The Advantage of Sealed Argon in LIBS Analyzers for PMI of Steel

Laser Induced Breakdown Spectroscopy (LIBS) has proven to be an important technology for Quality Control (QC) and Positive Material Identification (PMI) in material verification programs, especially for the steel industry. Accurate identification of the exact steel grade and composition can be paramount to safety when it comes to steel, which can be used in the construction, aerospace, and automotive industries, among others.

LIBS is the analytical technique that utilizes a highly focused laser that ablates the surface of a material in order to determine its chemical composition. The ablation causes a plasma to form, in which the light generated from the laser is broken down into specific wavelengths that each represent a single element. Similar to a fingerprint, every element of the periodic table emits a set of characteristic lines in the plasma.

LIBS is capable of measuring those alloying elements, including carbon. Carbon is one of the most important alloy ingredients in all families of stainless steels. Carbon in steel controls many physical properties, including hardness, strength, weldability and brittleness. Correct identification of carbon, low alloy and stainless steels in the field can indicate the appropriate grade to be used for specific applications or for design remediation.

LIBS has been around for many years and is a technique used primarily in laboratory equipment but is now available in a portable handheld analyzer, so analysis can be performed during the production process and on finished goods. (You can read how LIBS technology works in this free ebook, LIBS Technology for Non-Scientists.)

Of great importance in stainless steel analysis is the ability to differentiate between L and H grades. Low carbon, “L grade”, stainless steels contain < 0.03% carbon, while those with concentrations above this are considered high carbon, “H Grade,” stainless steels. Most element lines in steel that are analyzed by LIBS include various element wavelengths that are above 200 nm. Below 200 nm air absorption becomes an overriding problem. (A nm=nanometer is a unit of length equal to one billionth of a meter.) However, the 193nm carbon line is below that level and thus is highly absorbed in air.

This means there must be a very pure argon atmosphere in order to avoid the loss of signal with the carbon emission line at low levels of concentration such as L grade SS. (The “L” grades of stainless steel indicate low carbon, which is used to provide extra corrosion resistance after welding.) Argon is used to stabilize and promote plasma formation during LIBS analysis. Additionally, it flushes the volume around the plasma to allow the detection of carbon that emits short wavelengths.

Although high purity argon is necessary, with the new handheld design you don’t need the big, bulky tanks used with the OES systems. The handheld LIBS analyzer uses a small cartridge that screws right into the instrument. It’s always the same quality. That’s critical for good results, so you can identify carbon, and calculate the equivalency right online.

When it comes to argon, a best practice is to require a complete seal of the argon atmosphere in the burn chamber, with an indicator so the operator knows when the integrity of that seal is lost. Some LIBS units on the market do not provide that seal integrity indicator and instead rely on overpressure to avoid air mixing in with the argon. This is a compromised design which makes it convenient — but not integrous. The result is some air may mix in with the pure argon and degrade the signal of carbon. The bottom line is a reduced level of repeatability and reproducibility as well as loss of long-term stability, especially for the lowest levels of carbon. (Read Repeatability in LIBS Testing: Why Is It Critical?) For this reason users are often required to take many more tests – and still may obtain less reproducible results.

A design that is more demanding in terms of a perfect seal gives a more assured pure argon atmosphere every measurement. This avoids the possibility of carbon signal degradation issues.

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