I’m referring to the weld, not the food. Fillet welding is the process of joining two pieces of metal together whether they be perpendicular or at an angle. According to Wikipedia, “These welds are commonly referred to as Tee joints which are two pieces of metal perpendicular to each other or Lap joints which are two pieces of metal that overlap and are welded at the edges. The weld is aesthetically triangular in shape and may have a concave, flat or convex surface depending on the welder’s technique.”
Fillet welded joints are common in arc welding. The Welding Institute reports that although fillet welded joints are the mostly frequently used weld joints, they are also one of the most difficult to weld with consistency. Fillet welds require the correct materials, correct size and shape, and a higher heat input than a butt joint of the same thickness. The fillet welded joints must adhere to the correct weld size in relation to the required leg lengths or throat thickness. With less skilled welders these issues can lead to lack of penetration and/or fusion defects. Another difficulty is that the welds are commonly done in places that are difficult to inspect. One may be able to do some visual testing, but quality cannot be assessed and material used cannot be identified using an eyes-only verification process.
If those welds are done on pressure piping systems – like those used in the oil and gas industry – there could be a major problem if the weld is not comprised of the correct material. Steel, the common material for pipes, is known for its exceptional strength but its fatal flaw is its susceptibility to corrosion and rust. One way to protect steel from corrosion is to alloy it with other metals such as chromium (Cr) or nickel (Ni) or to coat it with metallic and non-metallic coatings to provide barrier protection.
Piping system fabricators, especially those in the oil and gas industry, who implement quality assurance programs, have found that the use of portable XRF instruments is making a critical difference when it comes to confirming the metal and alloy in these pipes, as well as in the brackets and other components of a pipeline. So, it makes sense that verifying the elements in the welding material is just as important.
A handheld X-ray fluorescent (XRF) analyzer is a common tool for positive material identification (PMI) in the oil and gas industry. XRF is a non-destructive analytical technique used to determine the elemental composition of materials. XRF analyzers determine the chemistry of a sample by measuring the fluorescent (or secondary) X-ray emitted from a sample when it is excited by a primary X-ray source. Each of the elements present in a sample produces a set of characteristic fluorescent X-rays (“a fingerprint”) that is unique for that specific element, which is why XRF spectroscopy is an excellent technology for qualitative and quantitative analysis of material composition. (You can read more about XRF technology here.)
However, accessing the welds themselves can be a challenge for PMI. As we discussed in a previous article (Are You Falling Short of 100% PMI Because You Can’t Reach The Welds?), first, there can be millions of welds in a pipeline because of the length the pipes run, the multiple components and the multiple weld points in each section. Second, welds can be located in inaccessible areas. There are many tight corners, a lot of clips, and corner welds that are difficult to reach.
Handheld analyzers that are small enough to access awkward corner welds or tight spots and be able to collect 100% of the necessary data must be utilized. Additionally, an exceptional camera function with pinpoint accuracy — shooting the x-ray at a specific analysis location — will help ensure true readings.
Here are pictures of a common tight area in a fillet weld on a spool piece. A 3mm collimator and camera were used to isolate and avoid weld dilution, which can give the PMI inspector confidence that the weld is being accessed and analyzed properly. In the second picture, follow the light from the nose of the XRF analyzer (above the numbers) and you will see how the instrument is focusing in onto the actual weld, which is then confirmed by the screen on the third photo.
A well-done PMI program — utilizing the finest of technology — can help ensure that pipeline failures due to off-spec welding material is a rare occurrence.