By "surface finish" do you actually mean surface texture or surface roughness? Technically, the term "surface finish" means some type of surface coating or treatment like painting, plating, anodizing, etc.
Also, what is the specific method of hardness testing? Some micro hardness tests use very small probes/loads and evaluating the results optically requires a very smooth surface at the test location.
but, wiki syas "Surface finish of the part and the indenter do not have an effect on the hardness measurement, as long as the indentation is large compared to the surface roughness. "
for 30T, what Ra needed?
thanks.
Specimen Surface Preparation. The degree of workpiece surface roughness that can be tolerated depends on the Rockwell scale being used. As a rule, for a load of 150 kgf on a diamond indenter, or 100 kgf on a ball indenter, a finish ground surface is sufficient to provide accurate readings. As loads become lighter, surface requirements become more rigorous. For a 15 kgf load, a polished or lapped surface usually is required.
Surfaces that are visibly ridged due to rough grinding or coarse machining offer unequal support to the indenter. Loose or flaking scale on the specimen at the point of indenter contact may chip and cause a false test. Scale should be removed by grinding or filing. Decarburized surface metal must also be removed to permit the indenter to test the true metal beneath.
ASTM E18-14 does not list a minimum surface finish for test pieces other than to state that "both the test surface and the bottom surface of the test piece should be smooth, even and free from oxide scale, foreign matter, and lubricants." The standard does state an Ra of 0.00005mm for indenter balls and <0.003mm for test blocks.
Note that the process used to obtain a given surface finish (e.g. shot peening, machining) can very well change hardness at the surface if you are measuring using microhardness (Knoop, Vickers) to measure. This may be useful information.
