Conversion tables for hardness numbers

Summary

• 1. The accuracy of conversion of hardness numbers from one scale to the other cannot be evaluated from the mean square errors of the measurements; we must take into account the zone of scatter of the hardness values which is due to the nonequivalence of the relationships examined and to the influence of overlooked systematic errors. The accuracy of conversion can be estimated from the mean square deviations σ_Δ of the results of individual series of measurements from the averaged curve.

• 2. An investigation of 12 conversion tables compiled by different authors indicates the presence of systematic errors in the compilation of each of them. Hence, none of the tables has any advantage over the others and none can be recommended. Their union into a single conversion table is equally impossible.

• 3. Conversion of hardness numbers from one scale to another is permissible for a tentative estimate of the properties of metals, but not for accurate work, and is only permissible for tables compiled for a restricted range of metals and alloys used in a particular branch of industry.