PMI Testing

Positive Material Identification (PMI) has become more prevalent and more practical in recent years. The development of portable x-ray fluorescence (XRF) units using x-ray tubes instead of isotopes has made it easier to verify alloys both at the time of final packaging and shipment, as well as at receiving and in the field.

Portable optical emission spectroscopy (OES) instruments have also become affordable, which permits analysis of even lower molecular weight alloying elements like carbon. However, the most accurate analyses of carbon, nitrogen and sulfur are still obtained by combustion techniques.

Testing protocols range from performing a spot check on a lot of material, to obtaining a complete chemistry for every piece. While many of these instruments have exceptional accuracy for portable hand-held units, they cannot match the accuracy of more sophisticated stationary units found in most test laboratories or at steelmaking facilities. These limitations should be taken into consideration when establishing acceptance criteria.

ElementsLimit or Maximum of Specified Range, %Tolerance Over the Maximum Limit or Under the Minimum Limit
Carbon to 0.010, incl 0.002
over 0.010 to 0.030, incl 0.005
over 0.030 to 0.20, incl 0.01
Chromium over 10.00 to 15.00, incl 0.15
over 15.00 to 20.00, incl 0.20
Nickel over 15.00 to 20.00, incl 0.10
over 15.00 to 20.00, incl 0.15

The variation in chemical analysis testing has always been recognized in the standardization of test methods. ASTM and AMS specifications establish the permitted variation over or under the specified limits for each element when conducting a product analysis. In ASTM this information can be found in standards such as A480 (Flat Products) or A484 (Bar). AMS specifications 2248 and 2269 detail similar allowances. The allowances are necessary based on interlaboratory testing variations of tests conducted to the same test methods. These allowances were established for the more sophisticated stationary instruments that offer the highest degree of accuracy. They do not incorporate variations that can exist within a heat of steel from one location to another. Table 1 shows some permitted variations for a few element ranges as they exist in ASTM A480.

Most modern XRF units also generate a standard deviation calculation which offers a guide as to how precise the measurement might be. Multiplying the standard deviation by six (six sigma) will give an indication of the range of values expected for the reading 93% of the time. This wide amount of potential variation in the reading often means that an acceptable chemistry might show up as being outside the acceptable range. For this reason alone, portable instruments should be used only to confirm the alloy type and should not be used as a basis for rejection.

There are many other reasons for the variation of results in chemistries when testing in the field. Part geometry and surface condition can be factors. In stainless and nickel alloys there can be minor surface variations due to normal processing such as annealing and pickling. Portable instruments do not penetrate below this surface layer which might be marginally reduced in chromium or molybdenum, or other elements. It may be necessary to remove several thousandths from the surface to get a more reliable indication of the base metal chemistry. This effect can also be avoided by taking the reading on a freshly cut edge or end.

The reported results also need to be adjusted for interference from other elements. As the alloy content changes, the interference on the reading from other elements also changes. The portable instruments have sophisticated algorithms that compensate for much of this interference, but these instruments are designed to work on a very wide range of alloys and compositions. Stationary laboratory instruments can be recalibrated using standards specific to the composition being examined to improve accuracy. Laboratories in producing mills are the most familiar with compensating for these variations with their alloys, which is why even independent laboratories occasionally have difficulty matching the mill's accuracy.

Rolled Alloys has portable XRF units in each of our facilities and we perform hundreds of tests every day. These instruments are used to verify the alloy identity. Acceptance or rejection of material on the basis of chemical analysis should only be done using certified laboratories with the most sophisticated equipment and techniques for the element in question. In case of a dispute or disagreement, the producing mill should always be involved.

Please contact Rolled Alloys Quality Assurance if you have questions about our PMI procedures or capabilities.