ISO 898-1 is the international standard that defines what a steel bolt's "grade" actually guarantees. When a wind turbine drawing calls out a class 10.9 bolt, it is ISO 898-1 that turns those two numbers into measurable, testable mechanical requirements — and that a mill certificate is written against.
§ 01 Scope of ISO 898-1
The full title is "Mechanical properties of fasteners made of carbon steel and alloy steel — Part 1: Bolts, screws and studs with specified property classes." It applies to fasteners with coarse and fine pitch threads, diameters up to 39 mm (with guidance beyond), made of carbon or alloy steel. It does not cover stainless or non-ferrous fasteners — those fall under ISO 3506.
It is part of a family: Part 1 covers bolts/screws/studs, while ISO 898-2 covers nuts. A correctly specified joint references both.
§ 02 What the standard specifies
For each property class, ISO 898-1 sets minimum (and where relevant maximum) values for a defined set of mechanical and physical properties. The most important are:
- Tensile strength (Rm) — the ultimate strength of the bolt material.
- Yield / 0.2% proof strength (Rp0.2) — the onset of permanent deformation.
- Proof load — a load the bolt must carry with no measurable permanent set.
- Hardness — a range, both minimum (strength) and maximum (to limit embrittlement risk).
- Elongation and impact — ductility and toughness, ensuring the bolt yields rather than shattering.
The two-number code is constructed directly from these: first digit × 100 = Rm in MPa; product of digits × 10 = yield in MPa. The mechanics of reading it are covered in what the bolt property class means.
§ 03 The property class table
| Class | Rm min (MPa) | Rp0.2 min (MPa) | Hardness HV (approx.) |
|---|---|---|---|
| 4.6 | 400 | 240 | 120–220 |
| 5.6 | 500 | 300 | 155–220 |
| 8.8 | 800 | 640 | 250–320 |
| 10.9 | 1000 | 900 | 320–380 |
| 12.9 | 1200 | 1080 | 385–435 |
Classes 8.8 and above are quenched and tempered alloy steels. For wind turbine structural connections, 10.9 is the standard choice; the reasons it is preferred over 12.9 are set out in Grade 10.9 vs 12.9 bolts.
§ 04 Testing and marking
The standard prescribes how each property is verified — tensile tests on full-size bolts or machined specimens, proof-load tests, hardness tests, and (for higher classes) impact testing. It also fixes the marking: the property class and a manufacturer's identification mark must be stamped on the head of bolts of suitable size.
For load-bearing turbine bolting, the marking alone is not sufficient evidence — the heat must be backed by an EN 10204 3.1 inspection certificate tying the delivered batch to actual test values. Structural bolting systems formalise this further: see EN 14399 vs ASTM A490.
§ 05 Nuts and the assembled set
A bolt is only as strong as its weakest mating part. ISO 898-2 defines nut property classes (8, 10, 12) so the nut can carry at least the proof load of the matching bolt. The rule is simple: the nut class must equal or exceed the bolt class number — a class-8 nut on a 10.9 bolt under-rates the joint and can strip before the bolt reaches its design load.
In practice, specify the complete set — bolt class, nut class, washer where required, coating, and 3.1 documentation — so the assembly performs as designed. ISO 898-1 gives you the language to do that precisely.