Grain Size Measurement

Grain Size MeasurementDwi Marta NurjayaGrain SizeGrain size: the dimensions of the grains or crystals in a polycrystalline metal exclusive of twinned regions and sub-grains when present. Grain size is usually estimated o r measured on the cross section of an aggregate of grains Common units are: average diameter, average area,number of grains per linear unit, number of grains per unit area, and number of grains per unit volume.ASTM Grain size numberASTM grain size number : a grain size designation bearing a relationship to average intercept distance at 100 diameters magnification according to the equation:

G = ASTM grain size number = 10.0 2 log2 L ,

Where, L Is the average intercept distance in millimeters at 100X magnification.Average grain diameter : the mean diameter of an equiaxed grain section whose size is representative of all the grain sections in the aggregate being measured.ASTM Grain size numberNAE = 2 G - 1

where, G = ASTM grain size number NAE = number of grains per square inch at 100X magnification.

Grain Size Equations Relating Measured Parameters to the Microscopically Determined ASTM Grain Size, G

ASTM Grain size numberMethods based on ASTM E112 Comparison Method Planimetric Method Intercept Method

Comparison MethodThe comparison method 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 plastic overlays, or an eyepiece reticle.

An eyepiece reticlesNote : Use only with a 10X objectiveComparison MethodWhen grain size estimations are made by the more convenient comparison method, repeated checks by individuals as well as by interlaboratory tests have shown that unless the appearance of the standard reasonably well approaches that of the sample, errors may occur. To minimize such errors, the comparison charts are presented in four categories as follows:Plate IUntwinned grains (flat etch). Includes grain size numbers 00, 0, 12, 1, 112, 2, 212, 3, 312, 4, 412, 5, 512, 6, 612, 7, 712, 8, 812, 9, 912, 10, at 100X.Plate IITwinned grains (flat etch). Includes grain size numbers, 1, 2, 3, 4, 5, 6, 7, 8, at 100X.Plate IIITwinned grains (contrast etch). Includes nominal grain diameters of 0.200, 0.150, 0.120, 0.090, 0.070, 0.060, 0.050, 0.045, 0.035, 0.025, 0.020, 0.015, 0.010, 0.005 mm at 75X.Plate IVAustenite grains in steel (McQuaid-Ehn). Includes grain size numbers 1, 2, 3, 4, 5, 6, 7, 8, at 100X.

Suggested Comparison Charts for Metallic MaterialsThese suggestions are based upon the customary practices in industry

Comparison chart RatingsLook at a properly etched microstructure, using the same magnification as the chart, and pick out the chart picture closest in size to the test specimen. If the grain structure is very fine, raise the magnification, pick out the closest chart picture and correct for the difference in magnification according to:G = Chart G + QQ = 6.64 Log10(M/Mb)

Where: M is the magnification used andMb is the chart magnificationPlanimetric MethodAlso known as Grain Counting Method or Jeffries MethodIn the planimetric procedure inscribe a circle or rectangle of known area (usually 5000 mm2 to simplify the calculations) on a micrograph, a monitor or on the groundglass screen of the metallograph. Select a magnification which will give at least 50 grains in the field to be counted.Planimetric Method ProcedureInscribe circle of known area, A, on a image with magnification, MCount the number of grains are completely within the area, Ninside Count the number of grains are partially within the area, Nintercept,Use the Jeffries formula and G formula to find the Grain size number

Planimetric MethodNA = number of grain per mm2f = Jeffries MultiplierNinside = number of grains inside the test circleNintercept = number of grains that intercept the test circle

Ninside = 68 Nintercept = 41

Jeffries Multiplier = (1002)/5000 = 2.0

Intercept MethodsAlso known as Heys/Hilliard/Abrams Intercept MethodsIntercept Methods are more convenient to use than the planimetric methods. These methods are amenable to use with various types of machine aids.Intercept procedures are recommended particularly for all structures that depart from the uniform equiaxed form.There is no direct mathematical relationship between the ASTM grain size number, G, and the mean lineal intercept, unlike the exact relationship between G, NAE , NA and A, for the planimetric methodIntercept MethodsApply a test line over the microstructure and count the number of grains intercepted or the number of grain boundary intersections (easier for a single-phase grain structure). After you count N or P, divide that number by the true line length to get NL or PL.If intersection apparently coinciding with the junction of three grains should be scored as 1.5

N = number of grains interceptedP = number of grain boundary intersectionsLT is the true test line length

Intercept MethodsMean Lineal Intercept = l

ASTM Test Pattern for Intercept Counting

The test line intercepted 5 whole grains and the line ends fell in two grains. These are weighted as an interception. So the total is 6 intercepts (N=6).Intercept counts (N)The test line has intersected 6 grain boundaries. The ends within the grains are not important in intercept counting. So, P=6 for the intercept count.Intersection counts (P)

Intersection counts (P)If the grain structure is not equiaxed, but shows some distortion of the grain shape, use straight test lines at various angles, or simply horizontal and vertical with respect to the deformation axis of the specimen.Circle Intercept (Abrams Three-Circle Procedure)The ASTM three-circle grid (three concentric circles with a line length of 500 mm). This test pattern averages the anisotropyD1 = 79.58 mmD2 = 53.05 mmD3 = 26.53 mm

Intercept counting are 41, 25 and 20 grainsIf LT = 11.4 mm

N = 41 + 25 + 20 = 86