What a Mill Test Certificate Actually Traces
September 27, 2026
A mill test certificate is the one document on a stainless order that nobody at the machine shop wrote. It comes from the steel producer, and that is exactly why it is worth reading rather than filing.
What the certificate actually is
When a mill melts a heat of stainless, it tests that specific melt and issues a certificate against it: the heat number, the chemical composition measured on that heat and the mechanical properties, usually tensile strength, yield strength, elongation and hardness, measured on a sample from it.
That certificate travels with the bar stock cut from that heat. A shop that buys a length of 316L round bar receives the mill certificate for the heat it was cut from, and when it machines your part from that bar, it can pass the same certificate to you with the heat number matched to your job traveler.
This is different from a specification. ASTM A276 for 316L bar gives a range for chromium, molybdenum, carbon and the rest. The mill certificate gives you the actual numbers measured on the heat your part came from, which sit somewhere inside that range, not at its edges by assumption. See the grade reference for the specification ranges themselves.
What to check on one
The heat number matches your parts. This is the entire point of the document. If the certificate shows a heat number that does not appear on your job traveler, packing slip or part marking, the certificate proves nothing about the parts in the box.
The grade and specification match your drawing. A certificate for 304 stock does not cover a 316L order, even when the numbers look close. Check the specification too: ASTM A276 bar and ASTM A479 bar are both round stock certificates but to different test methods.
Composition sits inside the specification range, not just close to it. Chromium, nickel, molybdenum and carbon are the ones that matter most for a 300-series grade. For a 316L part specifically, check carbon is at or below the L-grade limit, since that is the whole reason the L exists. See 316 against 316L against 316Ti.
Mechanical properties, if your drawing calls for them. Most machined-part drawings do not specify tensile or yield strength directly, because machining does not change them much on austenitic grades. A 17-4 PH part aged to a specific condition is the exception: check the certificate reflects the bar before ageing, since the shop’s own hardness test after heat treatment is the number that actually matters for that grade. See 17-4 PH machining conditions.
Who signed it, and against which standard. A certificate that cites EN 10204 names a type, usually 3.1, which means an independent body within the mill’s quality system issued it, not just the salesperson.
Where this comes up
Medical devices. A device file that requires heat traceability is asking for exactly this chain: heat number on the certificate, matched to a lot number on the part, matched to a record you can produce years later if a failure needs investigating. See ISO 13485 against ISO 9001 for when a device actually needs this level of paper.
Food and dairy equipment. Recall readiness is the underlying reason, the same logic as a food producer tracing an ingredient lot. See stainless for food contact.
Pressure and marine parts. A grade substitution error in a wetted, pressurised or safety-relevant part is the failure a mill certificate exists to prevent from being invisible.
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What to write in the RFQ
Mill test certificate required, heat number to match part marking. EN 10204 Type 3.1 acceptable.
That single line tells a shop the traceability is not optional and lets it price the sourcing accordingly, since not every stock length a shop holds comes with one. What to put in a stainless RFQ has the rest of the checklist a comparable quote needs.
A mill certificate is not proof your part is correct. It is proof of what melt it came from and what that melt measured. The shop’s own inspection is what proves the part itself is right. Ask for both when the part needs either.
For what a certificate adds to a quote and how to name its type, read the guide to a stainless steel machining quote.
Frequently asked questions
Is a mill certificate the same as a certificate of conformance?
No. A certificate of conformance is a supplier's statement that the part meets the order. A mill test certificate is the steel producer's own record of the melt, the heat number and the properties actually measured on that heat. Ask for the mill certificate when traceability matters, not a certificate of conformance from whoever cut the bar.
Can a shop supply a mill certificate for material it already had in stock?
Only if it kept the certificate that came with that specific bar and can match it to the heat number stamped or tagged on the stock. A shop buying remnant or unknown-origin bar cannot produce one after the fact, which is exactly the gap a certificate closes.
Do I need a mill certificate on every stainless part?
No. Most dry, indoor, low-consequence parts do fine without one. Ask for it when a device file, a food safety system or your own quality system requires heat traceability, or when a failure would need tracing back to a specific melt.
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Send the drawing. We read it, send one itemised quote within 24 business hours and place the work with a partner shop that cuts stainless.
Get a quote for your stainless partsFigures in this guide are published specification limits or published typical values for the alloy, not measurements of any part. Machinability is rated against free-machining carbon steel B1112 at 100 and PREN is chromium plus 3.3 times molybdenum plus 16 times nitrogen. The conventions are set out in thereference section and the editorial standards.