The ASTM E112 test determines the average grain size of metallic materials principally of single phase which is extendable to non-metallic materials too. It uses three basic methods of ‘planimetric approach’, ‘intercept approach’, and ‘comparison method’.
The test procedures in ASTM E112 are efficient for all varieties of metallic materials and non metallic materials having structure similar to their metallic counterparts. These are primarily efficient for single phase grain structures. However, the test procedures can also be extended to a specific grain structure in multiphase specimens. The test methods have higher efficacy for two-dimensional grain sections when exposed in the sectioning plane.
The ASTM E112 test uses three basic methods in estimation of grain size of almost all metals. This also extends its measuring standards to structures bearing similar geometry with metallic structures in the comparison chart provided in ASTM E112. Planimetric approach – This suggests computing the number of grains per cross section in a predefined area NA for quoting the ASTM grain size number G. Intercept approach – This counts the number of grains being intercepted per unit length of a test line, thus giving the ASTM grain size number G. This method is especially recommended for structures departing from the uniform equiaxed form. Comparison method – This method does not involve any measurements of number of grain size or the intercepts but uses a predefined set of graded images either in the form of clear plastic overlays, wall chart or an eyepiece reticle. These are suitable for completely recrystallised or cast materials with equiaxed grains.
Three different methods in ASTM E112 require varying specimens. Planimetric approach – It needs specimens inscribing a circle or rectangle of known area (usually 5000 mm2 for simplifying the calculations) on a micrograph, a monitor or on the ground glass screen of the metallograph. Intercept approach – It involves using specimens on which line intercepts could be drawn to yield at least 50 intercepts. Comparison approach – Specimens are to be compared with the photomicrographs of the appropriate standard grain-size series, or with suitable reproductions or transparencies of them.
ASTM E112 test procedure gives the ASTM grain size G as determined by either of the three approaches. While in dispute, the data obtained by the planimetric method will be considered as referee. The measured values are all expressed in SI units. Enlisted equivalent inch-pound values might be approximate. Planimetric approach – Precision of this test is proportional to the number of grains counted. Optimally, the specimen which produces a magnification of about 50 grains within the test circle is a good choice. Comparison approach – Specimen analysis in this method estimates grain size as the ASTM grain size number G, or grain diameter Intercept approach – Grain size estimations shall be made on three or more representative areas of each specimen section.