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.
EN 10204 defines material test certificate types. For wind turbine main structural fasteners and pressure-bearing pipe clamp hardware, type 3.1 is mandatory — the results are certified by the manufacturer's authorised inspection representative, independent of production. Type 2.1 (works certificate, not independently verified) is accepted only for secondary hardware. Without a 3.1 certificate, the material heat cannot be traced back to a specific melt and test batch, which is a fundamental supply chain risk for critical joints.
- Best for
- Procurement teams specifying documentation requirements for wind turbine fasteners, pipe clamps, and clamp hardware destined for primary structural or pressure-bearing applications
- Not suitable for
- Using 2.1 certificates on structural bolts, blade root studs, or main hydraulic clamp bodies — most wind OEMs and certification bodies explicitly require 3.1 for these categories
- Selection steps
- 1 — Classify component (primary/secondary) → 2 — Specify EN 10204 3.1 for primary structural and pressure-bearing items → 3 — State on PO → 4 — Verify certificate format on receipt → 5 — Archive with heat number for traceability
- RFQ information
- Component type and application, MTC level required (2.1 or 3.1), heat number traceability requirement, filing and archiving requirement
§ 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.
Before release, use the clamp test-evidence guide to confirm what each certificate and report actually proves.
Evidence and decision boundary
- Direct evidence
- ISO 10474 defines inspection-document types, ISO 404 general steel delivery requirements and ISO 9001 the management system rather than each delivered part.
- Engineering inference
- Link drawing, revision, BOM, heat, processes, coating, dimensions, tests, deviations and release records to the physical batch.
- Typical or indicative value
- Certificate level and dossier content are indicative until the order defines the standard, validation and retention requirements.
Primary sources checked
- ISO 10474:2013 - steel inspection documents
- ISO 404:2013 - steel delivery requirements
- ISO 9001 quality-management overview
Related commercial route: Compare wind-turbine clamp systems and RFQ inputs.
For the related release decision, use Do Wind Turbine Clamps Need REACH, RoHS and SCIP Documents?.