Hardness conversion chart
These conversions come from ASTM E140 and apply to carbon and alloy steel. They are approximate by definition: each scale measures a different thing, so a conversion is a correlation from test data rather than a formula.
Tensile strength in the last column is an estimate from Brinell, using the long-standing rule that ultimate tensile strength in ksi is roughly half the Brinell number. Do not use it for design.
| Rockwell C | Brinell (HB) | Vickers (HV) | Approx tensile (ksi) | Approx tensile (MPa) |
|---|---|---|---|---|
| 65 | 739 | 832 | - | - |
| 60 | 654 | 697 | 327 | 2,255 |
| 55 | 560 | 595 | 280 | 1,930 |
| 50 | 481 | 513 | 241 | 1,660 |
| 45 | 421 | 446 | 211 | 1,455 |
| 40 | 371 | 392 | 186 | 1,280 |
| 35 | 327 | 345 | 164 | 1,130 |
| 30 | 286 | 302 | 143 | 985 |
| 25 | 253 | 266 | 127 | 875 |
| 20 | 226 | 238 | 113 | 780 |
Rockwell B for softer metals
Rockwell C uses a diamond cone under 150 kgf and is meant for hardened steel. On anything softer than about 20 HRC the indentation is too deep for the reading to mean much, and you switch to Rockwell B, which uses a 1/16 inch ball under 100 kgf.
| Rockwell B | Brinell (HB) | Vickers (HV) | Typical material |
|---|---|---|---|
| 100 | 240 | 253 | Cold-worked alloy steel |
| 95 | 216 | 227 | Medium carbon steel, normalised |
| 90 | 185 | 195 | Hot-rolled bar |
| 85 | 163 | 171 | Mild steel plate |
| 80 | 145 | 152 | Low carbon sheet |
| 70 | 117 | 123 | Annealed low carbon |
| 60 | 95 | 100 | Soft brass, dead soft steel |
Which scale to use
Rockwell C for hardened and tempered steel, tool steel and case-hardened parts above about 20 HRC. Rockwell B for mild steel, annealed stock, brass and aluminium alloys.
Brinell suits castings, forgings and coarse-grained material, because the 10mm ball averages over a large area and is not thrown off by one hard grain. It leaves a visible dent, so it is not for finished surfaces.
Vickers works across the whole range from soft aluminium to carbide, which is why it is the reference scale in laboratories and the one the other scales are correlated against. The indent is tiny, so it also works on thin case layers and small parts.
