MTC vs. CoC: Why a Certificate of Conformance Is Not a Mill Test Certificate
Every quarter, somewhere, a buyer discovers that the "certificate" backing their pressure-system material is a Certificate of Conformance — a one-line declaration — rather than a Mill Test Certificate with actual heat-specific results. The material may be fine. The legal position is not.
What each document is
- CoC (Certificate of Conformance): the supplier's statement that the material "conforms" to the specification. No test results, no heat data, sometimes no heat number. Under EN 10204 it corresponds to type 2.1/2.2. It is a promise.
- MTC (Mill Test Certificate / Mill Test Report): the actual measured chemistry and mechanical properties of the specific heat your material came from, validated by the mill's independent inspection department (3.1) or a witnessed third party (3.2). It is evidence.
When a CoC is acceptable
For non-critical, non-pressure, non-safety applications where the specification allows it — architectural uses, general fabrication, prototypes. Many buyers accept CoCs for commercial-grade material and the practice is legitimate when everyone understands what they hold.
When it absolutely is not
- Pressure equipment under ASME, PED or GB codes — 3.1 is the floor; many EPC specs demand it contractually.
- Safety-critical service — subsea, aerospace, nuclear-adjacent: 3.2 witnessed is common.
- Anywhere failure analysis matters. If a part fails in service, the MTC is the document investigators reach for. A CoC tells them nothing and leaves you holding the liability.
The tell-tale signs on the document
A real 3.1 MTC lists: heat number, full chemical analysis with measured values, mechanical test results (tensile, yield, elongation, hardness where specified), the standard and revision, product form and dimensions, and the inspection department's signature. If your document is missing measured values, it is not a 3.1 — whatever the filename says.