Positive Material Identification (PMI) lets us confirm the grade of every incoming batch of metal against the purchase specification before it enters stock. It is a check over and above paperwork, labels and dimensional inspection.
How do you tell a super duplex stainless steel from an austenitic stainless steel, or a nickel alloy from a copper alloy, when they all look much the same? That is the problem faced across the supply chain. Kathy Stubbs, Group Technical & Quality Manager, explains our approach to material identification and traceability.
"To ensure material traceability, product conformance and, ultimately, the safety of people and equipment, we run PMI on all incoming stock material. Our PMI analysers are pre-loaded with our alloy specifications, so we can quickly determine the product grade and verify compliance with the purchase requirements. More than 99% of the time the results give us no cause for concern, but occasionally we identify incoming material that does not appear to be the correct product. This extra layer of checking lets us challenge the supplier confidently.
A worked example
Another supplier had offered some Aluminium Silicon Bronze to DefStan (Defence Standard) 02-879 Part 2 Annex D. The material arrived with conforming test certification. During a routine PMI analysis, however, we identified that it was not Aluminium Silicon Bronze but Nickel Aluminium Bronze, a very different alloy.
A repeat tensile test would not have revealed the non-conformance, because the two grades have very similar mechanical properties. Only a chemical analysis or a magnetic permeability test would confirm conformance, and neither is routinely carried out by many suppliers.
Aluminium Silicon Bronzes are non-magnetic, with a magnetic permeability of < 1.05, whereas Nickel Aluminium Bronze has a magnetic permeability of 1.50.
The supplier challenged the validity of our results, but the combination of PMI and magnetic permeability testing gave robust evidence, and the material was rejected. Supply to our own customer was delayed as a result, but that was far preferable to inadvertently supplying the wrong alloy for a critical application.
Investment in PMI detectors and a robust goods-inward process is not glamorous, but it delivers guaranteed traceability through our quality management system."