Technical data: mechanical properties, tolerances and coatings
The numbers a purchasing engineer or a QC inspector actually wants: property class requirements, thread and dimensional tolerances, coating thickness and corrosion performance, and the hydrogen embrittlement rules that apply to high-strength plated fasteners.
1. Mechanical properties — bolts, screws and studs (ISO 898-1)
| Property class | Nominal tensile Rm, min | Lower yield ReL, min | Elongation A, min | Typical core hardness | Notes |
|---|---|---|---|---|---|
| 4.8 | 400 MPa | 320 MPa | 14 % | — | General purpose, no hardness band specified |
| 5.8 | 520 MPa | 400 MPa | 10 % | — | Common for anchors and general fixing |
| 8.8 | 800 MPa | 640 MPa | 12 % | ≈ 250–320 HV | The structural workhorse; quenched and tempered |
| 10.9 | 1,040 MPa | 940 MPa | 9 % | ≈ 320–380 HV | Alloy steel; electroplated versions require de-embrittlement |
| 12.9 | 1,220 MPa | 1,100 MPa | 8 % | ≈ 385–435 HV | Highest strength; impact tested from M16; avoid hot-dip galvanizing |
| A2-70 / A4-70 | 700 MPa | 450 MPa | 0.4 d | — | ISO 3506-1; A2 = 304 family, A4 = 316 family |
| A4-80 | 800 MPa | 600 MPa | 0.3 d | — | Higher strength stainless; slightly reduced ductility |
Values are the minimum requirements of the named standards for the stated property class. Acceptance is against the mill test certificate of each production heat — request the MTC for the actual lot, and treat the figures above as the specification, not as our inspection result.
2. Mechanical properties — nuts (ISO 898-2)
| Nut style | Property class | Proof load stress | Matched bolt class |
|---|---|---|---|
| Style 1 (DIN 934, height 0.8 d) | Class 8 | 800 MPa | 8.8 |
| Style 1 (DIN 934) | Class 10 | 1,040 MPa | 10.9 |
| Style 1 (DIN 934) | Class 12 | 1,150 MPa | 12.9 |
| Also available — style 2 / higher nuts | Class 10 / 12 | Higher proof load for the same thread size | Where stripping must be excluded, e.g. high preload |
Matching rule: class 8 nuts with 8.8 bolts, class 10 with 10.9, class 12 with 12.9. A stronger nut does not make a weaker bolt safe — the joint fails where the design puts the load.
3. Thread and dimensional tolerances
| Item | Standard value | Comment |
|---|---|---|
| External thread tolerance | 6g (standard), 6f on request | Coating thickness can push a 6g thread towards the 6h limit — a real issue for hot-dip galvanized nuts, solved by over-tapping or by using galvanizing with a specified clearance |
| Internal thread tolerance | 6H (standard), 6G on request | Hot-dip galvanized nuts are normally tapped oversize to suit the coating |
| Thread pitch, coarse (standard) | M8 × 1.25 · M10 × 1.5 · M12 × 1.75 · M16 × 2 · M20 × 2.5 · M24 × 3 | Fine pitch available on request (e.g. M16 × 1.5) for vibration or thin-wall applications |
| Product grade / mechanical requirements | Grade A (d ≤ 24 mm), B (d > 24 mm), C (general) | Grade affects dimensional tolerance, not strength — quote both, as drawings often mix them |
| Marking | Property class + manufacturer mark, required from M5 upward | Head marking is the primary identity of the part; specify if you need your own mark plus class |
| Thread inspection | Go / no-go gauges (6g / 6H) | Reported in the dimensional inspection record |
4. Surface finish and corrosion performance
| Finish | Standard | Typical coating thickness | Corrosion indication | When to specify it |
|---|---|---|---|---|
| Zinc plated, clear, trivalent | ISO 4042 | 5–12 µm (8 µm typical) | Neutral salt spray (ISO 9227) to first white rust typically tens of hours — always verified by test report | Dry indoor, general machinery, cost-sensitive |
| Zinc plated, yellow, trivalent | ISO 4042 | 5–12 µm | Better than clear passivation; still not a substitute for hot-dip galvanizing outdoors | Mild outdoor exposure, industrial interiors |
| Hot-dip galvanized | ISO 1461 | Local minimum ≈ 45 µm (typically 50–70 µm) | Evaluated by coating mass / thickness, not by salt spray — HDG is the outdoor and structural standard | Structural steel, solar, anchors, marine-adjacent, 10–25 year exposure |
| Zinc flake (Dacromet / Geomet) | ISO 10683 | 4–10 µm | Long neutral salt spray without white rust for many systems; no hydrogen embrittlement risk of the electroplating type | Automotive, high-strength classes 10.9 / 12.9, tight tolerance joints |
| Stainless, passivated | ASTM A967 | N/A | Corrosion resistance set by the alloy, not a coating | A2-70 / A4-80 where media, salt or hygiene preclude coatings |
| Black oxide, oiled | — | < 1 µm | Minimal corrosion protection; appearance and light oil retention only | Machine tools, tooling, sockets where tolerance is critical |
Salt-spray hours are system-dependent: thickness, passivation, sealing and thread geometry all move the number. Treat any quoted figure as needing a test report against the specific finish you are buying.
5. Hydrogen embrittlement — the rule that saves high-strength orders
Electroplating plus high strength equals embrittlement risk
Property classes 10.9 and above (and 12.9 always) must not be electroplated and shipped without an agreed de-embrittlement treatment. Typical practice under ISO 4042 / ASTM B850: bake within a few hours of plating, commonly 190–230 °C for four hours or more, with the cycle documented. Hot-dip galvanizing and zinc flake carry a materially lower risk, but for 12.9 the safest recommendation remains to avoid hot-dip galvanizing altogether.
- What we supply: process records including the plating date, bake start time and duration for any electroplated 10.9 / 12.9 order.
- What you should ask for: a hydrogen embrittlement statement per lot, plus a bending or proof-load test where the application is safety critical.
- Where it bites: a bolt that passes every dimensional check and fails in service weeks later — embrittlement is not visible on the part.
6. Torque and preload — indicative values
| Size (class 8.8, zinc plated, dry) | Indicative tightening torque |
|---|---|
| M8 | ≈ 25 N·m |
| M10 | ≈ 49 N·m |
| M12 | ≈ 86 N·m |
| M16 | ≈ 210 N·m |
| M20 | ≈ 410 N·m |
| M24 | ≈ 710 N·m |
Indicative only, assuming a friction coefficient of roughly 0.14 and approximately 65 % of yield preload. Lubrication, coating, washer type and joint stiffness change the right value substantially. For structural connections, follow the project specification and the assembly's declared K-class — not a catalogue table.
Frequently asked questions
Can you supply 12.9 bolts hot-dip galvanized?
It is technically possible but we advise against it for safety-critical joints: the zinc bath temperature interacts unfavourably with high-strength alloy steel. For 12.9 we recommend zinc flake (ISO 10683) or black oxide, or a change of property class.
Why does my hot-dip galvanized nut not fit the bolt?
Thick zinc changes effective thread dimensions. The usual answer is over-tapped nuts or a controlled clearance class. It is a design decision made before ordering, not something to fix with force on site.
Do your salt spray claims come with reports?
Yes — coating thickness and neutral salt spray reports are issued per production lot on request through the same laboratories that test our mechanical properties.
Need these values confirmed for your specification?
Send the standard, property class, size range and coating — we reply with price, the document list and lead time within 48 hours.