The Mill Test Certificate is the single most important document that travels with your pipe fittings. It is your evidence that the material is what you paid for. Here is how to read one — and how to spot a bad one.
3.1 vs 3.2: What the Numbers Mean
EN 10204 defines inspection document types:
- Type 3.1 — issued by the manufacturer’s own quality department, independent from production. This is the industry standard for fittings.
- Type 3.2 — countersigned by an independent third party (e.g. SGS, BV, or the buyer’s inspector) who witnesses the testing. Specified for critical projects, at extra cost and lead time.
The Sections That Matter
1. Heat number. The key to traceability. The heat number on the certificate must match the number stamped or stenciled on the fitting itself. No match = no traceability = reject the batch.
2. Material grade and standard. Check the exact designation: ASTM A234 WPB, not just “carbon steel”. Watch for lookalikes — WPB and WPL6 are different products.
3. Chemical composition. For A234 WPB, key limits include carbon ≤ 0.30% and manganese 0.29–1.06%. Values should sit comfortably inside the limits; numbers hugging the maximum on every element are a warning sign.
4. Mechanical properties. For WPB: tensile strength 415–585 MPa, yield ≥ 240 MPa, elongation per the standard. Heat-treated condition should be stated (e.g. normalized).
5. Dimensions and quantity. The certificate should list the actual sizes and piece counts of your order, not a generic range.
Red Flags We Check Before Shipment
- Heat number on paper doesn’t match the product marking
- Chemistry identical across multiple heats (copy-paste certificates)
- Missing heat treatment condition
- Certificate issued by a trader with no manufacturer name
- Scanned signature layered over obviously edited numbers
As a supplier we verify every certificate against the physical goods during pre-shipment inspection, and PMI (spectrometer) testing is available when a project demands extra assurance. If a seller hesitates to provide the MTC before you pay the balance — walk away.
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