Why Hardness Matters
Hardness testing is the quickest way to verify a material’s heat treatment and estimate its tensile strength without destroying the part. In practice, you’ll encounter three scales constantly: Brinell (HB), Rockwell B (HRB), and Rockwell C (HRC).
Conversion Table — Steel
These conversions are approximate and apply to carbon and alloy steels. They’re based on ASTM E140.
| Brinell (HB) | Rockwell C (HRC) | Rockwell B (HRB) | Approx. Tensile Strength (ksi) |
|---|---|---|---|
| 614 | 58 | — | — |
| 587 | 56 | — | — |
| 547 | 53 | — | — |
| 495 | 50 | — | 260 |
| 461 | 48 | — | 243 |
| 415 | 45 | — | 220 |
| 375 | 40 | — | 200 |
| 341 | 37 | — | 180 |
| 311 | 33 | — | 163 |
| 285 | 30 | — | 149 |
| 262 | 27 | — | 137 |
| 241 | 23 | 99 | 126 |
| 217 | 20 | 97 | 116 |
| 197 | — | 93 | 105 |
| 179 | — | 89 | 96 |
| 163 | — | 86 | 87 |
| 149 | — | 81 | 80 |
| 137 | — | 77 | 74 |
| 121 | — | 71 | 66 |
| 111 | — | 65 | 60 |
Which Scale to Use?
| Scale | Range | Best For |
|---|---|---|
| Rockwell C (HRC) | 20–70 HRC | Hardened steels, tool steels, heat-treated parts |
| Rockwell B (HRB) | 60–100 HRB | Soft steels, brass, aluminum, copper alloys |
| Brinell (HB) | 90–650 HB | Castings, forgings, raw stock — widest range |
Quick Conversion Rules of Thumb
- Tensile strength (ksi) ≈ Brinell × 0.5 — rough but useful for quick estimates
- HRC 20 ≈ HB 240 — the crossover point where Rockwell C becomes reliable
- HRC < 20: Don’t use Rockwell C — readings are unreliable below this range, switch to HRB or Brinell
Common Hardness Values
- Mild steel (1018, hot rolled): ~130 HB / 72 HRB
- Medium carbon (1045, normalized): ~180 HB / 89 HRB
- 4140 quench & temper: 28–34 HRC (typical spec)
- Tool steel (D2, hardened): 58–62 HRC
- Grade 8 bolt: 33–39 HRC
- Case-hardened surface: 58–63 HRC
Related: Machinability of Metals | AISI/SAE Steel Numbering System
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