Question

Provide a summary of the ASTM E112–12 standard’s definition of the grain size number G. Your summary should give the precise mathematical definition for G. Your summary should also supply a conceptual description of G; this description should not require significant technical background to understand. • Provide a summary of the Lineal Intercept Procedure described in the ASTM E112–12 standard. Your summary should give sufficient detail that someone could use it to compute the grain size number G without any other information.

Overall what exactly is G? What is ASTM E112-12? What is the lineal intercept procedure?

Answer 1

The grain size number G is defined mathematically by the formula, ;

where N_AE is the number of grains per square inch at 100x magnification; G is the ASTM grain size number.

A metal structure is an aggregate of 3D crystals of varying sizes and shapes. Even if all these crystals were identical in size and shape, the grain cross sections, produced by a random plane through such a structure, would have a distribution of areas varying from a maximum value to zero, depending upon where the plane cuts each individual crystal. Clearly, no two fields of observation can be exactly the same.

Linear intercept procedure:

You can estimate the average grain size by counting the number of grains intercepted by one or more straight lines.

Make counts first on three to five blindly selected and widely separated fields to obtain a reasonable average for the
specimen.

An intercept is a segment of test line overlaying one
grain. An intersection is a point where a test line is cut by a grain boundary. Either may be counted, with identical results in a single phase material. When counting intercepts, segments at the end of a test line which penetrate into a grain are scored as half intercepts. When counting intersections, the end points of a test line are not intersections and are not counted except when the end appears to exactly touch a grain boundary, when 1/2 intersection should be scored. A tangential intersection with a grain boundary should be scored as one intersection. An intersection apparently coinciding with the junction of three grains should be scored as 1 1/2 .With irregular grain shapes, the
test line may generate two intersections with different parts of the same grain, together with a third intersection with the intruding grain. The two additional intersections are to be counted.