Engineering reference
Bolt Property Class — Mechanical Properties (ISO 898-1)
Tensile, yield, hardness (HV / HRC) and elongation for each metric strength class — the hardness targets we hit when we harden fasteners to class.
How to read this
The class number encodes strength: for 8.8, tensile Rm ≈ 8×100 = 800 MPa and yield ≈ 0.8×800 = 640 MPa. Hardness (Vickers/Rockwell) is the shop-floor proxy we control during heat treatment to land the class. Values are ISO 898-1 minimums; the exact grade and hardness are confirmed per drawing and lot.
| Class | Tensile Rm (MPa) | Yield (MPa) | Vickers HV | Rockwell HRC | Elong. A (%) |
|---|---|---|---|---|---|
| 4.6 | 400 | 240 | 120–220 | — | 22 |
| 4.8 | 420 | 340 | 130–220 | — | 14 |
| 5.6 | 500 | 300 | 155–220 | — | 20 |
| 5.8 | 520 | 415 | 160–220 | — | 10 |
| 6.8 | 600 | 480 | 190–250 | — | 8 |
| 8.8 | 800 | 640 | 250–320 | 22–32 | 12 |
| 9.8 | 900 | 720 | 290–360 | 27–36 | 10 |
| 10.9 | 1040 | 940 | 320–380 | 32–39 | 9 |
| 12.9 | 1220 | 1100 | 385–435 | 39–44 | 8 |
Nut classes (e.g. 8, 10, 12) are proof-load-matched to bolt classes 8.8, 10.9, 12.9 — a class-8 nut suits an 8.8 bolt.
Reference minimums only. High-strength classes (10.9, 12.9) are hardness-controlled and hydrogen-embrittlement sensitive after plating — see the hydrogen-bake tool.