A stamping says what someone believes the metal is. Positive material identification is the check on that belief, and it is the last line of defence before a wrong alloy reaches a weld or a pressure boundary.
PMI is easy to oversell. It is routinely described as “proving the material is correct,” when what it actually proves is narrower: that the surface composition is consistent with a named grade. The distinction matters more than it sounds, because the failure mode PMI is meant to catch — a grade mix-up — usually leaves a perfectly readable mark and a perfectly wrong composition.
What the specification actually requires
ASME SA-961, the common requirements standard for steel flanges, forged fittings, valves and parts, carries an explicit PMI supplementary requirement. Reading it precisely is instructive, because it tells you what the standards body considers the risk to be.
S60.1 states that forgings shall receive positive material identification “to ensure that the purchaser is receiving forgings of the correct material grade prior to shipment.” The stated purpose is narrow and specific: to ensure that no material grade mix-up has occurred during the manufacturing and marking of the forgings.
Note what that clause is aimed at. It is aimed at the manufacturing and marking chain — the point where a bar gets mis-fed, a heat gets split, or a stamp gets pressed onto the wrong end. It is not aimed at verifying a mill test report, and it is not a substitute for one.
The three requirements that are easy to miss
| Clause | Requirement | Why it changes practice |
|---|---|---|
| S60.2 | PMI examination in accordance with ASTM E1916 | Names a specific practice standard, so the method is defined rather than ad hoc |
| S60.3 | Quantity examined shall be 100 % of the forgings | Not a sample. Every forging is examined. |
| S60.4 | All forgings not of the correct material grade shall be rejected | A wrong result is not a concession item |
| S60.5 | Method of forging marking after examination to be agreed | Re-marking after PMI is a contract decision |
The 100 % requirement in S60.3 is the one most often lost in translation. A sampled PMI programme is not an S60 programme. Sampling belongs to a different kind of inspection, and if you are buying to SA-961 with S60 invoked, every forging gets examined.
Method capability: what each technique can and cannot read
PMI is not one test. It is a family of techniques with very different sensitivities, and the choice determines which elements you learn about and which you remain guessing at.
| Method | Elements read reliably | Blind to | Typical field role |
|---|---|---|---|
| XRF (handheld alloy analyser) | Cr, Ni, Mo, Mn, Si, Cu, Nb, and most alloying additions | Carbon, nitrogen, and low-level sulphur/phosphorus | Default first-line screen for austenitic and nickel alloys |
| Optical emission spectrometry (spark) | Carbon and the full alloy matrix including light elements | Local surface only; requires a clean conductive spot | Workshop confirmation where carbon matters |
| Handheld XRF with carbon capability | Adds an estimated carbon figure | The carbon figure is inferred, not measured directly | Separating L from H, or 304 from 316 in urgent cases |
| Density / hardness spot checks | Coarse alloy-family discrimination | Cannot separate neighbouring grades within a family | Supplementary only |
| Laser alloy identification | Broad alloy-family identification | Not a substitute for composition measurement | Rapid sorting of mixed stock |
Read the second column carefully against the third. Handheld XRF is the workhorse of field PMI, and it is genuinely good at the elements that distinguish stainless families from each other. It is blind to carbon. That single limitation drives most of the interesting failures.
The failure mode this creates: the L grade problem
The clearest example comes from SA-312 chemistry itself. Compare the carbon limits and the alloying additions as printed in SA-312 Table 1:
| Grade | UNS | Carbon max % | Chromium % | Nickel % | Molybdenum % |
|---|---|---|---|---|---|
| TP304 | S30400 | 0.08 | 18.0–20.0 | 8.0–11.0 | — |
| TP304L | S30403 | 0.035 | 18.0–20.0 | 8.0–13.0 | — |
| TP316 | S31600 | 0.08 | 16.0–18.0 | 10.0–14.0 | 2.00–3.00 |
| TP316L | S31603 | 0.035 | 16.0–18.0 | 10.0–14.0 | 2.00–3.00 |
Now look at what a handheld XRF sees. TP304 and TP304L have identical chromium and overlapping nickel ranges — the L grade actually permits a higher nickel ceiling. TP316 and TP316L are worse: chromium, nickel and molybdenum ranges are identical. The only difference between them is carbon, and carbon is exactly what XRF cannot measure.
So a field PMI result that reads 17% Cr / 12% Ni / 2.5% Mo is equally consistent with TP316 and TP316L. The instrument has done its job correctly and the answer is still ambiguous. Anyone who treats a passing XRF reading as proof that a pipe is the L grade is reading past what the method can support.
Two further traps sit in the same table. TP304L and TP316L carry a footnote in SA-312 Table 1 raising the carbon maximum to 0.040 % for small outside diameter or light wall — so a specimen measuring 0.038 % carbon is still conforming. And the stabilised grades, TP321 and TP347, are identified by titanium and niobium levels that are XRF-visible, which is why a stabilised grade is a much easier PMI target than an L grade.
Why PMI exists at all: the marking chain is the weak link
ASME SA-999 sets out the general marking and certification requirements that make PMI necessary in the first place. The certification clause requires the manufacturer or supplier to furnish a certificate of compliance, dated and identifying the certifying organisation — and that organisation is responsible for the contents whether or not it actually signed the document. The test report must then carry a defined list: heat number, heat analysis, product analysis where specified, tensile properties, bend or flattening test results, hydrostatic test pressure, nondestructive electric test method and impact results.
Each of those is a link. A heat number transcribed wrongly, a bar loaded into the wrong press, a stamp reused on a second heat, a certificate covering a lot that was split — each breaks traceability without breaking anything visible. PMI is the physical check that the metal in your hand corresponds to the designation on its stamp, at the moment before that metal enters the fabrication sequence.
This is why S60.5 requires the marking method after examination to be agreed between manufacturer and purchaser. If PMI runs before marking, the marking is unverified. If it runs after, the marking reflects a confirmed composition. The order matters, and the clause leaves the choice to the contract rather than to habit.
Practical field procedure
A defensible PMI routine on receipt of stainless or nickel alloy pipe or forgings looks like this:
- Verify before you test. Confirm the mark against the packing list first. A test on an unidentified component produces a result you cannot attach to anything.
- Measure the full alloy matrix, not one element. Cr alone cannot separate 316 from 304 reliably across a wide composition field. Read Ni and Mo together.
- Treat carbon results with the method that produced them. If carbon is inferred rather than measured, say so on the report.
- Escalate to a carbon-capable method when L versus H matters, and confirm by a method independent of the first.
- Check the alloying additions that confirm stabilising. Titanium and niobium readings distinguish TP321 and TP347 from unstabilised grades and from each other.
- Correlate to the certificate. PMI confirms the metal; only the certificate confirms the test results. Record both references on the same line.
- Photograph the stamp before it is removed by grinding, cutting or welding. The mark is evidence and it is destroyed by the first operation that follows.
What PMI is worth on the order
Put plainly: PMI proves the alloy family is present on the surface you measured. It does not prove heat treatment, it does not prove mechanical properties, it does not prove the product came from a qualified source, and for L versus H grades it does not prove carbon at all. It is a powerful, cheap, fast check that catches the single most expensive category of error — the wrong alloy in the system — and it is a poor substitute for the certificate package that supports every other claim.
For our own material reference, the chemistry behind this article is set out on the stainless pipe and tube range page, and the Incoloy 800/800H seamless pipe page shows how a nickel alloy resolves against a handheld analyser. If you want PMI arranged as part of a supply, or the certificate package to go with it, send the grade list to our contact page or WhatsApp +86 186 2137 7760 and we will price it.
