How Hardness Conversion Works
Hardness is how well a material resists being pushed in by a sharp object. Different scales use different indenters and loads, so a reading on one scale does not match any other scale directly. Conversion tables let you line up readings between scales. The most common table is ASTM E140 for carbon and alloy steels.
The Five Main Scales
Rockwell C (HRC) uses a diamond cone at 150 kg for hard steels. Rockwell B (HRB) uses a steel ball at 100 kg for soft steels. Brinell (HB) uses a large steel ball at 3000 kg for castings and bulky parts. Vickers (HV) uses a small diamond pyramid for anything. Knoop (HK) uses a long diamond for coatings and thin layers.
Why Conversions Are Not Exact
The scales measure slightly different things. HRC measures depth of penetration. HB measures indent surface area. HV measures indent diagonal. On the same part, two scales can give readings that convert within a few percent of each other. The tables are empirical, built from thousands of matched measurements on known steels.
Pro tip: For tight drawing specs, pick one scale and stick with it. Specifying "45 HRC" is cleaner than "428 HB or 446 HV". The shop measures in the scale you specified, so conversion error does not stack up.
Hardness to Tensile Strength
For plain carbon and low-alloy steels, tensile strength in psi is about 500 times the Brinell hardness. So a steel at 250 HB has roughly 125,000 psi tensile. This rule works best between 150 and 400 HB. Do not use this rule for stainless, aluminum, brass, or any non-ferrous metal.
Which Scale Should I Specify?
For hardened steel parts above 20 HRC, specify HRC. For annealed steels, brass, and bronze below 100 HRB, specify HRB. For large castings, forgings, and stock, specify HB. For thin coatings, case depth, or microhardness, specify HV or HK. Always add the load (e.g., HB 3000 kgf) when using Brinell.