The Dangers of Using Intercept Methods for the Characterize Non-unimodal Grain Structures

It is well-known that grain size has an impact on mechanical properties of materials. Therefore, it is critical that the proper method is used to when performing a metallographic characterization of grain size. The ASTM Committee F42 on Additive Manufacturing Technologies’ Subcommittee F42.01 Test Methods Work Item # 85120 purposes a method for assessing grain size distribution unique to the Additive Manufacturing (AM) Industry. The F42 .01 method is being proposed even though there are ample methods for characterizing grain sizes and grain size distributions under the jurisdiction of ASTM E04 Committee on Metallography’s subcommittees E04.08 Grain Size and E04.14 Quantitative Metallography. The test methods available under E04.08 are E112-25 Standard Test Methods for Determining Average Grain Size, E930-18 Standard Test Methods for Estimating the Largest Grain Observed in a Metallographic Section (ALA Grain Size), E1181-02(2023) Standard Test Methods for Characterizing Duplex Grain Sizes; and under E04.14, E1382-97(2023) Standard Test Methods for Determining Average Grain Size Using Semiautomatic and Automatic Image Analysis.

A copy of the purposed F42 test method was received, reviewed, and tested on several images. Four images were provided by the F42.01 subcommittee, and another was taken from published article that provided planimetric grain size distribution data. This presentation carefully walks through the proposed new test method; the results it yields and contrasts those results with image analysis results based on planimetric grain size measurements which reveals the dangers in using the wrong grain size measurement method and how it will mischaracterize a grain size distribution.