We were called out to do some testing of some real world materials with a big inspection company (not just CRMs, like our imposter competitors want you to believe).
One sample of interest was a piece of L-grade stainless. We shot it at the spot labeled “Location 1” in the picture. The result was about 0.03% carbon. The client said it is supposed to be 0.01% according to their spark OES unit. So we ground the spot around Location 1 again, re-shot, and once again got about 0.03% carbon. We then shot our CRMs and obtained the expected results, about 0.014% on a 316L CRM, about 0.025% on a 304L CRM, etc. Shot the Location 1 again, and got 0.031% again. Repeatable, but not explainable…yet.
The OES result of 0.01% carbon was obtained at the region called Location 2. The OES burn is no longer visible because we decided to grind that area, and test the same spot as the OES tested. Now with the Z-200 LIBS we obtained 0.009% on the first analysis, and we obtained 0.010% on the next. Here, repeat shots with the handheld LIBS gave 0.009% carbon and 0.011% carbon. The two LIBS burn marks are visible in the figure. Now we’ve got excellent agreement with the OES results, once we tested the same location.
So what’s the story? If you look at the material carefully, part of it is slightly gold tinted color, rather than the usual silvery metal. We also did a test (not labeled by an arrow) in a different part of the sample, on the “golden colored” area. Again the carbon result was closer to 0.03%. Why the difference? As it turns out, the area around Location 1 had been cut with a tungsten carbide blade, thus we believe depositing carbon deep into the metal matrix and creating the high carbon result. LIBS results near the OES burn area were repeatedly in the 0.01% range in agreement with the OES result. Repeated tests in the discolored area were repeatedly between 0.025-0.035% carbon.
This is a great example of how important not only is sample prep, but also understanding what else has been done with the material in question. Had the owner of the sample not remembered the material had been cut with a carbide blade, it could have easily been misclassified as straight grade rather than L-grade material. Once we tested the area that had not been contaminated by the saw blade, we repeatedly measured the same carbon value (within 10 ppm) of the OES.
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