Grain Size Analysis
Grain size measurement is carried out using the Metallurgical Analysis module of I-MEASURE PLUS or I-MAX PLUS.
The grain size number can be obtained by measuring
Grain intercept lengths
Number of grain intercepts
Number of intersections
Grain boundary length per unit area
Grain areas per unit area
Applicable Standards: ASTM E112, ASTM 1382
Measurement Criteria
The average grain size can be determined by the number of grains per unit area, the grain boundary length per unit area, the grain areas, the number of intercepts or boundary intersections, or the intercept lengths.
Measurements are performed on grains observed within the microscopic field, and repeated operations are required to ensure accuracy.
When using an image analyzer, borides, carbides, nitrides, and other phases at grain boundaries may be mistakenly detected; these should be excluded before measurement.
Foreign substances and misalignment in the microscope or camera system can cause image distortion; proper alignment and illumination adjustments are essential for clear imaging.
Unless the grain structure is extremely coarse, magnification should be selected so that at least 50 grains are included in the field of view.
However, if the image contains too many grains, calculation accuracy may decrease.
For statistical reliability, at least 5–20 fields of view should be analyzed.
1. Linear Intercept Method
Draw a set of straight lines or circular chords across the image.
Measure the intercept lengths where the lines cross grains.
Exclude chords whose endpoints are not both located inside a grain.
Continue until at least 500 intercepts are collected from randomly selected fields of view.
Formula:
l̄ = L_T / (P × M)
Where:
Lₜ = Total line length
P = Number of intercepts
M = Magnification
Grain Boundary Threshold
Threshold adjustment is used to clearly distinguish the grain boundaries.
The proper threshold should be set to ensure stable detection.
Flatten Background: Corrects image distortion caused by uneven illumination (brightness) and generates a new, adjusted image.
Display Option
Different display patterns can be applied to highlight the detected grain boundaries.
Choose the option that best visualizes the analysis.
Test Pattern Selection
Select a suitable test pattern (e.g., circle, line, or grid) to measure the intercepts.
Patterns should be placed randomly on the image fields.
Measurement starts immediately once a pattern is selected.
The measurement results are displayed as shown in the image above.
Del: Deletes the most recently measured result.
Reset: Clears all measured values and resets the system.
Up to 5 fields of view can be measured repeatedly.
Report
Exports the results to an Excel document.
2. Planimetric Method
In this method, a defined test area such as a circle or rectangle is used for counting grains.
n₁ = Number of whole grains completely inside the test area
n₂ = Number of grains intersected by the boundary
Aₜ = Test area
Formula:
Grain density per unit area: Nₐ = (n₁ + n₂/2) / Aₜ
ASTM Grain Size Number: G = 3.322 × log₁₀(Nₐ) − 2.954
The greater the number of counted grains, the higher the precision of the G value.
Report
Exports the results to an Excel document.
