What is PMI Testing?
PMI (Positive Material Identification) testing is the analysis of materials to determine the chemical composition of a metal or alloy at particular (usually multiple) steps of alloy manufacturing or in-process alloy installation. Knowing the exact composition and grade of an alloy enables suppliers, plant workers, and other responsible parties in the chain of custody of components to match alloy specifications that are chosen for their specific properties such as heat resistance, corrosion resistance, durability, etc. Having the right alloy in the right place is essential in places like petroleum refineries and chemical plants, because the right alloy with the right properties is often all that stands between a safe, efficient operation and lost time and revenue.
Stainless steel grades differ by chemical composition. The key differences between the various grades are mainly the amount of Fe (iron), Cr (chromium), Ni (nickel), Mo (molybdenum), and Cu (copper). Some grades are very different from each other and have different properties; for example, 400 series stainless steels are magnetic while 300 series stainless steels are not. Other grades are more similar with very similar properties, and it is much harder to tell the difference between them without the aid of technology. For example, 304SS has 18-20% Cr and 8-10.5% Ni, whereas 316SS has 16-18% Cr and 10-14% Ni. Although these alloys are very close in composition, they respond differently to corrosive chemicals and higher temperatures making it necessary to be certain you are using the correct alloy for the job intended.
But my material already has a mill stamp on it identifying the material?
Mixing up material, stamping the wrong grade, losing the paperwork or having the wrong paperwork are all scenarios that have happened and continue to happen in the metals industry. It´s never done on purpose, but there are circumstances that cause these issues making it necessary for PMI to be performed at multiple stages of delivery and use.
In process piping, for example, 41% of the largest losses in the hydrocarbon processing industry are a direct result of failures in piping systems. One of the first-pass, most effective ways to prevent such failures is to conduct PMI to insure that specs are met where corrosion-resistant alloys are necessary, since corrosion is one of the leading causes of piping failures.
A handheld XRF PMI gun can analyze, identify and display grade and composition of a material in less than 5 seconds without creating any marks on the material or destroying the sample in any way.
The method is utilised for quality control and safety compliance, and is an integral part of both production and asset integrity management across many industries including oil and gas, power, chemical, pharmaceutical, nuclear, aerospace and fabrication .
Positive material identification can prevent potential product failure in manufacturing. At oil and gas, power generation and pharmaceutical plants, pre-service and in-service inspection of critical components and welds with PMI can prevent breakdown and its costly consequences.
- Ensure products/components have been manufactured using the correct alloy
- Find potentially mixed-up alloys
- Identify if the wrong material has been used
- Ensure material conforms to the correct standard and specification (both customer and industry)
- Ensure welded components have used the correct filler material
Positive material identification is performed using either of the two techniques below:
- X-ray Fluorescence (XRF) analyser: This is the most common method and the portability of the hand-held equipment allows NDT SERVICE to perform PMI on-site at our customers’ premises. The device scans the metal material and identifies its key elements. However, it cannot detect carbon and some lighter elements and is not suitable for identification of pure carbon steel materials.