINING AVERAGE GRAIN DETERMINING AVERAGE GRAIN SIZESIZE

TERMINOLOGIESTERMINOLOGIES

Grain : The area within the confines of the original boundary observed on the 2-dimensional plane of polish or that volume enclosed by the original boundary in the 3-dimensional object.

ASTM grain size number : the ASTM grain size number, G, is defined as :

NAE = 2G-1

where NAE is the number of grains per square inch at 100X magnification.

Grain boundary intersection count : Determination of the number of times a test line cuts across, or is tangent to, grain boundaries.

Grain intercept count : determination of the number of times a test line cuts through individual grains on the plane of polish.

Intercept length : The distance between two opposed, adjacent grain boundary intersection points on a test line segment that crosses the grain at any location due to random placement of the test line.

Grains in steel at 100x magnification

Grain boundary intersection count

SIGNIFICANCE AND USESIGNIFICANCE AND USE 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.

In the metallographic laboratory, analyzing grains in metallic and alloy samples is important for quality-control. Most metals are crystalline in nature and contain internal boundaries, commonly known as "grain boundaries".

When a metal or alloy is processed, the atoms within each

growing grain are lined up in a specific pattern, depending on the crystal structure of sample. With growth, each grain will eventually impact others and form an interface where the atomic orientations differ.

It has been established that the mechanical properties of the sample improve as the grain size decreases.

Therefore, alloy composition and processing must be carefully controlled to obtain the desired grain size. 

After metallographic sample preparation, grains in a specific alloy are often analyzed via microscopy, where the size and distribution of these grains can demonstrate the integrity and quality of the sample

Generalities of Application Generalities of Application

It is important using that methods, to recognize estimation of average grain size is not a precise measurement. Metal structure is an aggregate of 3-D crystal of varying size and shapes.

The size and location of grains in a microstructure are normally completely random. No nominally random process of positioning a test pattern can improve the randomness, but random process can yield poor representation by concentrating measurement parts of specimen.

SamplingSampling Specimen should be selected to represent average

condition within a heat lot, treatment lot, or to assess variation anticipated across or along a product or component , depending on nature of material being tested and purpose to study.

Specimen should not be taken from areas affected by shearing, burning, or other processes that will alter the grain structure.

Test SpecimensTest Specimens If the grain structure is equiaxed then any specimen

orientation is acceptable. The presence of equiaxed grain structure in wrought specimen can only determined by examination of a plane of polish parallel to the deformation axis.

If the grain structure on longitudinal oriented specimen is equaixed, then grain size measurement on this plane or other will be equivalent within the statistical precision of test method.

If the grain structure is not equaixed but elongated, then grain size measurements on specimen with different orientation will vary. In this case grain size should be evaluated on atleast two of three principle planes.

The surface to be polished should be large enough in area to permit measurement of at least five field at the desired magnification. In most cases, except for thin sheet or wire specimens, a minimum polished surface area of 160 mm square is adequate.

The specimen shall be sectioned, mounted, ground, and polished according to the recommended procedure. The specimen shall be etched using a reagent, as given in practice E 407, to delineate most, or all of the grain boundaries

Calibration Calibration

Use a stage micrometer to determine the true linear magnification for each objective, eyepiece and bellows or zoom setting to be used within error of 2%.

Use a ruler with a millimeter scale to determine the actual length of straight test lines or the diameter of test circles used as grids.

Preparation of PhotomicrographsPreparation of Photomicrographs

When photomicrographs are used for estimating the average grain size, they shall be prepared in accordance with Guide E 883.

Methods of grain size Methods of grain size measurementmeasurement

COMPARISON METHODCOMPARISON METHOD

In former times, and even still in practice today, most laboratories would analyze grains via the "Chart Comparison" method.

Here, operators perform a visual estimation of the grain size by comparing a live image under an optical microscope to a micrograph chart, often posted on the wall near the microscope.

◦ Comparison of the grain structure to a series of graded images

Wall chart Clear plastic overlays An eyepiece reticle.

The following chart was used to make this image

METHODS FOR DETERMININGTHE AVERAGE GRAIN SIZE

ASTM METHODS E 112PCN 12-501 120-10

Plate 1BUntwinned Grains

100X

◦ Repeatability and reproducibility of ±1 grain size number.

◦ Specimens consisting of equiaxed grains.

To minimize errors, the comparison charts are presented in four categories as below :

◦ Plate I—Untwinned grains (flat etch).◦ Plate II—Twinned grains (flat etch)◦ Plate III—Twinned grains (contrast etch)◦ Plate IV—Austenite grains in steel

Examples of Grain size standards Examples of Grain size standards from Plates I, II, III, IVfrom Plates I, II, III, IV

Untwinned Grains(Flat Etch) from Plate I. Grain size no. 3 at 100x

Twin Grains(Flat Etch) from Plte II. Grain size no.3 at 100x

Twin Grains(Contrast Etch) from Plate III. Grain size 0.090 mm at 75X

Austenite Grains in steel from Plate IV. Grain size no. 3 at 100X

The table below lists a number of materials and the comparison charts that are suggested for use in estimating theis average grain sizes.

The estimation of microscopically determined grain size should be made by direct comparison at the same magnification as the appropriate chart.

The photomicrograph of the test specimen is compared with the photomicrographs of the standard chart, and the photomicrograph which most nearly matches the specimen image is selected.

This estimated grain size is reported as the ASTM grain size number.

PLANIMETRIC PROCEDUREPLANIMETRIC PROCEDURE◦ Involves an actual count of the number of grains within a

known area.

◦ Number of grains per unit area, NA, is used to determine the ASTM grain size number, G.

◦ Repeatability and reproducibility of ±0.25 grain size units

◦ Require marking off of the grains as they are counted.

PROCEDUREo Inscribe a circle or rectangle of known area on a micrograph

or on the ground glass screen of the metallograph.o Select a magnification which will give at least 50 grains in the

field.o When the image is focussed properly, count the number of

grains within this area.o Therefore, the number of grains per square millimeter at 1X,

NA ,is calculated from

o Where, Ninside = No. of grains included completely within the known area

Nintercepted = No. of grains intersected by the circumference of the area

f = Jeffries’ multiplier

Relationship Between Magnification Used and Jeffries’Multiplier, f, for an Area of 5000 mm2 (f= 0.0002 M2)

The ASTM grain size number, G, can be calculated from NA from

GENERAL INTERCEPT METHODGENERAL INTERCEPT METHOD◦ Actual count of the number of

grains intercepted grain boundary, per unit length of test line

◦ Lineal intercept length, used to determine the ASTM grain size number, G.

◦ Repeatability and reproducibility are less than ± 0.5 grain size units.

◦ Faster than the planimetric method for the same level of precision.

◦ Recommended for structures that depart from the uniform equiaxed form

ASTM No. 0 has a mean interceptsize of 32.00 at 100X.

lo = 32.00mm

Heyn Lineal Intercept Procedure

◦ The number of grains intercepted by one or more straight lines sufficiently long to yield at least 50 intercepts.

◦ The precision of grain size estimates by this method is a function of number of grain intercepts counted(hence, either a longer test line or a smaller magnification is used).

◦ Either intercept or intersection may be counted.

◦ When counting intercepts, segments at the end of a test line which penetrate into the grain are considered half intercepts.

◦ When counting intersections, the end points of a test line are not counted as intersections, except when it exactly touches a grain boundary(½ intersection).

◦ A tangential intersection is considered as 1 intersection.

◦ An intersection coinciding at the junction of 3 grains is considered 1½.

◦ In case of non-equiaxed grains, test lines require averaging of values made at variety of orientation.

Circular Intercept Method

◦ Automatically compensate for departures from equiaxed grain shapes

◦ Ambiguous intersections at ends of test lines are eliminated.

◦ Most suitable for use as fixed routine manual procedures for grain size estimation in quality control.

◦ There are 2 circular intercept methods: Hilliard Single-Circle Procedure Abrams Three-Circle Procedure

Hilliard Single Circle Procedure

◦ Any circle size of known circumference may be used. Circumferences of 100,200, or 50 are usually convenient.

◦ The test circle diameter should never be smaller than the largest observed grain.

◦ A small reference mark is usually placed at the top of the circle to indicate the place to start and stop the count.

◦ Apply the selected circle to the microscopic image at a convenient magnification and count the intersections of the circle with grain boundaries.

◦ The precision of the measurement increases as the number of counts increases.

Abrams Three-Circle Procedure

◦ The test pattern consists of three concentric and equally spaced circles having a total circumference of 500mm

◦ Successively apply this pattern to at least 5 blindly selected and widely spaced fields, separately recording the count of intersections per pattern for each of the tests.

◦ Examine the grain structure and select a magnification that will yield 40-100 intercepts or intersections.

◦ For most grain structures, a total count of 400-500 intercepts over 5-10 fields produce better than 10% relative accuracy.

◦ After applying the test circles, the total grain boundary intersections are counted by a manually operated counter.

◦ For each field count, calculate NL or PL according to:

where Ni and Pi are the number of intercepts or intersections counted on the field, L is the total test line length(500mm) and M is the magnification.

Statistical AnalysisStatistical Analysis

No determination of average grain size can be an exact measurement.

Thus, no determination is complete without also calculating the precision within which the determined size may, with normal confidence, be considered to represent the actual average grain size of the specimen examined.

It is assumed that the normal confidence to represent the expectation that the actual error will be within the stated uncertainty 95% of the time.

Many specimens vary measurably in grain size from one field of view to another, this variation being responsible for a major portion of the uncertainty.

So, after the desired number of fields have been measured, mean value of NA or l from the individual field values is calculated according to

Next, standard deviation of individual measurements is calculated.

Then, calculate 95% confidence interval, of each measurement according to :

Table listing values of t as a function of n

Specimens with non-equiaxed Specimens with non-equiaxed Grain shapesGrain shapes If the grain size was altered by processing so that the

grains are no longer equiaxed in shape, grain size should be made on longitudinal(l), transverse(t), and planar(p) oriented surfaces for rectangular bar, plate or sheet type materials.

For round bars, radial longitudinal and transverse sections are used.

If directed test lines are used for the analysis, measurements in the 3 principal directions can be made using only two of the three principal test planes.

Planimetric Method :

◦ When the grain shape is not equiaxed but elongated, make grain counts on each of the three principal planes, i.e., longitudinal, transverse and planar oriented surfaces.

◦ Determine the number of grains per mm2 at 1X on the three planes, NAl , NAt , NAp , and calculate the mean number of grains per unit area NA from :

Intercept Method: For the case of randomly determined values of PL or NL on

the three principal planes, compute the average value according to :

or

Alternatively, calculate ll , lt , lp from the PL or NL values on each plane.

Additional information on grain shape may be obtained by determining lparallel(0°) and perpendicular(90°) to the deformation axis on a longitudinally oriented surface. The grain elongation ratio or anisotropy index, AI, can be determined from,

The mean value of l for the measurements in the three principal test directions is obtained by averaging the directed NL or PL values and then computing l from this mean value; or by calculating directed l values in each of the principal directions and then averaging them

ReportReport The test report should document all of the pertinent

identifying information regarding the specimen, its composition, specification designation or trade name, date of test, heat treatment or processing history, specimen location and orientation, etchant and etch method, grain size analysis method, etc, as required.

List the number of fields measured, the magnification, and field area. The number of grains counted or the number of intercepts or intersections counted, may also be recorded

A photomicrograph illustrating the typical appearance of the grain structure may be provided

List the mean measurement value, its standard deviation, 95% confidence interval, percent relative accuracy, and the ASTM grain size number.

Precision and BiasPrecision and Bias The precision and bias of grain size measurements depend

on the representativeness of the specimens selected and

the areas on the plane of polish chosen for measurement.

The relative accuracy of the grain size measurement improves as the number of specimen taken from the product increases.

The relative accuracy improves as the number of fields sampled and the number of grains or intercepts counted increase.

Bias in measurements will occur if specimen preparation is inadequate. The true structure must be revealed and the grain boundaries must be fully delineated for best precision and freedom from bias.

In accurate determination of the magnification of the grain structure will produce bias.

If the grain structure is not equiaxed in shape, measurement of the grain size on only one plane will bias test results.

When using the comparison chart method, the chart selected should be consistent with the nature of the grains(i.e. twinned or untwinned, or carburized and slow cooled) and the etch(flat etch or contrast etch) for best precision.

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