ASTM E112 Grain Size Analyzer

What is ASTM E112 Grain Size Analysis?

ASTM E112 (“Standard Test Methods for Determining Average Grain Size”) is the definitive standard for measuring grain size in metals and alloys. Grain size directly affects mechanical properties — yield strength, hardness, ductility, and fatigue resistance all depend on it. The Hall–Petch relationship predicts that finer grains produce stronger metals.

Materials scientists, quality control engineers, and metallurgists use grain size measurements for incoming inspection, process control, failure analysis, and compliance with specifications in aerospace (AMS), automotive (IATF 16949), and nuclear (ASME) industries.

How the Intercept Method Works

The Heyn intercept method is the most objective of the three ASTM E112 approaches. A set of straight test lines of known total length is placed on the micrograph. Each grain boundary crossing is counted. The mean intercept length is:

where L is the total test line length (corrected for magnification) and N is the total number of boundary intersections. The ASTM grain size number G is then calculated from:

where ℓ is in millimeters. A higher G number means finer grains. Most engineering alloys fall between G = 1 (coarse) and G = 14 (ultrafine).