CONSIDERING THE APPROPRIATENESS OF THE INTERNATIONAL STANDARDS FOR HARDNESS TESTS

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Metadata	Details
Publication Date	2017-03-28
Authors	Takashi Yamamoto
Institutions	Yamamoto Hospital

Abstract

Not only because ISO standards for hardness tests have a large impact on the revision of corresponding industrial standards, but also because of the recent tendency of ISO to tighten regulations, the authors investigated technically the appropriateness of some important ISO standards for hardness tests as follows . (1)Regarding the spherical tip geometry of a Rockwell diamond indenter, ISO 6508 Part 2 specifies that the conical angle shall be 120 ± 0.35° and the radius of curvature R shall be 0.200 ± 0.01 mm, and specifies a detailed method for measuring these dimensions. ISO 6508 Part 3 specifies tighter tolerances, or 120 ± 0.10° and 0.200 ± 0.005 mm, respectively, and also assumes using a standardized indenter. The results of a study made with commercially available diamond indenters on how the indenter tip, or the radius of curvature R, influences the hardness values obtained. According to the results, it can hardly be concluded that the radius of curvature R has a significant effect on hardness measurements even for the standard block of 60 HRC, which has a relatively shallow indentation. (2) A Vickers diamond indenter often becomes a subject of ISO discussions regarding its tip shape. Concerning the line of conjunction that tends to appear at the indenter tip, ISO 6507 Part 2 requires the length of the line to be between 0.5 µm and 2.0 µm, depending on the test load, and ISO 6507 Part 3 requires it to be between 0.25 µm and 1.0 µm, depending on the test load. Therefore, if the line of conjunction which has a length of ad is produced at the tip of a Vickers diamond indenter when the indentation’s diagonal length is d , the error in Vickers hardness will be ( d’/d ) 2 =1 +a 2 . Calculating the error term of the equation, you find that an error in Vickers hardness measurements attributable to the line of conjunction measuring one percent of the diagonal length d is only 0.01%. (3) It is proposed to use a Vickers or Brinell indentation made on a standard block as a “reference” when examining the measuring microscope. However, when you try to use contrast to observe the surface profile of a specimen with an optical microscope, the boundary between light and dark is subject to the brightness, the illumination method and the numerical aperture (NA), the aperture stop and focusing of an objective lens.

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