For a load-bearing wind turbine bolt, the marking on the head tells you what the bolt claims to be. The mill test certificate is what proves it. A correctly specified EN 10204 3.1 certificate ties each batch of fasteners back to a tested heat of steel — and without it, you are trusting a stamp rather than a measurement.
§ 01 What a mill test certificate is
A mill test certificate (also called a mill test report, MTR, or inspection certificate) is a document issued by the manufacturer that records the actual measured properties of the delivered product: the chemical composition of the steel heat and the mechanical test results — tensile, yield, hardness, and where relevant impact. It is the evidence that the fasteners meet the property class declared on them, such as class 10.9 to ISO 898-1.
§ 02 The EN 10204 document types
EN 10204 defines the types of inspection document a manufacturer can issue. The number tells you who tested the product and how independent that testing was:
| Type | Based on | Validated by | Traceable to batch? |
|---|---|---|---|
| 2.1 | Declaration of compliance | Manufacturer (no test results) | No |
| 2.2 | Test report | Manufacturer (non-specific tests) | No |
| 3.1 | Specific test results | Manufacturer's authorised inspector (independent of production) | Yes |
| 3.2 | Specific test results | Manufacturer + independent / customer inspector | Yes |
The key dividing line is between the "2.x" documents (general statements, not tied to your specific batch) and the "3.x" documents (specific test results, traceable to the actual delivered items). For wind turbine structural fasteners, 3.1 is the normal minimum requirement; 3.2 adds a second, independent witness and is specified for the most critical applications or where a customer or third party must co-sign.
§ 03 What a 3.1 certificate contains
A complete 3.1 certificate ties the physical fastener back to tested values. Expect to see:
- Heat / cast number — the identifier linking the bolt to a specific batch of melted steel.
- Chemical analysis — the alloy composition of that heat (C, Mn, Cr, Mo, etc.).
- Mechanical results — measured tensile, yield/proof, elongation, hardness; impact values if required.
- Standard and property class — e.g. ISO 898-1 class 10.9.
- Coating / process data — where relevant, plus the inspector's stamp and signature.
§ 04 The traceability chain
Traceability means an unbroken paper trail from the installed bolt back to the steel mill. In a wind project this chain typically runs: steel heat → fastener manufacturing lot → 3.1 certificate → delivery / batch marking → installation record. If a problem is ever found — a coating fault, a property out of spec, a field failure — traceability lets you identify exactly which other bolts came from the same heat and lot, scope the issue, and act on it.
This is why traceability is not paperwork for its own sake: it is the mechanism that turns a single discovered defect into a bounded, manageable batch recall instead of an unknown fleet-wide risk. It complements the structural-bolt documentation required under systems like EN 14399 and ASTM A490.
§ 05 What to require in an RFQ
To make sure you receive usable documentation, state it explicitly at enquiry stage:
- Specify "EN 10204 3.1" (or 3.2 for critical lots) in the RFQ notes — do not assume it is included by default.
- Require the heat number to appear on the certificate and be linkable to the delivered batch.
- Ask for certificates to be supplied with the shipment, not on later request.
- For coated bolts, require the coating and (if applicable) friction-class data on or alongside the certificate.
A supplier that handles this routinely is a good signal of quality control overall — see how to choose a wind fastener supplier.