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November 16, 2012
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Harnessing The Power Of PMI In Reliability Investigations

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A non-destructive form of positive material identification can be a shortcut to uncovering the root cause of bad welds, parts failure and more. It also can be an effective way to verify on-spec conditions in your equipment and systems.

 

When the maintenance manager at a Midwestern water treatment plant was confronted with a wet floor in the facility’s pump room, his crew was perplexed. The team could see that a cracked pump housing was the culprit and knew they could repair it with weld filler material. What they didn’t know was the type of material that had been used in the manufacturing of the pump’s housing—key information that would let them select the proper weld filler. After a brief discussion, the next step was obvious: Contact a materials testing laboratory to determine the material used in the pump.

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Fig. 1. Positive material identification using X-ray fluorescence (XRF) analysis can provide material validation at any point in the manufacture of a finished product. 

Defining PMI 
Materials testing labs use a process called “positive material identification” (PMI) to determine the makeup of metallic alloys. As in crime-scene investigations, where a number of tools are used to help unravel mysteries, PMI is one means of tracing a material back to its original Material Test Report (MTR), which is a certified chemical analysis that identifies metallic components. Originally, PMI was a destructive technique performed using wet chemical analysis methods for comparison to material-grade compositional requirements. The process was highly accurate—but labor-intensive, time-consuming and costly.  

As technology has progressed, advances in technology and electronics have led to portable, handheld X-ray analyzers that are capable of providing valuable chemical information that was previously only available using fixed laboratory equipment. Portable X-ray fluorescence (XRF), for example, provides a fast, easy, non-destructive analysis with minimal capital investment (see Sidebar below). Originally designed as a means of identifying high-alloyed corrosion-resistant materials used in the petroleum industry, this technology has demonstrated the ability to provide analysis for most metallic engineering materials used across industry today.  

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Fig. 2.  Handheld XRF devices offer a fast, easy way to conduct a non-destructive analysis with minimal capital investment.

 

XRF analyzers work by exposing a material sample to a beam of X-rays generated from an internal source. The X-rays interact with the sample surface, locally exciting atoms that emit energy after excitation. This emitted energy is characteristic of the elements present in the sample and is identifiable as a characteristic energy level. The intensity of these unique energy levels represents the relative concentration of each element of interest that is collected on a solid-state detector. This approach can gather qualitative and often quantitative information. Modern software packages have the ability to identify hundreds of commercially recognizable alloy grades. 

PMI using XRF analysis has become an important tool for quality-assurance applications in nearly all areas of industry, but especially in petroleum processing, nuclear building materials, foundry and scrap. Quality systems are increasingly demanding complete traceability for all materials used in production. This can mean that at any point in the design, manufacture or assembly of components into a final product, material traceability may be required on demand. When traceability for production machinery is required and not available, material validation using portable XRF may be indicated to provide a PMI link to an original MTR. 

Most modern instruments are designed to permit spot analysis for applications. This is ideal for limited-access areas and for inventory control and material-sorting processes that are based on differences in chemistry. Material validation of incoming stock and final assemblies demonstrate that proper materials are being used. And while the XRF-PMI technique is best suited for high-alloy carbon steel, stainless steel, nickel and cobalt materials, it can analyze other materials. Rapid quantification with reliable results is possible with minimum sample preparation, such as a light surface abrasion. 

XRF techniques can also be used to recreate lost information, such as stock-traceability paperwork. The instruments used in this type of PMI can provide comparison to hundreds of alloy grades and families. 

Industry-accepted procedures and work instructions for PMI using portable XRF devices define acceptance and rejection criteria and aspects to the testing requirements. These considerations include minimum required radiation safety training and safe practices to be used when operating this equipment.

XRF limitations and successes 

PMI using portable XRF has broad applications—but also some limitations. These include the inability to provide analysis of light elements such as carbon. This is an issue in analysis of carbon steel, where carbon may be the only element separating grades of material, allowing for identification of the alloy family or materials. Other light elements, including Mg, Al, Si, P and S, suffer from poor limits of quantification, which may impose further restrictions on the effective application of the technique for some materials.

Still, XRF-PMI remains a highly effective way to verify most materials, as its many successes prove. Here are a few of the many mysteries that have been solved by XRF-PMI:

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Fig. 3. PMI analysis of a leaky water-treatment plant pump verified that the wrong weld filler had been used around the footing and inner surface of the cover, thus preventing the maintenance crew from making a repair weld without heat-treating (which the crew couldn’t do). Verdict: The plant had to replace the pump.

The case of the leaky water pump…

Let’s return to the water treatment plant’s pump problem. In this case, the crew removed the pump and took it to the materials testing laboratory to verify the weld material around the footing and the inner surface of the cover for filler-material compatibility to the base material. PMI testing techniques showed the housing to be nickel-based Inconel 825. The analysis also showed the wrong weld filler had been used originally, which caused corrosion and the subsequent leak after just a few years of service. Because the characteristics of Inconel 825 prevented making a repair weld without heat-treating—which the crew couldn’t perform—the plant had to purchase a replacement pump.

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Fig. 4. Before going online, a new coal plant needed to ensure that its critical components had been built to specified requirements (which included verification  of base materials and weld fillers). Quick, onsite PMI analyses of each panel of a 30-section fabricated meter- housing body and every six inches of weld material confirmed that this component passed with flying colors. The unit went into service immediately.

The case of the critical pressure vessel… 

In preparation to go online, a new coal plant needed to ensure that its critical components had been built/fabricated to specified requirements—which included verification of base materials and weld fillers. Of particular interest was a fabricated meter housing body with more than 30 fabrication sections and welds. Because this critical component would be in daily operation in severe conditions, management ordered a quick, onsite PMI analysis. Each panel and every six inches of weld material was verified to ensure compliance. After the analyses confirmed that the unit passed all requirements, the plant was able to put it into service immediately. 

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Fig. 5. When an aftermarket performance-motorcycle fabrication shop had trouble making bends in formed metal components,  PMI analysis helped trace the problem back to two mixed lots of steel in the shop’s stock room.

The case of the suspected mixed-material flat stock… 

When an aftermarket performance-motorcycle fabrication shop was having trouble making bends in formed metal components, a lab was called in to check for mixed material in the stock room. The lab assisted by restoring traceability of two mixed lots of steel. The material intended for the work was alloy steel 4130 (UNS G41300), which contains numerous elements, including 0.80% to 1.10% chromium. Because XRF analysis can detect chromium, as well as nickel and molybdenum, in significant quantities, the process enabled the lab to sort the shop’s materials using this element alone. It determined that some stock contained lower levels of chromium than required (0.03% to 0.05%), and was set aside for other uses.

Putting PMI to work in your operations

Whether monitoring incoming and outgoing materials, reverse engineering, contract satisfaction or product repair, portable XRF provides the ability to quickly verify material-grade composition using  non-destructive testing. As an indisputable value-add in an era of high-quality expectations, counterfeit parts and product traceability, this type of PMI could easily become one of your favorite investigative tools. MT 

Greg Mann is Chemical Analysis Group Leader/Materials Specialist, for Anderson Laboratories, Inc., in Greendale, WI. Telephone: (414) 421-7600; or email: gregm@andersonlabs.com.

Michael Porfilio is Director of Operations at Anderson Laboratories. Telephone: (414) 421-7600; email: mikep@andersonlabs.com.

 

A Look Inside X-Ray Fluorescence

X-ray fluorescence (XRF) spectrometry is a technique for the analysis of elements that has broad application in both science and industry. It’s based on the principle that individual atoms, when excited by an external energy source, emit X-ray photons of a characteristic energy or wavelength. By counting the number of photons of each energy emitted from a sample, an element may be identified.

XRF technology is an outgrowth of Wilhelm Röntgen’s discovery of high-energy radiation, which he dubbed X-rays, in 1895. In 1913, English physicist Henry Moseley advanced the concept when he constructed an X-ray spectrometer to measure the frequency of certain types of X-rays produced from tubes with different electrode materials. 

Moseley’s results showed that the ordering of the wavelengths of the X-ray emissions of the elements happened to coincide with the ordering of the elements by atomic number. This relationship is now known as Moseley’s Law. 

X-rays can be used to identify elements because of the characteristic radiation emitted from the inner electronic shells of the atoms. This radiation is comprised of X-ray photons whose specific energies permit the identification of their source atoms. The X-ray photons are emitted during fluorescence, which is the emission of an X-ray photon that occurs when atoms in a tested material are rearranged by the external energy source. By detecting this photon and measuring its energy, an element can be determined.

While the first commercially pro-duced X-ray spectrometer was developed in the 1950s, modern, handheld XRF instruments have revolutionized the field. Capable of analyzing solid, liquid, and thin-film samples for major and trace components, the analysis is rapid and sample preparation is usually minimal or not required. As noted in the accompanying article, handheld analyzers allow XRF technology to be used onsite to quickly gather key information, offering a low-cost alternative to the more detailed, but time-consuming laboratory techniques that involve fixed machinery and sample destruction. In the industrial maintenance and reliability arena, XRF technology is used most often on metals—to determine components of alloys in parts, equipment and raw stock—and on chemicals, especially petrochemicals. 

Growth opportunities for these systems were advanced significantly with passage of the European Union’s Regulation of Hazardous Substances (RoHS) directive and the Waste Electrical and Electronic Equipment (WEEE) directive, which restrict the use of certain metals in products. 

Sources: University of Missouri, National Science Foundation, Northern Arizona University

More About Anderson Laboratories, Inc.

Established in 1939, Anderson Laboratories is an independent materials testing facility located in Greendale, WI. Specialties include chemical analysis, mechanical testing, welding procedure and performance qualification, failure analysis, environmental and corrosion testing, as well as on-site evaluations. Analyses are performed by skilled personnel with a combined experience of more than 70 years, and instrumentation is programmed for the majority of engineering alloys used in modern manufacturing. Anderson’s tensile capabilities range from several grams to 200,000 pounds force, both in tension and compression. Computer-controlled testing equipment can determine many properties in both English and Metric units. Compliance to the appropriate ASTM, ASME, EN, JIS and other applicable specifications is considered of vital importance in obtaining accurate mechanical testing results. Anderson Laboratories is accredited to the International Organization for Standardization (ISO) 17025 through the American Association for Laboratory Accreditation (A2LA) and Det Norske Veritas (DNV). Learn more at www.andersonlabs.com.

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