Metal Structure Evaluation Method Through Grain Size Analysis

Metal structures are polycrystalline bodies having crystal structures composed of many grains. Between grains, there is an area with distorted atomic arrangement, whose boundary is called a grain boundary.
Even with metals and alloys that are the same, their grains are arranged in a specific pattern by heat or other treatment as their atoms grow, resulting in a different grain boundary formation compared to before treatment. As grain sizes change, the mechanical properties also change. Therefore, material grain sizes changed by heat and other treatments are an important factor in evaluating mechanical properties.



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Visual comparison using standard diagrams: The engineer estimates the grain size number by visually comparing the metal structure sample magnified with a microscope against the standard diagram.


Visual comparison using grain size charts: A grain size chart for an ocular lens, in which an image of the grain size pattern is inserted, is used with a metallurgical microscope to enable visual inspection with both the magnified sample and the standard diagram in the field of view.


Common Grain size measurement specifications are:


  • ASTM E112 – Standard Test Methods for Determining Average Grain Size.
  • ASTM E407 – Standard Practice for Micro-etching Metals and Alloys.
  • ASTM A923 – Standard Test Methods for Detecting Detrimental Intermetallic Phase in Duplex Austenitic / Ferritic Stainless Steels.
  • ASTM E3 – Guide for Preparation of Metallographic Specimens.
  • ASTM E1181 – Standard Test Methods for Characterizing Duplex Grain Sizes.


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