Mill certificates and material traceability: what the paper proves

What a mill certificate is

A mill certificate is a document from the material producer that states the grade, the chemical composition and the mechanical properties of a heat or lot. It is the standard way to show that the metal met the specification before it was cut, formed or machined.

It is not a certificate for the finished part. It describes the material at the mill, before the supplier's process. The gap between the two is where most traceability problems live.

The certificate is also a commercial document. It is used in disputes, in audits and in safety cases. A missing or vague certificate can hold up a shipment even when the material is fine.

The certificate types

EN 10204 is the European standard for inspection documents for metallic products. The type number tells you who issued the document and whether it contains test results. The phrase mill certificate can mean different things in different industries, so read the type, not the title.

Type 2.2 is often mistaken for 3.1 because both contain numbers. The difference is that a 2.2 test may not have been taken from the delivered item, only from the same heat or lot. For a critical part, that is not enough.

Reading the certificate

Start with the grade and the condition. A 6061 aluminium supplied in T6 is not the same as the same alloy in O condition, even though the chemistry looks similar. Check the standard named on the certificate against the standard on the drawing.

Then read the heat number, the chemical composition and the mechanical results. Check the units, the test direction and the location of the test. A tensile result from a longitudinal specimen does not describe the transverse direction. A hardness value without a scale is not a result.

Check the date and the issuer as well. A certificate that is older than the stock, or issued by a trader rather than the mill, needs a question. The name on the letterhead should match the producer named in the chain.

Heat, lot and coil

A heat is a single melt. One heat can be cast into several ingots, rolled into many coils and slit into several strips. A lot is the quantity treated as one unit for inspection or delivery, and it may combine parts of a heat or several heats.

The certificate usually follows the heat. The coil tag and the packing list connect the heat to the delivered goods. If the supplier cuts a length from a coil and does not record the coil number on the remnant, the chain breaks at the saw.

What it does not prove

A certificate does not prove that the bar in the crate came from the heat on the paper. That link depends on the supplier's goods in controls, the labelling and the lot records. A certificate can be correct and still be attached to the wrong box.

It also does not prove the heat treatment of the finished part, the dimensions, the plating or the weld. If the supplier heat treats after machining, the mill condition is not the final condition. Those steps need their own records and, where the risk justifies it, their own tests.

A certificate can also be genuine and still be irrelevant. If the drawing calls for a different condition, a different test or a different standard, the paper does not close the requirement. Read the drawing first, then the certificate.

Incoming inspection and verification

At goods in, check the grade, the condition, the size and the quantity against the purchase order. Check the heat number on the certificate against the tag on the material. Keep a copy with the receiving record, and note any discrepancy before the material is issued.

Where a mix-up would be costly, verify the alloy on the part. Positive material identification can separate alloy families, and a hardness check can confirm a temper. These are screening checks, not a full chemical analysis, but they catch the common errors.

Sampling is not a substitute for the paper trail. A chemistry check on one piece does not prove the rest of the lot, and a certificate does not prove the piece. Use both when the risk is high.

Building the traceability chain

Traceability is a chain of records, not a single document. The purchase order names the grade. The certificate arrives with the goods. The goods in record links the heat to the lot. The route card carries the lot through cutting, forming and finishing. The shipping record links the finished part to the lot.

Mark the remnants as carefully as the raw stock. A cut piece with no lot number cannot be traced back, and a remnant returned to stock without a label can contaminate the next job. The label is part of the process, not an office task.

Digital records help, but the label on the material still matters. A barcode that only exists in the ERP system is no use at the saw. Print the lot number on the tag and on the route card, and make the operator confirm it at the point of use.

A buyer checklist

Ask for the certificate with the first article and with each lot where the requirement applies. Check that the certificate type matches the purchase order, that the grade and condition match the drawing, and that the heat number matches the tag.

Keep the certificate, the receiving record and the route card together. If a question arrives months later, the pack should let someone follow the part from the heat to the shipment without a search through email.

• Certificate type and issuer.
• Grade, condition and standard.
• Heat number against the tag.
• Chemistry and mechanical results within the specification.
• Heat treatment record, if the supplier performed it.
• Lot number on the route card and the shipping record.