1. Sample surface
The surface of the sample shall be smooth and flat without oxide skin, decarburization layer and dirt. The roughness of the sample surface will result in lower test value and higher data dispersion.
For forgings and castings with unsmooth surface, the surface of the workpiece shall be polished smooth with a portable grinder and sandpaper before testing.
2. Sample thickness
The specimen shall be of sufficient thickness, otherwise the hardened deformation area at the bottom of the specimen indentation will spread to the contact surface with the anvil, causing deformation of the specimen support surface and inaccurate test results. Chinese standard stipulates that different scales or test conditions shall be selected for samples with different thicknesses, and there shall be no visible deformation trace on the back of the sample after the test. For Rockwell hardness test, for diamond indenter, the sample thickness shall not be less than 10 times of the residual indentation depth; for ball indenter, the sample thickness shall not be less than 15 times of the residual indentation depth; For Brinell hardness test, the thickness of the sample shall not be less than 8 times of the indentation depth.
3. Curved specimen
There is a certain deviation between the hardness value measured on the curved sample and that measured on the plane sample. Therefore, a correction value shall be added or subtracted from the test results. A correction value shall be added when testing on the convex cylinder surface, and a correction value shall be subtracted when testing on the concave cylinder surface.
The Brinell hardness test shall not be conducted on the curved surface. For the curved surface sample, the test part shall be polished flat (the size of a coin is enough), and then the test shall be conducted.