ASTM E 112 GRAIN SIZE MEASURING METHODS full standard, mecanical

STANDARD TEST METHOD FORSTANDARD TEST METHOD FOR
DETERMINING AVERAGE GRAINDETERMINING 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 ApplicationGeneralities 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

CalibrationCalibration
 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 measurementMethods of grain size measurement

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 DETERMINING
THE AVERAGE GRAIN SIZE
ASTM METHODS E 112
PCN 12-501 120-10
Plate 1B
Untwinned 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 standardsExamples 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.

PROCEDURE
o 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 intercept
size 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-equiaxedSpecimens 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.

ASTM E 112 GRAIN SIZE MEASURING METHODS full standard, mecanical

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