Grain Size Analyzer — method & reference

Manual intercept grain sizing per ASTM E112. Calibration is per-image (µm/px); the default is 1.0 µm/px, i.e. uncalibrated — set the real scale for every image, either by typing the µm/px at import or by deriving it with the 📏 scale-bar tool.

How it works

A test grid of known total length is drawn over the microstructure. You click every point where the grid crosses a real grain boundary (skip twins & scratches — that judgment is what makes the count defensible). The mean lineal intercept length is:

and the ASTM grain-size number G follows the E112 Table 6 relation for mean lineal intercept (ℓ in mm):

Higher G = finer grains (each whole number ≈ a doubling of grains per unit area). Negative G is valid for coarse grains (a computed G = −1 is ASTM designation “00”).

Two methods

• ◎ Abrams three-circle (primary, the E112 referee grid): three concentric, equally-spaced circles with diameters in the canonical 1 : 2 : 3 ratio (Fig. 5). Closed loops remove orientation bias and end-point ambiguity. A test-line/boundary tangency counts as 1; a triple-junction is scored as 2 (sanctioned by E112 §14.3.2.2).
• ▤ Heyn lineal: four horizontal + four vertical straight lines (≥4 orientations, §13.4). Simple and traceable.

Each method stores its own clicks per image, so you can measure a field both ways and compare.

Recommended practice (ASTM E112)

• 40–100 intercepts per field (§14.3.2.1); aim for a total of ~400–500 over ≥ 5 fields (§14.3.2). The N readout warns below 40.
• The summary reports mean ℓ, SD, 95% CI = t·s/√n (Student's t, Table 7) and %RA = CI/ℓ̄·100. Target %RA ≤ 10% (§15.6) — add fields until it's met.
• G is computed from the mean ℓ across fields, never by averaging G numbers (§18.7).
• Twin boundaries are ignored (§3.2.2). Two-phase: grain size = the matrix phase (§17.1); decide a rule and keep it.
• A valid CI needs ≥ 5 fields; fewer is flagged ⚠ in the table.

Keyboard

Z undo · D mark done & next · ←/→ prev/next image · 1/2 switch method · Ctrl+click multi-select · Shift+click select range (click first, Shift+click last) · Esc clear selection

References

• ASTM E112-13, Standard Test Methods for Determining Average Grain Size, ASTM International.
• H. Abrams, “Practical Applications of the Three-Circle Intercept Grain Size Method,” Met. Trans. (1971).
• ASTM E1382, Standard Test Methods for Determining Average Grain Size Using Semiautomatic and Automatic Image Analysis.
• G. F. Vander Voort, Metallography: Principles and Practice, ASM International.