Mill certificate versus test certificate: what each proves

A purchase order asks for "a test certificate with the lot" and the crate arrives with a mill certificate for the coil. Both sides believe the requirement was met, and neither document answers the question the buyer actually had: what did this lot of parts test at?

Mill certificate versus test certificate is a distinction of issuer and sample, not of format. A mill certificate is issued by the steel producer and describes the heat it made. A test certificate is issued by a laboratory or by the fabricator and describes the results of tests on a stated sample, which may be a piece cut from the delivered sheet or a part after forming and welding. The first travels with the material. The second travels with the evidence.

The difference matters at three points: when a customer audits the certificate against the drawing, when a part fails in service and the record has to show what was verified, and when a structural or railway specification names the document it will accept. This article sets out who issues each document, what each one proves, and which one to write into the purchase order for a given part.

What separates a mill certificate versus test certificate?

The issuer and the sampling basis. A mill certificate comes from the party that made the steel and reports results from the heat or lot it delivered. A test certificate comes from a party that performed a test, and its value depends on whether that party is independent, whether the sample is identified, and whether the test method is accredited.

The terminology is worth fixing because it causes most of the arguments. Buyers say "mill certificate", "mill test certificate", "MTC" and "MTR" for the same document. They say "test certificate" for two different things: an inspection certificate issued under EN 10204 or ISO 10474, and a laboratory report on a sample. Those are not interchangeable, and a purchase order that does not name the document type gets whichever one the supplier has on file.

The formal hierarchy under ISO 10474:2013 runs from a declaration of compliance, through a test report based on non-specific inspection, to an inspection certificate based on specific inspection, and finally to a certificate validated by an inspector independent of both the manufacturer and the purchaser. Each step buys more independence and costs more time.

The documents, who issues them, and what they prove

The table sets out the documents a sheet metal buyer will meet. The Ganesh column states what can travel with a lot from our works and what has to be arranged with an external party, so the requirement can be written before the order is placed.

The rows that get confused are the first and the last. A declaration of compliance is a statement, not a result. A certificate of conformity covers the part and the process, and it is worth having, but it says nothing about yield strength or chemistry.

What a mill certificate can and cannot prove

A 3.1 certificate proves that the delivered lot was tested and that the results meet the grade in the standard named on it. It ties a heat number to a set of tensile, chemical and sometimes impact results. That is exactly what a traceability requirement needs, and it is why the document is requested on structural work.

It cannot prove four things that buyers routinely assume:

• That the part is to grade after forming. Cold working raises yield strength and lowers ductility. A coil that tested at 28% elongation can be at 12% in a stretch-flanged corner.
• That the thickness in the crate matches the certificate. Thickness is a separate measurement, and coil thickness varies within the permitted band. Weighing a blank is the quickest check.
• That the weld is sound. The certificate says nothing about the heat affected zone, the filler, or the procedure.
• That the coating is correct. Zinc mass and paint film thickness are measured on the finished part, not on the steel.

That is the boundary between a material document and a part document. The mill certificate article covers how to read the fields on a 3.1, including the trap in an impact grade letter.

What a test certificate adds

A test certificate adds a sample. The laboratory receives a piece of known identity, tests it by a named method, and reports the result with the method number and, on an accredited report, the measurement uncertainty. The report is only as good as the sample identification: a report that says "CRCA, 2 mm" with no heat number, no part number and no date cannot be tied to the crate in front of the auditor.

Accreditation is the part buyers check least and should check first. An ISO/IEC 17025 report carries the laboratory's accreditation symbol, the test method, the sample description and the result. A report without the symbol is still a test, but it is not an accredited test, and a customer specification that says "accredited laboratory" will reject it.

The tests worth paying for depend on the part:

• Tensile test to ISO 6892-1 when the forming route depends on the actual properties of the coil rather than the grade minimum.
• Charpy impact when the specification names a test temperature, because the grade letter is a promise and the test is the evidence.
• Coating mass or thickness when the part is outdoor and the warranty depends on the zinc.
• Weld macro or hardness traverse when a procedure has to be qualified on the production thickness.
• Salt spray when a finish is being compared, remembering that hours in a cabinet rank finishes against each other and do not predict years outdoors.

How the sampling rule changes the answer

Sampling is where a certificate earns or loses its value. A tensile test is taken from a defined position in the product, and a heat may cover many tonnes. If the certificate reports one test for a 20 t heat and the delivered lot is 2 t, the result is still a 3.1 certificate for that lot, but the statistical confidence rests on the mill's sampling plan, not on the 2 t.

Charpy testing follows the same logic with a number attached: three specimens, an average of 27 J, and no single value below 70% of the average, which is 18.9 J. A certificate that reports only the average hides the scatter. When the part is cyclically loaded in cold service, ask for the individual values.

The practical rule is to match the sampling to the consequence:

• A non-safety bracket on a painted enclosure needs a 3.1 certificate and a thickness check.
• A structural member in a machine frame needs 3.1 plus the impact values if the drawing names them.
• A railway or pressure component needs 3.2, with the inspector's stamp on the document.
• A finish that carries a warranty needs a coating test on the finished part, because the coil certificate says nothing about the film.

What travels with a Ganesh lot

The certificate pack is assembled from the coil paperwork inward. A lot of CRCA or hot-rolled parts carries the mill's 3.1 inspection certificate with the heat number, and the works order records the heat number against the parts made from that coil. That link is what an auditor asks for: not the certificate alone, but the certificate plus the record that shows which parts used it.

Where the drawing or the customer specification calls for a third-party inspection, the 3.2 certificate is arranged with the external inspector before the lot is released, and the inspection time sits in the schedule rather than at the end. Where a laboratory test is required, the sample is cut, identified and sent to an ISO/IEC 17025 accredited laboratory, and the report is issued by that laboratory. We do not issue accredited test reports ourselves, and a quotation that implies otherwise is a quotation to check.

The works runs ISO 9001:2015 and ISO 14001:2015, with PPAP and first article inspection available where the programme requires them. The first article is a dimensional and visual record of the part, which is the document that proves geometry. It does not replace the material certificate, and the material certificate does not replace it.

What to write in the purchase order

A certificate requirement is only enforceable if it names four things:

1. The document type and standard, for example "inspection certificate 3.1 to ISO 10474" rather than "mill certificate".
2. The tests required, by method, for example tensile to ISO 6892-1 and Charpy at 0 °C.
3. The sample and the traceability link, including the heat number on the certificate and on the packing list.
4. The independence level, meaning whether the certificate has to be countersigned by a third-party inspector, and who arranges and pays for that inspector.

Add a fifth line for the parts that carry a finish: the coating test and the acceptance figure, measured on the finished part. A powder coating at 60–80 µm is a process window, and the certificate that proves it is a coating thickness report, not a steel certificate.

A purchase order that says only "test certificate required" will be satisfied by whatever document the supplier has, and that is how a 2.2 report ends up in a file that needed a 3.1. The standards and compliance hub collects the document definitions that sit around this one, including the certificate of conformity and the calibration chain behind the measurements.

When a certificate is standing in for a measurement

The most expensive misuse of a certificate is using it to avoid measuring the part. A material certificate says nothing about hole position, bend angle, flatness or weld distortion. A supplier who ships a certificate instead of a first article has proved the steel and left the geometry unproven.

The same applies to a certificate of conformity. It is a declaration that the parts were made to the drawing, and it is worth having for the shipment, but the evidence behind it is the inspection record. For a repeat order, the record that matters is the one showing the critical dimensions were measured on the lot, at the frequency in the control plan, with the gauge that was in calibration.

Where the part is welded, the document set widens again. A weld procedure qualification, a welder qualification and a visual or dimensional inspection record sit alongside the material certificate, and none of them substitutes for another. Welding inspection and traceability sets out how those records join up.

When neither document is the right requirement

A certificate is the wrong requirement when the risk is dimensional, when the finish is the warranty issue, or when the part is a prototype and the cost of the document exceeds the cost of the parts. In those cases the correct requirement is a first article inspection report, a coating test on the finished part, or a dimensional layout, and the certificate is an extra that proves less than the buyer thinks.

It is also the wrong requirement when the standard is named but the grade is not. "IS 2062 certificate" on a drawing that does not state E250A, B or C leaves the impact question open, and the certificate will be correct for whichever grade the mill supplied. The grade, the standard and the document type belong on the same line.

For a part that carries a structural or safety function, the sequence is: grade and standard on the drawing, document type in the purchase order, third-party inspector named, and the heat number carried through to the packing list. Send that requirement with the drawing to our engineering team and we will confirm which certificate will travel with the lot and which tests need an external laboratory.