Guide · Updated June 2026

What Is a Mill Test Certificate (MTC / EN 10204 3.1)? — Material Traceability for Stainless Steel Washers

A Mill Test Certificate (MTC) — also called a material test certificate, mill certificate, or inspection certificate — is a document issued by a metal producer that records the actual laboratory test results for a specific production heat (melt). It proves that the material in a delivery has measured chemical composition and mechanical properties, tied to an identified cast number. When a buyer specifies EN 10204 3.1, they are asking for the highest-confidence version of this document: one where the test data come from the exact material delivered and are validated by the manufacturer's authorised inspection representative, who is independent of the production department.

For stainless steel washers, an MTC is how you confirm that a batch stamped "316" actually contains the chromium, nickel and molybdenum that defines 316 — not a cheaper grade substituted somewhere in the supply chain.

1. The EN 10204 Certificate Types — What Each One Actually Means

EN 10204 is the European standard that defines the categories of inspection documents for metallic products. The four document types most frequently encountered in fastener and washer procurement are 2.1, 2.2, 3.1 and 3.2. They differ in two dimensions: whether the test data are specific to the delivered material, and who validates the certificate.

The key distinction between 2.2 and 3.1 is not who signs — it is whether the test data are tied to the specific delivered heat. A 2.2 document may contain real test results, but those results may come from a representative sample of the product type, not from the cast used for your order.

2. What an EN 10204 3.1 Certificate Contains

A properly issued 3.1 MTC for stainless steel coil or strip typically includes the following fields. These are worth understanding because a missing field is a sign the certificate is incomplete or possibly falsified.

Heat / Melt Number (Cast Number)

The heat number is the unique identifier of the steelmaking batch — the specific charge of molten metal from which the material was cast. All subsequent certificates, test reports, and delivery records should reference this number. The heat number is the foundation of traceability: it is what allows you to connect a finished washer back to the chemical analysis at the mill.

Chemical Composition

The certificate lists the measured content of each alloying element, typically expressed as weight percent. For stainless steel washer grades:

• 304 (UNS S30400 / 1.4301): Cr 18.0–20.0 %, Ni 8.0–10.5 %, C ≤ 0.08 %, Mn ≤ 2.0 %, Si ≤ 0.75 %, P ≤ 0.045 %, S ≤ 0.030 %
• 316 (UNS S31600 / 1.4401): As 304 plus Mo 2.0–3.0 %; Ni 10.0–14.0 %
• 316L (UNS S31603 / 1.4404): As 316 with C reduced to ≤ 0.03 % — the low carbon specification that prevents chromium carbide precipitation in weld heat-affected zones

The Mo column is particularly important: its presence at 2–3 % is what distinguishes 316 from 304. A washer labelled 316 with no molybdenum recorded on the MTC is a red flag.

Mechanical Properties

Yield strength (Rp0.2), tensile strength (Rm), and elongation at break (A) are the standard mechanical values for flat-rolled stainless. Hardness may also be reported. These values confirm the material was correctly processed (not over-cold-worked, not under-annealed) and give the buyer the data needed to verify conformance with the relevant product standard.

Standard / Grade Conformance Statement

The certificate states which standard the material was tested against — for example ASTM A240/A240M (flat-rolled plate/sheet/strip), EN 10088-2, or JIS G4304/G4305 — and the grade designation (e.g. 304, 316L, 1.4404). This is the baseline against which all the numerical values are assessed.

Authorised Signature

For a 3.1 certificate, the document must be signed or otherwise validated by the manufacturer's authorised inspection representative. This individual must be independent of the production department — EN 10204 is explicit on this point. A certificate signed only by a production manager does not meet 3.1 requirements.

3. Why Material Traceability Matters for Stainless Washers

Grade Substitution is a Real Risk

304 and 316 stainless look identical to the naked eye. Their surface finish, weight, and appearance after stamping are indistinguishable. The only way to know which grade a washer is — without testing — is through documented traceability to a melt that was chemically analysed. In supply chains with multiple intermediaries (mill → service centre → fastener manufacturer → distributor → buyer), material identity can be lost or deliberately misrepresented at any step. An MTC that carries a heat number through the chain, supported by PMI on the finished parts, closes that gap.

Discover that a "316" washer is actually 304 after it has been installed in marine hardware or a food-processing line, and the cost of replacement — plus any consequential damage from premature corrosion — typically far exceeds any savings on the original purchase price. See our 304 vs 316 stainless guide for a detailed comparison of corrosion performance in different environments.

Export Compliance and Audited Supply Chains

Many end-customer quality systems — ISO 9001, IATF 16949 for automotive, AS9100 for aerospace, PED for pressure equipment — require documented evidence that purchased materials conform to specification. An MTC is typically the accepted form of that evidence. Without it, a buyer cannot pass a supply chain audit, cannot demonstrate due diligence in a product liability situation, and cannot reliably trace a field failure back to a root cause.

HUIHUI's ISO 9001:2015 quality system (cert No. 74326Q00174R001) provides the documented, traceable process — from incoming material inspection, through stamping and dimensional inspection, to final shipment — that supports these requirements.

Full-Thickness, Full-Spec Material

An MTC also functions as a check against under-gauge coil — material rolled thinner than specified to reduce weight (and cost) per piece. Because washers are typically sold by count rather than weight, an under-gauge coil produces washers that pass a visual check but carry less material and may fall below dimensional tolerances for thickness. At HUIHUI, incoming coil is inspected to verify thickness against the certificate before production begins.

4. MTC vs PMI vs Certificate of Conformance

These three documents are frequently confused or used as if they are interchangeable. They are not — each answers a different question and they complement rather than substitute for each other.

MTC (Mill Test Certificate, EN 10204 3.1)

A paper record of what was measured at the mill when the heat was produced. It proves the material from that melt met specification at the time of manufacture. It does not prove that the parts in a specific box were made from that melt — that connection must be maintained through the production record and, if needed, confirmed by PMI.

PMI (Positive Material Identification)

A physical measurement on the actual finished parts — typically using a handheld XRF analyser. The XRF instrument measures the elemental composition of the metal surface in seconds and reports the grade. PMI answers the question "is this specific part the alloy it is supposed to be?" without relying on paperwork. It can detect grade substitution or mixing that an MTC cannot catch after the fact. The limitation is that XRF measures surface composition; it does not replicate the full laboratory analysis of a melt and has wider measurement uncertainty than a wet chemistry or spark OES analysis.

Certificate of Conformance (CoC)

A declaration — usually a single page — that states the goods conform to a named specification. It contains no test data and no heat reference. A CoC is appropriate for low-criticality commercial supply where a statement of conformity is sufficient, but it provides no traceability and no independent evidence. Under EN 10204, a CoC most closely resembles a 2.1 document.

For stainless washer procurement in regulated or audited supply chains, the appropriate combination is: an EN 10204 3.1 MTC (paper traceability from the melt) plus PMI on the finished parts (physical confirmation that the parts match the certificate). Either alone leaves a gap that a determined audit will find.

5. How to Request an MTC with Your Order

Requesting an MTC is straightforward — the key is specifying the requirement clearly at the time of order, not after shipment, because the certificate must accompany the delivery documentation and reference the same heat numbers.

• State the requirement in your RFQ or purchase order: "EN 10204 3.1 MTC required" or "Mill Test Certificate to EN 10204 3.1, covering chemical composition and mechanical properties."
• Specify the grade: 304, 316, or 316L — and the product standard if relevant (ASTM A240, EN 10088-2, etc.).
• State if PMI is also required: "XRF PMI on finished washers" or "PMI report with shipment."
• Request the MTC before or with the shipment, not after — once goods have been shipped, retrospectively obtaining a valid 3.1 certificate is difficult and the result may not be genuine.

HUIHUI sources coil from documented melts and can provide EN 10204 3.1 MTCs covering heat number, chemical composition and mechanical properties for 304, 316 and 316L orders. PMI is available on request. Both can be arranged at the quotation stage — simply include the requirement in your inquiry. Our full product range is available with these documentation options. See also the guides index for related technical articles.

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