Recently, hardness measurement has gained importance as an inspection to guarantee castings produced in foundries, because it is rapid and usually non-destructive. However, only a few study which deal with the conversible relationship of various hardness measurements for gray cast iron have been published up to date.
  This study is a basic investigation on the conversible relationship of hardnesses in gray cast iron which may be utilized in the hardness inspection and on some problems associated with various hardness measurements in gray cast iron. The test method which was employed for this study, was H_B (Brinell hardness), H_S (Shore hardness) and H_RB (Rockwell hardness; B scale) based on JIS. The specimens used for this study were produced by using 15 kW high frequency induction furnace, and their carbon equivalent range fell between 3.5 and 4.4. The hardness range of the specimens used was from H_R 90 to H_B 250.
  The results of the experiment are summarized as follows :
  (1) It was confirmed that hardness value of gray cast iron is mainly influenced by the dimensions and testing site of the specimens, casting quality, and testing conditions.
  (2) The variation in individual hardness value in gray cast iron is greater in the relatively soft iron, and it decreases with increasing hardness.
  (3) In order to increase the accuracy of hardness measurement, the surface of the specimens should be polished with No. 320 emery polishing paper for H_B and H_S testing, and No. 120 emery paper is recommended for H_RB testing.
  (4) The hardness conversion between gray cast iron and plain carbon steel is very different. Therefore, when the conversion value for plain carbon steel is applied to gray cast iron, considerable conversion error is caused due to the difference in quality.
  (5) The following regression equation was calculated by the computer for hardness correlation between H_B and H_S, as well as B_B and H_RB.
  Correlation between B_B and H_S :
      H_B=−104.7+8.483H_S
  Correlation between H_B and B_RB :
      H_B=−213.8+13.052H_RB−0.18733H_RB²+0.001026H_RB³

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