Positive Material Identification, or PMI, is an analysis technique used to verify the material chemistry of metal alloys.
The surface of the alloy is excited through electrical stimulation and the resulting waves of the light spectrum are analyzed. Each segment of the spectrum holds many indications of the elements present; these lines are referred to as element spectral lines. When a spectrometer looks at these element spectral lines the instrument can determine what percentage of each element is present in the material due to the intensities of the spectral lines.
Ensuring the alloy falls within the designated specifications is significant as the alloy mix determines properties such as tensile strength, weight load, wear capabilities, and weld compatibility.
The end use of the material will determine the specific grade of alloy and more specifically the exact mix of each alloy component needed for that end-use application. PMI provides verification through the metal process from smelting through fabrication to ensure the proper alloy is used for a given application. Traceability through the process via PMI testing provides companies with verified results they can pass along through the supply chain.
What PMI Is Used For
- To answer questions about a part which is part of an assembly or is just too large for shipping
- When material identification or documentation can no longer be found
- To test a sample which cannot be cut or altered for routine testing
- When there are questions regarding samples, which are simply too costly to destroy
- To test a mixed lot of materials that seems to not be what was purchased
TYPES OF PMI EQUIPMENT
Handheld devices commonly available are optical emission spectrometers, x-ray, and laser instruments. These devices are typical of the “point and click” variety and require very little operator training or surface preparation.
The drawback of handheld devices is they only produce qualitative results that provide base-level information such as a material is alloy 316, but do not offer specific percentages of each element within the sample. Handheld devices cannot provide truly accurate information on levels of Carbon, Phosphorous, Sulfur, and Nitrogen in the sample, but are good for sorting and are widely used in scrap yards as a sorting tool.
Mobile and Laboratory OES provide material chemistry information through quantitative analysis and are capable of determining the composition of the alloy sample. Even within each alloy are varying ranges of elemental acceptability and the specific percentages of each element are determined by the end-use application. Mobile and Laboratory equipment can provide this level of information through more sophisticated technology than you will find with handheld devices.
Laboratory OES instruments are capable of providing very specific results to a further degree of accuracy than mobile devices. However, they are large and not capable of being transported into the field for testing purposes. When PMI testing is needed on the shop floor or in the field, we turn to mobile devices.
Mobile PMI Limitations
There are a few things that need to be recognized with mobile instruments.
Severe jarring of the instruments can cause instrument failure. The component which analyzes the spectral line, the CCD detector, can be moved if the instrument is jarred too much. This can cause the instrument to read incorrectly. Mobile devices are larger than handheld devices and care needs to be taken during transport.
Mobile devices can perform general sorting types of testing such as identifying the difference between Steel 316 and 304. However, they can also perform much more precise measurements such as slight differences in carbon between two very similar grades of alloy. Such as splitting “L” Grade – the process of separating two grades that have identical specifications aside for one variation. The variation being that one requires “Low” Carbon.
When a more precise analysis is needed the ambient air needs to be negated and Argon must be used to perform the test. In these instances, an operator must ensure when working in the field enough argon is available to finish the job.
But even with these limitations, mobile OES instruments can provide valuable information needed to ensure the alloy is the correct composition.
Third Party Testing
Occasions may arise when a company needs to have parts tested, and either does not have the capabilities to do so or does not have the time.
Verichek Technical Services, Inc has both on-site and in-house testing capabilities to meet your individual needs. Our staff of technical engineers will test your alloy pieces and provide the verification you need to continue the work. Our quick response time diminishes downtime and allows your process to continue flowing.