Positive material identification (PMI) is essential to verify the grade and composition of stainless steel before releasing into production. Using the wrong grade can damage machining equipment, result in field failure or flout industry regulations. Verifying the grade is especially critical where a low carbon, or L-grade, steel is necessary to reduce corrosion of welded joints and brittleness in the final application.
In recent times, we’ve seen a trend towards LIBS analysis, with the argument that it’s more convenient for on-site PMI testing as light-weight, handheld LIBS analyzers are capable of measuring carbon content. In our experience however, handheld LIBS doesn’t give you the level of accuracy you need to be able to identify and sort your low-carbon steel with confidence in demanding environments.
Carbon is not the only element that gives different properties to grades of stainless steel. Checking for other elements against the specification is just as important as the carbon content. You need high accuracy and low levels of detection for a wide range of elements, including refractory metals, such as: molybdenum, niobium and tungsten, phosphorus and sulfur. This is needed for grades like SS 416 that promise improved machinability. And there’s a growing interest in the addition of boron to increase hardenability; this peaks at around 10 ppm and requires highly accurate analysis to verify. Handheld LIBS analysis isn’t currently able to detect all these elements at the accuracy needed for reliable materials identification.
The carbon equivalent (CE) content concept is used on ferrous materials, typically steel and cast iron, to determine various properties of the alloy when more than just carbon is used as an alloyant, which is typical. The idea is to convert the percentage of alloying elements other than carbon to the equivalent carbon percentage, because the iron-carbon phases are better understood than other iron-alloy phases. Most commonly this concept is used in welding, but it's also used when heat treating and casting cast iron.
Here are just some different models for the calculations:
If you are using the CE concept to estimate properties based on material composition, then you’ll need to accurately measure the concentrations of various alloying elements, such as: manganese, chromium, molybdenum and nickel. LIBS analysis would struggle to give the level of detection necessary to inform your calculation, and you’d find it difficult to predict the weldability of the material.
The use of duplex steel is on the increase, and this specialist grade requires careful identification and separation. While duplex steel includes various elements at precise levels, it’s typically differentiated by nitrogen content. Handheld LIBS analysis can’t test for nitrogen, so is unable to sort your duplex steels.
For the complexities of low-carbon steel grade verification, spark-OES is the reliable and accurate technique that will give you results you can trust. We can understand why handheld LIBS is attractive to those with lots of on-site testing to do. But LIBS simply doesn’t give the accuracy required for the critical compositional elements. However, we have a solution. We have developed the only truly portable spark-OES analyser on the market.
Hitachi High-Tech’s PMI-MASTER Smart is ideally suited for PMI and on-site safety inspections at oil and gas refineries, power plants and permanent structures, such as bridges. It provides very low levels of detection (LOD); in combination with the UVTouch probe, the LOD is 30 ppm. It’s able to detect a wide range of elements, including: phosphorous and sulfur, nitrogen for duplex steel identification, and boron at LOD of 5 ppm.
The PMI-MASTER Smart is truly portable, weighs only 15kg and is compact enough to be easily carried to the point of analysis. It can perform up to 300 analyses with a single battery and requires no warm-up. If you need the ultra-low LOD of the UVTouch probe, then a 20-minute purge is necessary. If you can accept a LOD of 200 ppm, then the standard probe with no purge is more than adequate.
Our PMI-MASTER range is trusted by global oil and gas providers and inspection companies. It comes complete with a comprehensive GRADE Database with over 360,000 materials of almost all worldwide alloy configurations in circulation. You’ll also find in the instrument library most CE calculations, so you can choose the one you need.
PMI-MASTER Smart gives you the accuracy and peace of mind of spark-OES, with the convenience of a mobile device. For more information on using mobile spark OES for your application, get in touch with us today.
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