Question 1

Which hardness scale should I specify on engineering drawings?

Accepted Answer

Use HRC for hardened steels (heat-treated parts, tools, springs) above 20 HRC. Use HB for softer steels, castings, and non-ferrous metals below 350 HB. Use HV for thin sections, case depths, and when a wide range is needed on a single scale. Always specify the scale and test conditions.

Question 2

Are hardness conversions exact?

Accepted Answer

No. ASTM E140 conversions are empirical approximations valid only for carbon and alloy steels. Accuracy degrades for stainless steels, non-ferrous metals, and at extreme ranges. The tables have ±2 HRC typical uncertainty. For critical specifications, test directly in the required scale rather than converting.