Updated:
How to read the two numbers
Under ISO 898-1 (equivalent to JIS B 1051), the property class of carbon and alloy steel bolts is written as X.Y:
- First number × 100 = nominal tensile strength in MPa. Example: 8.8 → 800 MPa.
- Second number × 10 = yield-to-tensile ratio in percent. Example: 8.8 → yield ≈ 80% × 800 = 640 MPa.
- Yield strength is the load beyond which the bolt stretches permanently – the key number for joint design.
| Class | Nominal tensile (MPa) | Nominal yield (MPa) | Typical material | Typical use |
|---|---|---|---|---|
| 4.6 | 400 | 240 | Low-carbon steel | Light structures, simple fixtures |
| 8.8 | 800 | 640 | Medium-carbon steel, Q&T | General machinery – the most common grade |
| 10.9 | 1000 | 900 | Alloy steel, Q&T | Highly loaded joints, automotive, tooling |
| 12.9 | 1200 | 1080 | Alloy steel (e.g. SCM435), Q&T | Molds, machine tools, socket head cap screws |
The table shows nominal values for easy reference. ISO 898-1 minimum values are slightly higher – e.g. class 10.9: tensile ≥ 1040 MPa, yield ≥ 940 MPa; class 8.8 above M16: tensile ≥ 830 MPa. Use the full standard tables for design.
Stainless bolts: what do A2-70 and A4-80 mean?
Stainless fasteners follow ISO 3506 instead. A2 is the 304 family; A4 is the 316 family with molybdenum for better corrosion resistance. The number after the dash × 10 is the minimum tensile strength: -70 = 700 MPa, -80 = 800 MPa. A2-70 has a yield strength of about 450 MPa – clearly below a steel 8.8 bolt (640 MPa) – so it cannot replace 8.8, 10.9 or 12.9 bolts in load-bearing joints.
Choosing the matching nut
- 8.8 bolt → class 8 nut; 10.9 → class 10; 12.9 → class 12.
- A nut may be a higher class than the bolt, never lower – otherwise threads strip before the bolt reaches its load.
- Pair A2-70 bolts with A2-70 nuts to avoid galvanic corrosion between dissimilar metals.
Common mistakes
- Using 12.9 everywhere "to be safe": it is harder but less ductile, and sensitive to hydrogen embrittlement if electroplated without baking.
- Replacing a 10.9 bolt with stainless because "stainless is better": it resists rust, but its strength is much lower.
- Tightening by feel: high-grade bolts need the correct torque for their size and friction to reach the design clamp load.
When ordering, specify the standard (DIN 912, ISO 4762…), size M×L, property class, finish (black oxide, zinc, Dacromet…) and whether a mill test certificate is required. Duc Huy Intech supplies classes 4.6 to 12.9 and stainless A2/A4 to DIN, ISO and JIS, with certificates on request.