CNC部品の材料証明書：MTRとトレーサビリティ
Short answer: a material certificate (MTR, mill test report) is the mill's written proof of what your metal is — chemistry, heat number, mechanical properties and the standard it was made to. For CNC parts that go into medical, aerospace, food or safety-critical equipment, you should be able to trace every finished part back to one heat of material. A factory that cannot produce an MTR for your batch is not quoting you a material, it is quoting you a hope.
Most machined parts never need a certificate: a cosmetic bracket in 6061 does not need its heat number recorded. But the moment a part carries a load, touches a patient, flies, or feeds a production line where a wrong alloy means a recall, traceability stops being paperwork and becomes engineering. As an ISO9001:2015 factory in Dongguan machining parts for export daily, we buy material with certificates, keep heat records on the batches that need them, and attach the MTR to the shipment. This guide explains what an MTR actually contains, what traceability you can reasonably demand, and how to ask for it so you get a document you can trust.
What an MTR Actually Is
An MTR — mill test report, material test certificate, or EN 10204 3.1 certificate — is issued by the material producer, not the machine shop. It states the heat or lot number, the chemical composition in percent, mechanical properties from the actual test (yield, tensile, elongation, sometimes hardness), and the specification the material conforms to, such as ASTM B221 for aluminum or ASTM A276 for stainless bar. The key word is traceability: the heat number on the MTR links a bar of metal back to the ladle it was poured from.
| MTR element | What it tells you | Why it matters |
|---|---|---|
| Heat / lot number | Which production batch the metal came from | The anchor for all traceability |
| Chemistry (C, Cr, Ni, etc.) | Alloy composition in weight percent | Confirms grade, predicts weldability and corrosion |
| Mechanical properties | Yield, tensile, elongation | Proves the temper or condition you spec'd |
| Standard reference | ASTM, EN, GB number | Defines what "conforming" means |
| Mill and date | Who made it and when | Source record for audits and recalls |
A certificate without a heat number is a piece of paper, not traceability. A certificate that lists chemistry but no mechanical properties cannot prove your 6061 is actually -T6. Reading an MTR is mostly checking that the numbers match your specification — and our material certificates guide walks through the fields line by line with a real example.
When You Actually Need Traceability
Traceability costs money — a small premium on certified material and real work in record-keeping — so the engineering question is when it pays. Medical device parts: essentially always, because an implantable or patient-contact part must be traced to material and process records for regulatory reasons. Aerospace: always, usually with EN 10204 3.1 and full process documentation. Food and beverage contact, chemical processing, offshore: yes, because a wrong grade corrodes and fails. Safety-critical structural parts in any industry: yes. Everything else — enclosures, brackets, cosmetic covers, prototypes for fit testing — usually does not need per-heat traceability, and paying for it raises your BOM for no engineering benefit.
| Application | Certificate level typical | Batch traceability |
|---|---|---|
| Aerospace, medical implants | EN 10204 3.1 + full records | Heat-level, documented |
| Medical instruments, food contact | 3.1 or certified material | Heat-level |
| General industrial, structural | 2.2 or mill cert on request | Lot-level |
| Prototypes, cosmetic parts | None needed | None |
The honest factory tells you which tier your part actually sits in rather than selling you certificates you do not need. If you are not sure, ask: we will tell you when traceability is engineering and when it is decoration — the same straight answer we give on tolerances.
How a Factory Keeps Material Straight
Traceability is only as good as the factory's material control. The chain runs: buy certified material with an MTR → tag each bar or lot with its heat number on receipt → keep the tag or transfer the number when stock is cut → record which lot machined which job → attach the MTR and lot reference to the packing list. The break point in most shops is the middle: material arrives certified, then the tags fall off and "some 316" becomes "316." That is how a wrong alloy reaches a customer.
Ask a factory three questions to test their control. Do you buy from mills or from traders without certificates? (No certificate upstream means no certificate downstream, no matter what they promise.) How do you identify material in the rack — painted heat codes, tagged remnants, a stock ledger? And can you show the lot number that produced my specific batch of parts? A factory that answers with a system — not a promise — can trace your parts. One that answers vaguely will produce a certificate for "material like yours," which is a different thing entirely.
What You Should Put in Your RFQ
Traceability requirements belong in the RFQ, not discovered at delivery. Write three lines: material grade and condition (for example, 316L stainless, ASTM A276, solution annealed); certificate requirement (EN 10204 3.1 with heat number, or mill cert on request); and whether parts need batch-level marking or just a documented lot reference. If the part itself must carry identity — laser marking a heat code or serial on each piece — say so, because it changes machining flow and cost. If you only need the MTR in the shipment file, say that instead and keep the part clean.
Two more details buyers miss. First, certificates should match the actual quantity and size of material: a certificate for 20 mm bar does not cover parts machined from 30 mm bar even if the grade is identical, because they are different lots. Second, when the drawing calls a tight tolerance or a critical fit, ask whether the machinist records the actual measured values — that is dimensional traceability, delivered by the CMM report rather than the MTR, and our CMM inspection guide explains what a proper report contains. Dimensional records and material records are two halves of the same traceability story; the parts are only fully documented when both exist.
The Factory Side: What We Actually Do
In our Dongguan shop, material control is a standing process rather than a special request. Certified stock is received with its MTR, logged by grade and heat, and identified in storage; jobs that require traceability are machined from a recorded lot, and the MTR plus lot reference ships with the parts. The ISO9001:2015 system — described in our certification guide — is what makes this repeatable rather than dependent on one conscientious employee. When you ask us for material certificates, the answer is a document trail that matches your part numbers, not a scanned file of someone else's metal. That is the difference between paperwork and traceability, and it is exactly the difference your auditor or your customer will check.
Frequently Asked Questions
Q: What is the difference between an MTR, a mill certificate and EN 10204 3.1?
A: An MTR and mill certificate are the same document — the mill's test report for a specific heat. EN 10204 3.1 is the European standard tier that adds an independent inspection statement; 3.1 certificates are typical for aerospace and medical parts, while 2.2 documents conformance without independent inspection. For most industrial parts a standard MTR with heat number is sufficient.
Q: Can a CNC factory issue a material certificate itself?
A: No. Only the material producer can issue a genuine MTR, because only the mill tested the heat. A factory's job is to buy certified material, preserve the certificate, and link it to your specific parts through lot records. If a shop offers to "make" you a certificate, that is a red flag, not a service.
Q: Do I need material certificates for every CNC part?
A: No. Cosmetic brackets, enclosures and fit-test prototypes rarely justify the cost. You need traceability when a part is safety-critical, regulated (medical, aerospace, food contact), or where a wrong alloy causes failure. Ask your factory which tier your part needs and pay only for real requirements.
Q: How do I verify that a certificate actually matches my parts?
A: Check that the heat or lot number on the MTR matches the lot reference on the factory's material records and packing list, and that grade, size and standard match your RFQ. A certificate for a different bar size or a different supplier's stock is not traceability for your batch.
Q: How do I request material certificates when getting a CNC quote?
A: State the requirement in the RFQ: grade and condition, certificate level (for example EN 10204 3.1 or mill cert), and whether parts need individual marking. Send the request with your drawing to sc@bquq.com or WhatsApp +86 13713157787, and the quotation will itemize any certificate cost and confirm the material control plan within 12 working hours.
Related Resources
- Material certificates guide — reading MTR fields line by line with a worked example.
- CNC machining and turning services — certified-material machining with per-batch inspection records.
- About BQUQ — an ISO9001-certified source factory in Dongguan with controlled material purchasing and storage.
- Contact us — send drawings and certificate requirements for a quote within 12 working hours.
Authored by the BQUQ Engineering Team. BQUQ is an ISO9001-certified source factory in Dongguan, China, running CNC machining, metal stamping, custom springs, heat sink and collet lines under one roof. Send drawings to sc@bquq.com or WhatsApp +86 13713157787 for a quote within 12 working hours. www.bquq.com
