The 4 Methods for Analysing Carbon in Steel: Which is Best?
In past articles, we discussed the importance of identifying the carbon content in steel because the addition of carbon into steel helps increase properties such as corrosion resistance, weldability, ductility, and hardness but could cause unexpected consequences. Today we will discuss the current methods used for carbon verification.
- Mill Test Reports (MTR)/Manufacturer Specifications.
In certain cases, the MTR can be unreliable or incorrect due to mismarking (a mix-up in the labelling process), unknown supplier, or other reasons. It is best practice to perform a second validation of the MTR to verify goods. Catching the issue at the forefront of inspection – using elemental analysis – avoids the costly problem of determining goods have been developed out of specification after adding value during the fabrication process. Oftentimes, the resulting occurrence must be scrapped entirely. (We’ve previously written about why you can’t always trust the steel mill test report.)
- Third Party Lab.
Sending samples to a third party laboratory to verify material composition confirms content, but takes time (sometimes up to 2 weeks), causes downtime, slows production, and adds costs.
- Mobile Optical Emission Spectroscopy (OES).
With OES technology, an electric arc or spark is used to vaporize a little bit — actually, a fair bit — of the surface of the metal sample, which forms a plasma. Atomized wavelengths are found and are converted into concentration units. Analysis time and sample preparation need to be considered with this technology.
Typically analysis takes about 30 seconds or more, and it always requires sample preparation because the surface must be representative. We’re talking about highly sensitive measurement, measuring down into the parts per million range. So any kind of contamination on that surface will change the measurement dramatically. Even touching your finger to the surface after you’ve prepared it can change the amount of carbon by thousands of parts per million (because of the oils in your hand). So it’s critical to make sure that you have surface that represents the alloy and not the contamination.
That very long cable can also be an issue to those working in an industrial environment. The cable can be run over (commonly by forklifts), get tangled, etc. Damage to the cable will cause extensive downtime and costly delays. If you want a wider range of mobility, you need a longer cable, which of course could cause more tangles and chances to be run over. Even with a longer cable you may not get to the highest shelf, or if you are on a catwalk in a refinery or working in a ditch on a pipeline, you may not be able to reach your material with the instrument.
- Laser Induced Breakdown Spectroscopy (LIBS).
LIBS is a new analytical technique using a highly focused laser to determine the chemical composition of materials — well, it’s not new to the lab, but it’s new to the field. It uses a high-powered micro laser to ablate a small portion of the sample. Compared to portable OES, the amount of material ablated on the sample by the handheld LIBS analyser is a fraction of the total amount. After you ablate it, the technique and analysis is pretty much the same as optical emission.
The difference between the LIBS and OES though, is LIBS is available in a handheld form. It uses a micro laser, instead of the power-hungry electric spark that requires power from a large battery, or plugging it into a wall socket, and is capable of measuring elements, including carbon, in the field for material identification. Handheld LIBS can operate for up to 6 hours with only a small rechargeable Li-Ion battery, similar to those used on your home power tools.
What I like about LIBS, is its powerful analysis capabilities, meaning it’s got a lab quality, or in many cases, near-lab quality result that is directly in your hands. When operating a handheld LIBS analyser be sure to follow your local laser safety regulations. Also, although the high purity argon is necessary, you don’t have the big tanks used with the OES systems. The handheld LIBS analyser 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, calculate the equivalency right online.
All in all, I like the high accuracy, repeatability, precision, portability, and ease of use of the LIBS analyser. If you don’t have accuracy, or if you can’t repeat that accuracy over time, or you can’t reach all your material that needs to be verified, then you really don’t have a viable technology.