Hardness & surface roughness
Approximate conversion tables for steel and the ISO 1302 roughness grades.
Hardness conversion (steel, approximate)
| HRC | HV | HB | Rm ≈ (N/mm²) |
|---|---|---|---|
| 20 | 238 | 226 | 760 |
| 25 | 266 | 253 | 850 |
| 30 | 302 | 286 | 970 |
| 35 | 345 | 327 | 1,100 |
| 40 | 392 | 371 | 1,250 |
| 45 | 446 | 421 | 1,430 |
| 50 | 513 | 481 | 1,640 |
| 55 | 595 | – | 1,900 |
| 60 | 697 | – | – |
| 62 | 746 | – | – |
| 64 | 800 | – | – |
| 65 | 832 | – | – |
| 66 | 865 | – | – |
| 68 | 940 | – | – |
Per ASTM E140 / ISO 18265 for unalloyed and low-alloy steels. Conversions are approximate; HB is not valid above ~ 50 HRC. Rm ≈ 3.2 × HV.
Surface roughness grades
| Grade | Ra (µm) | Ra (µin) | Rz ≈ (µm) | Typical process |
|---|---|---|---|---|
| N1 | 0.025 | 1 | 0.1–0.18 | Lapping, superfinishing |
| N2 | 0.05 | 2 | 0.2–0.35 | Lapping, superfinishing |
| N3 | 0.1 | 3.9 | 0.4–0.7 | Polishing, honing |
| N4 | 0.2 | 7.9 | 0.8–1.4 | Fine grinding, honing |
| N5 | 0.4 | 15.7 | 1.6–2.8 | Grinding |
| N6 | 0.8 | 31.5 | 3.2–5.6 | Grinding, fine finishing |
| N7 | 1.6 | 63 | 6.4–11.2 | Finish milling / turning |
| N8 | 3.2 | 126 | 12.8–22.4 | Milling, turning |
| N9 | 6.3 | 248 | 25.2–44.1 | Rough milling / turning |
| N10 | 12.5 | 492.1 | 50–87.5 | Roughing |
| N11 | 25 | 984.2 | 100–175 | Sawing, rough turning |
| N12 | 50 | 1,968.5 | 200–350 | Flame cutting, forging |
Rz is roughly 4 to 7 × Ra for machined surfaces; the ratio depends on the process.
Values are guide values for orientation. Always check them against the machine, clamping and workpiece. No liability for results.