Scope
ISO 898-1 specifies the mechanical properties — tensile, yield, hardness, ductility, and specific Charpy / surface integrity tests — for bolts, screws, and studs in metric sizes manufactured from carbon and alloy steel. It does not specify dimensions; pair it with a dimensional spec like DIN 931 / ISO 4014 (hex head bolt, partial thread) or DIN 933 / ISO 4017 (hex head bolt, full thread).
The companion spec for nuts is ISO 898-2. For stainless steel fasteners, see ISO 3506.
How to read property class designations
A class designation like "10.9" encodes both tensile and yield strength in a simple format:
- First number × 100 = minimum tensile strength in MPa
- First number × second number × 10 = minimum yield strength in MPa
- The second number is also the ratio of yield to tensile (e.g. 0.9 = 90%)
So property class 10.9 means:
- Min. tensile = 10 × 100 = 1000 MPa (≈ 145 ksi)
- Min. yield = 10 × 9 × 10 = 900 MPa (≈ 130 ksi)
- Yield ratio = 90%
Once you know the rule, every callout self-decodes — there's no lookup table required.
The full property class table
| Class | Min. Tensile (MPa) | Min. Yield (MPa) | Min. Elongation | Hardness (HV) | Inch equivalent |
|---|---|---|---|---|---|
| 4.6 | 400 | 240 | 22% | 120–220 | A307 Grade B |
| 4.8 | 420 | 340 | 14% | 120–220 | — (cold-formed) |
| 5.8 | 520 | 420 | 10% | 130–240 | — |
| 6.8 | 600 | 480 | 8% | 190–250 | — |
| 8.8 | 800 | 640 | 12% | 245–335 | SAE Grade 5 |
| 9.8 | 900 | 720 | 10% | 290–360 | — |
| 10.9 | 1040 | 940 | 9% | 320–380 | SAE Grade 8 |
| 12.9 | 1220 | 1100 | 8% | 385–435 | (no inch equivalent) |
(Test temperature, sampling rules, and supplementary requirements — the bulk of the spec — are in the standard itself.)
When property class 12.9 matters
Property class 12.9 has no common inch equivalent. SAE Grade 8 lands at about 150 ksi tensile (≈ 1040 MPa, between classes 10.9 and 12.9). 12.9 is widely used in:
- Socket head cap screws (where head geometry doesn't compromise the joint)
- High-performance automotive engines and drivetrains
- Specialty industrial machinery requiring tight, compact, high-load joints
12.9 is not recommended for hot-dip galvanizing — hydrogen embrittlement risk is significant at that strength level. Use mechanical galvanizing (B695) or a zinc-flake system instead.
Marking
ISO 898-1 requires the property class to be permanently marked on the bolt head. The marking format is the same as the callout: 8.8, 10.9, 12.9. Smaller heads (M5 and below) may use a clock-face mark (one dot at the position corresponding to the property class). All bolts must also carry the manufacturer's identification mark.
Related specifications
- ASTM F568M — US-harmonized adaptation; mechanically interchangeable with ISO 898-1
- ISO 898-2 — Companion nut specification (covers nut classes 4, 5, 6, 8, 10, 12)
- ISO 3506 — Stainless steel mechanical properties (A2, A4, A5 grades)
- SAE J429 — Inch counterpart for SAE Grades 2, 5, 8